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1. If the javac command fails review the JAVA INSTALLATION INSTRUCTIONS To run the program you supply it with the x and y coordinates of two points on a plane For this version of Distance only integer points are supported The command sequence is java Distance x Yo X1 Y1 to compute the distance between the points xo Yo and x y For example the command java Distance 0 3 4 0 will compute the distance between the points 0 3 and 4 0 and print the following Distance between java awt Point x 0 y 3 and java awt Point x 4 y 0 is 5 0 The command java Distance 4 5 11 19 will compute the distance between the points 4 5 and 11 19 Distance between java awt Point x 4 y 5 and java awt Point x 11 y 19 is 20 518284528683193 We ll explain this strange looking output and also show how to improve it later 8 2 Detailed Program Structure and Overview As promised we will now provide a detailed description of this Java program We will discuss the syntax and structure of the program and the meaning of that structure 1 Chapter 6 on page 35 2 HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMINGS2FINSTALLATION 52 Detailed Program Structure and Overview 8 2 1 Introduction to Java Syntax The syntax of a Java class is the characters and symbols and their structure used to code the class using Unicode characters A fuller treatment of
2. Now suppose you define a comparator that can compare Animals class AnimalComparator implements Comparator lt Animal gt int compare Animal a Animal b Since Dogs are Animals you can use this comparator to compare Dogs also Comparators for any superclass of Dog can also compare Dog but comparators for any strict subclass cannot 123 Generics Comparator lt Animal gt myAnimalComparator new AnimalComparator static int compareTwoDogs Comparator lt super Dog gt comp Dog dogl Dog dog2 return comp compare dogl dog2 The above code is valid because the Animal class is a supertype of the Dog class Use of a class that is not a supertype would cause a compilation error CATEGORY JAVA PROGRAMMING FR PROGRAMMATION JAVA TYPES G N RIQUES 2 HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVAS20PROGRAMMING 3 HTTP FR WIKIBOOKS ORG WIKI PROGRAMMATIONS20JAVAS2FTYPES 20GS5E9N oe E9IRIQUES 124 22 Defining Classes 22 1 Fundamentals Every class in Java can be composed of the following elements fields Fields are variables that hold data specific to each object For example an employee might have an ID number They are also called member variables There is one field for each object of a class member methods Member methods perform operations on an object For example an em ployee might have a method to issue his paycheck or to acces
3. 8 HTTP EN WIKIPEDIA ORG WIKI C 2B 2B 9 HTTP EN WIKIPEDIA ORG WIKI VISUALS20BASIC 10 HTTP EN WIKIPEDIA ORG WIKI PYTHON 20 28PROGRAMMING 20LANGUAGES29 11 HTTP EN WIKIPEDIA ORG WIKI RUBY 20 28PROGRAMMING 20LANGUAGES29 Necessary prerequisites 2 2 2 As an author or contributor If you intent on writing content for this book you need to do the following e When writing content for this book you can always pose as an anonymous author however we recommend you sign in into the Wikibooks website when doing so It becomes easier to track and acknowledge changes to certain parts parts of the book Furthermore the opinions and views of logged in users are given precedence over anonymous users Once you have started contributing content for this book make sure that you add your name to THE AUTHORS AND CONTRIBUTORS LIsT BE BOLD and try to follow the CONVENTIONS for this Wikibook It is important that the conventions for this book be followed to the letter to make content consistent and reliable throughout 2 3 Necessary prerequisites 2 3 1 Installing the Java platform For in depth information see the chapter on THE JAVA PLATFORM In order to make use of the content in this book you would need to follow along each and every tutorial rather than simply reading through the book But to do so you would need access to a computer with the Java platform installed on it the Java platform is the basic prerequisite
4. 173 30 11 MINIMIZE THE USE OF THE NEW TYPE INT SYNTAX FOR CREATING ARRAYS OF OB Contents 30 12 CHECK ALL REFERENCES OBTAINED FROM UNTRUSTED METHODS 30 13 COMPARING STRING VARIABLE WITH A STRING LITERAL 30 1 4 SEEALSO ii bb a ed ee eee hook Sea ele 31 LINKS 31 1 EXTERNAL REFERENCES 32 LICENSE 33 GNU FREE DOCUMENTATION LICENSE 34 AUTHORS LIST OF FIGURES 31 2 EXTERNAL LINKS diia Soh eek ee ee Ba Ge Wik Ge es oe ea ea 177 ata o 177 a 177 181 183 185 193 Contents 1 Introduction The beautiful thing about learning is nobody can take it away from you Learning a computer programming language is AKIN TO a toddler s first few steps towards an upright walk You stumble and you fall but when you start walking you take upon it as SECOND NATURE Learning to program is similar in many ways to learning to walk However once you start programming you never cease to evolve Learn one programming language and you know them all This book is a concentrated effort in trying to get you to take your baby steps towards program ming in the Java programming language Although this book is primarily meant to act as a learning aid for beginners it can equally be helpful as a reference manual and guide for the in termediate and experienced programmers The book is laid out in such a manner that with each successive chapter the complexity of programming constructs increase Thus programmers
5. public static void main String args Distance dist new Distance intValue args 0 intValue args 1 intValue args 2 intValue args 3 dist printDistance The main method invokes the final method intValue four times The intValue takes a single string parameter and returns the integer value represented in the string For example intValue 3 will return the integer 3 22 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FSTRINGS20CONCATENATION 57 Understanding a Java Program The intValue method The intValue method delegates its job to the Integer parselInt method The main method could have called Integer parselnt directly the intValue method simply makes the main method slightly more readable private static int intValue String data return Integer parselnt data This method is PR t va r E22 since like the fields point 0 and point it is part of the internal im plementation of the class and is not part of the external programming interface of the Distance class Static vs Instance Methods Both the main and intValue methods are STATIC METHODS The static keyword tells the compiler to create a single memory space associated with the class Each individual object in stantiated has its own private state variables and methods but use the same static methods and members common to the single class object created by the compiler wh
6. public AClass counter 1 public int getNumber0fObjectsCreated return counter The counter variable is incremented each time an object is created Public static variable should not be used as these become GLOBAL variables that can be accessed from everywhere in the program Global constants can be used however See below static public final String CONSTANT_VAR Const e Static methods can be used for utility functions or for functions that do not belong to any particular object For example 137 Using Static Members public Match public static int addTwoNumbers int parl int par2 return parl par2 25 3 Danger of static variables Use static variables only for e data sharing be careful e defining global constants Use static methods for e utility functions Using static variables and or method for other purposes goes against object orientation and will make your code either harder to read or maintain 25 4 External links STATIC INTERACTIVE JAVA Lesson CATEGORY JAVA PROGRAMMING 1 HTTP JAVALESSONS COM CGI BIN FUN JAVA TUTORIALS MAIN CG1I SES A07898 CODE STC amp SUB FUN 2 HTTP EN WIKIBOOKS ORG WIKI CATEGORYS3AJAVA S20PROGRAMMING 138 26 Destroying Objects Unlike in many other object oriented programming languages Java performs automatic garbage collection any unreferenced objects are automatically
7. queues provide additional insertion extraction and inspection opera tions There are FIFO first in first out and LIFO last in first out queues java util BlockingQueue lt E gt waits for the queue to become non empty when retrieving an element and waits for space to become available in the queue when storing an element Best used for producer consumer queues 29 4 1 Set The basic implementation of the Set interface is the HashSet interface s Collection AbstractCollection interface AbstractSet Set e Class gt Class CopyOnwriteArraySet EnumSet interface SortedSet sj D l oe Since 15 Figure 19 java util TreeSet lt E gt Elements are sorted not synchronized null not allowed java util HashSet lt E gt Not synchronized Allows the null elements java util CopyOnWriteArraySet lt E gt Thread safe a fresh copy is created during modification operation Add update delete are expensive 152 Set or List or Queue java util EnumSet lt E extends Enum lt E gt gt All of the elements in an enum set must come from a single enum type that is specified explicitly or implicitly when the set is created Enum sets are represented internally as bit vectors java util LinkedHashSet lt E gt Same as HashSet plus defines the iteration ordering which is the order in which elements were inserted into the set Detecting duplicate objects in Sets Set cannot
8. BASTIE BE BE HONIK LEN22 HMAILIT 01000 PAGE26 185 Authors BIGDUNC BRENTP23 CARSRACBOT CECLAUSON COLFULUS COLINDAVEN CooLBoY1234 CSPURRIER DALLAS1278 DAN POLANSKY DARKLAMA 2 DARKXXXXILLUSION DAvIDBOURGUIGNON DAVIDCARY DERBETH DEVOURERO DICKDICKDICK DIRK HUNNIGER DIRK GENTLY Djg DMONEGO DRAwWDE83 ELI VERBUYST 4 ELLMIST 720 ERVINN 937 WO 5 ES O5 QI QI COD a Rae co Pee wo 22 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER BIGDUNC 23 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER BRENTP 24 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER CARSRACBOT 25 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER CECLAUSON 26 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER COLFULUS 27 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER COLINDAVEN 28 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER COOLBOY1234 29 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER CSPURRIER 30 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER DALLAS1278 31 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER DAN_POLANSKY 32 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER DARKLAMA 33 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER DARKXXXXILLUSION 34 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER DAVIDBOURGUIGNON 35 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLEUSER DAVIDCARY 36 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER DERBETH 37 HTTP EN WIKI
9. BigObject That ShouldNotBeSerializedWithAButton bigOne Button aButton new Button Nested class to implement the action listener class MyActionListener implements ActionListener public void actionPerformed ActionEvent e 81 Nested Classes System out println Hello There MyApp aButton addActionListener mew MyActionListener Using anonymous classes is especially preferable when you intend to use many different classes that each implement the same Interface CATEGORY JAVA PROGRAMMING 1 HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVAS20PROGRAMMING 82 14 Access Modifiers 14 1 Access modifiers You surely would have noticed by now the words public protected and private at the beginning of class s method declarations used in this book These keywords are called the access modifiers in the Java language syntax and can be defined as Quote Keywords that help set the visibility and accessibility of a class its member variables and methods The following table shows what Access Modifiers are appropriate for classes nested classes member variables and methods Class Nested Method Interface Interface class or Mem method ber vari signature able public visible sameasits sameasits visible visible from any class class from any from any where where where protected N A its class its class N A N A and its and its subclass subclass and fro
10. Note that we can not add anything to this collection We can only take out elements of type Object from it So what is the use of this variable if we can not add anything to the collection it represents The use of this new construct will be clear when you want to create a generic method that takes any collection 122 Wildcard Types public static void printElements Collection lt gt anycoll Iterator lt gt iter anycoll iterator while iter hasNext System out print iter next Wildcard for a specific type of classes lt extends ClassName gt specifies a restriction on the types of classes that may used For example to create a collection that may only contain Serializable objects specify Collection lt extends Serializable gt serColl new ArrayList lt String gt The above code is valid because the String class is serializable Use of a class that is not serializ able would cause a compilation error The following collection can only contain objects that extend the class Animal class Dog extends Animal Create Animal Collection variable Collection lt extends Animal gt animalColl new ArrayList lt Dog gt lt super ClassName gt specifies a restriction on the types of classes that may be used For example to declare a Comparator that can compare Dogs you use Comparator lt super Dog gt myComparator
11. features operations list are defined by the SuperClass class Interface inter mew ClassName features operations list are defined by the interface In the car analogy the created car may have different Type of drivers We create separate user manuals for them Average user manual Power user manual Child user manual or Handi capped user manual Each type of user manual describes only those features operations ap propriate for the type of driver The Power driver may have additional gears to switch to higher speeds that are not available to other type of users When the car key is passed from an adult to a child we replacing the user manuals that is called Type Casting In Java casts can occur in three ways up casting going up in the inheritance tree until we reach the Object up Casting to an interface the class implements down casting until we reach the class the object was created from Type and Type Casting will be covered in more details later at JAVA PROGRAMMING TYPES module 11 4 Multiple classes in a Java file Normally a Java file can have one and only one public Java class However a given file can contain additional non public classes public class OuterClass class AdditionalClass Because they have the package default access specifier the AdditionalClass can be accessed only in the same package These additional classes compile to separa
12. 177 Links FREE JAVA ADVANCED JAVA Books FREE JAVA AND J2EE EBOOKs JAVA BOOKS AVAILABLE FOR FREE DOWNLOADS ROEDY GREEN S JAVA amp INTERNET GLOSSARY A comprehensive reference that s also an excel lent starting point for beginners C2 JAVA LANGUAGE NETBEANS IDE ECLIPSE IDE ZEUS FOR WINDOWS IDE OFFICIAL JAVA HOME SITE ORIGINAL JAVA WHITEPAPER COMPLETE JAVA PROGRAMMING TUTORIALS JAVAPASSION JAVA COURSE The Javapassion Site Java Course driven by Sang Shin from Sun BEANSHELL Interpreted version THE JAVA LANGUAGE SPECIFICATION THIRD EDITION This book attempts a complete spec ification of the syntax and semantics of the language THE JAVA VIRTUAL MACHINE SPECIFICATION SECOND EDITION and AMENDMENTS A PURE JAVA DESKTOP JAVAPEDIA PROJECT Bruce Eckel Thinking in Java Third edition HTTP WWW MINDVIEW NET BOoKs TIJ Bruce has an C C free book available on line too JAVAGAMEDEVELOPMENT Daily news and articles on Java Game Development JAVA CERTIFICATIONS SITE SCJPR SCWCD SCBCD JAvA 5 0 SCEA JAVA PROGRAMMING FAQS AND TUTORIALS 8 MORE RESOURCES JAVA LESSONS 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 WWWwW FREEBOOKCENTRE NET JAVATECH JAVACATEGORY HTML WWWwW BESTEBOOKSWORLD COM DEFAULT ASP CAT 55 WWW TECHBOOKSFORFREE COM JAVA SHTML WWW MINDPROD COM JGLOSS JGLOSS HTML c2 comM CGI WIK
13. 6 80 JGREATOR I Z site Sal he eck ei lel ee Bt ee a a 6 9 ECLIPSE X z 6 A AR ae WS S W eR ee Gna Se Hee GS we 6 10 NETBEANS ocea ma he u Ge u RW Ae Be EO en Pe es Da Da 6 11 BLUEJ v bb us hw ku q oe eR A Ho be we LS RS 0 12 KAWA dee aah de Se BW ow ae oe ae wh Sw Pee Noe ea S 6 395 GANT A a arte rene ed OG ea A ee ater se etal her ated Bt BS A hte te tbe 6 14 THE VT COMPILER t u K ack ee ea ee Ra a BA ESE yusa ES gt UQ NO gl 13 13 14 17 17 17 20 21 21 25 29 33 33 35 35 36 36 37 37 37 38 38 38 39 39 39 40 42 HI Contents 10 11 12 13 14 15 IV EXECUTION 7 1 JSE CODE EXECUTION LS asr a as A aa S we ee a Eh wc eae 7 2 JZEE CODEEXECUTION cis Cees ea a Ae A ee AA 7 3 JINI ote a Galea we gue A A AA A ER PE w a Ges ae UNDERSTANDING A JAVA PROGRAM 8 1 THE DISTANCE CLASS INTENT SOURCE AND USE 8 2 DETAILED PROGRAM STRUCTURE AND OVERVIEW 8 3 COMMENTS IN JAVA PROGRAMS ee SYNTAX 9 1 UNICODE a 6a 55405 5 a S a we arn a Oe SS lS a arig 9 2 TATERALS a S Soest A Silu s u Reh wapu en Oe td Soin Moe 9 3 BLOCKS Ste fe A Sie Bloc Spec EO DMD o ada font Z 9 4 WHITESPACES 4 6 0 eh 6 24 4 3 26 Se whee eee es SIRES Se S ka Ee 9 5 REQUIRED WHITESPACE 1 a sea ee 9 6 INDENTATION 2 0 6 68 s s eb 8 SER Re Quq HS Ee LEN A ss STATEMENTS 10 1 WHAT EXACTLY ARE STATEMENTS 0 ee ee 10 2 WHERE DO YOU FIND STATEMENT
14. BOOLEAN AND NULL PRIMITIVES 17 1 2 Object Reference Types 17 1 3 Arrays 17 2 Java as hybrid language On the one hand Java is a strongly typed language in that all expressions and declarations have types either declared or inferred and the language only allows programs which adhere to the type constraints Therefore most type errors are caught and detected by the Java compiler Some type errors can still occur at runtime because Java supports a cast operation which is a way of changing the type of one expression to another However Java performs run time type checking when doing such casts so an incorrect type cast will cause a runtime exception rather than succeeding silently and allowing data corruption On the other hand Java is also known as a hybrid language While supporting object oriented OO programming Java is not a pure OO language like SMALLTALK or RuBY Instead Java offers both object types and PRIMITIVE TYPES Primitive types are used for boolean character and numeric values and operations This allows relatively good performance when manipulat ing numeric data at the expense of flexibility For example you cannot subclass the primitive types and add new operations to them 17 3 Examples of Types Below are two examples of Java types and a brief description of the allowed values and opera tions for these types Additional details on each are available in other modules 17 3 1 Example int The PRIMITIVE
15. Java first edition JSE Java Standard Edition had has the following in its belt e GUI capabilities AWT Swing Network computing capabilities RMI e Multi tasking capabilities Threads With JSE the following Java code executions are possible Computer JVM Java Application Data base Figure 9 Figure 1 Stand alone execution Stand alone Java application 1 HTTP EN WIKIPEDIA ORG WIKI JAVAS20REMOTES20METHODS20INVOCATION 43 Execution Figure 1 Stand alone application refers to a Java program where both the user interface and business modules are running on the same computer The application may or may not use a database to persist data The user interface could be either AWT or Swing The application would start with a main method of a Class The application stops when the main method exits or if an exception is thrown from the application to the JVM Classes are loaded to memory and compiled as needed either from the file system or from a jar file by the JVM Invocation of Java programs distributed in this manner requires usage of the command line Once the user has all the class files he needs to launch the application by the following com mand line where Main is the name of the class containing the main method java Main Java jar class libraries Utility classes framework classes and or third party classes are usually packaged and dis tributed in Java jar
16. Only override the hashCode method if you want to eliminate value duplicates Do not override the hashCode 3 method if you know that the values of your objects are dif ferent or if you only want to prevent adding the exactly same object Beware that the hashCode 19 may be used in other collection implementaions like in a Hashtable to find an object fast Overriding the default hashCode method may affect per formance there e the default hashCodes are unique for each object created so if you decide not to override the hashCode method there is no point overriding the equal method as it won t be called SortedSet The SortedSet interface extends the Set Interface All elements in the SortedSet must implement the Comparable Interface furthermore all elements must be mutually comparable Note that the ordering maintained by a sorted set must be consistent with equals if the sorted set is to correctly implement the Set interface This is so because the Set interface is defined in terms of the equals operation but a sorted set performs all element comparisons using its compare method so two elements that are deemed equal by this method are from the standpoint of the sorted set equal 8 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FAPIS2FJAVA LANG OBJECTS 23HASHCODE 28 529 520METHOD 9 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FAPIS2FJAVA LANG OBJECTS 23HASHCODE 28 29 520METHOD 10 HTTP EN
17. T T E T E T TP WIKI ECLIPSE ORG I 2 3 3 For Linux Installation using Terminal TP EN WIKIPEDIA ORG WIKI DEBUGGING TP DOWNLOAD NETBEANS ORG NETBEANS 7 0 1 FINAL BUNDLES NETBEANS 7 0 1 ML WINDOWS EXE rp NETBEANS ORG COMMUNITY RELEASES 70 INSTALL HTML TP WWW ECLIPSE ORG DOWNLOADS DOWNLOAD PHP FILE TECHNOLOGY EPP DOWNLOADS LEASE INDIGO SR1 ECLIPSE JAVA INDIGO SR1 WIN32 ZIP amp R 1 TP WWW ECLIPSE ORG DOWNLOADS DOWNLOAD PHP FILE TECHNOLOGY EPP DOWNLOADS LEASE INDIGO SR1 ECLIPSE JAVA INDIGO SR1 WIN32 xX86_64 ZIP amp R 1 ECLIPSE INSTALLATION Downloading and installing the Java platform on Linux machines in particular Ubuntu Linux is very easy and straight forward To use the terminal to download and install the Java platform follow the instructions below 1 Download instructions For Ubuntu go to Application Acces sories gt Terminal Alternatively you can press Alt to open the Run Application window At the prompt type xterm or gnome terminal to open the Terminal window At the prompt write the following sudo apt get install openjdk 7 jdk openjdk 7 jre openjdk 7 doc All Java software should be installed and instantly available now Alternatively you can manually DOWNLOAD THE JAVA SOFTWARE from the Oracle website 24 HTTP WWW ORACLE COM TECHNETWORK JAVA JAVASE DOWNLOADS INDEX HTML 10 Necessary prerequisites For the convenience of our readers the
18. files These jar files need to be put in the CLASSPATH of the java program from which these classes are going to be used If a jar file is executable it can be run from the command line java jar Application jar Client Computer Server Compu Browser En Application Server lt html gt lt APPLET name Hello codebase Data base lt APPLET gt lt html gt Figure 10 Figure 2 Applet Execution Java Applet code Figure 2 Java Applets are Java code referenced from HTML pages by the lt APPLET gt tag The Java code is downloaded from a server and runs in the client browser JVM Java has built in support to render applets in the browser window 2 HTTP EN WIKIPEDIA ORG WIKI HTML 44 JSE code execution Sophisticated GUI clients were found hard to develop mostly because of download time in compatibilities between browser JVM implementations and communication requirements back to the server Applets are rarely used today and are most commonly used as small sepa rate graphic like animation applets The popularity of Java declined when Microsoft withdrew its Java support from INTERNET EXPLORER default configuration however the plugin is still available as a free download from JAVA COM More information can be found about applets at the APPLET CHAPTER in this book Also Wikipedia has an article about JAVA APPLETS Client Server applications The client server applications c
19. the structure of such data and operations associated with it Any data be it numeric or a sentence of words can have a different data type For in stance just to define different types of numbers in Java there are about six simple types available to programmers some define whole numbers integers numbers and others define numbers with a decimal values floating point numbers Java also gives freedom to programmers to cre ate complex and customizable data types We will deal with complex data types in later chapters 17 1 Data Types in Java Java is considered a strongly typed programming language in that it is obligatory for all data expressions and declarations within the code to have a certain type associated with it This is either declared or inferred and the Java language only allows programs to run if they adhere to type constraints As we have discussed above you can have types that define a number or types that define tex tual content within your program If you present a numeric type with data that is not numeric say textual content then such declarations would violate Java s type system This gives Java the unique ability of type safety Java checks if an expression or data is encountered with an incor rect type or none at all It then automatically flags this occurrence as an error at compile time Most type related errors are caught by the Java compiler hence making a program more secure and safe once compiled completely
20. them needs to be referenced by typing in the whole package name For Example package com mycompany myapplication mymodule import com yourcompany yourapplication youmodule SameClassName SameClassName yourObjectRef new SameClassNanme com hercompany herapplication hermodule SameClassName herObjectRef new com hercompany herapplication hermodule SameClassName 75 Packages The Java package has one more interesting characteristic the package name corresponds where the actual file is stored on the file system And that is actually how the com piler and the class loader find the Java files on the file system For example the class com yourcompany yourapplication yourmodule YourClass is stored on the file system in the cor responding directory com yourcompany yourapplication yourmodule YourClass Because of this package names should be lowercase since in some operating systems the directory names are not case sensitive 12 2 Wildcard imports It is possible to import an entire package using an asterisk import javax swing While it may seem convenient it may cause problems if you make a typographical error For ex ample if you use the above import to use JFrame but then type JFraim frame new JFraim the Java compiler will report an error similar to Cannot find symbol JFraim Even though it seems as if it was imported the compiler is giving the error report at the first mention of JFrai
21. 1 Java applications can be written once and run anywhere This feature of the Java platform is commonly abbreviated to WORA in formal Java texts 21 The Java Platform Native Unix Application Native Windows Application Native Java Application Java Runtime Java Runtime Environment Environment Windows Machine Unix Machine Figure 4 4 1 1 Executing native Java code or byte code Native Java applications are preserved in a special format called the BYTE CODE Byte code remains the same no matter what hardware architecture operating system or software platform it is running under On a file system Java byte code resides in files that have the class also known as a class file or the jar also known as a Java archive extension To run byte code the JRE comes with a special tool appropriately named java Suppose your byte code is called SomeApplication class Ifyou want to execute this Java byte code you would need to use the following command in Command Prompt on Windows and Terminal on Linux or Mac OS java SomeApplication class If you want to execute a Java byte code with a jar extension say SomeApplication jar you would need to use the following command in Command Prompt on Windows and Terminal on Linux or Mac OS java jar SomeApplication jar 1 HTTP EN WIKIPEDIA ORG WIKI JAVAS20BYTECODE 22 Java Runtime Environment JRE Note Not all Java class files or Jav
22. 1 4 version of the exam JAVA LANGUAGE SPECIFICATION 3RD EDITION THINKING IN JAVA4 Java 5 SDK DOCUMENTATION Java 5 SDK DOCUMENTATION IN CHM FORMAT Java 5 API DOCUMENTATION THE JAVA TUTORIAL Sun Developer Network NEW TO JAVA CENTER A SIMPLE JAVA TUTORIAL 9 TWO SEMESTERS OF COLLEGE LEVEL JAVA LECTURES FREE JAVA LESSONS INTERACTIVE JAVA PROGRAMMING TUTORIALS BASED ON EXAMPLES JAVA TUTORIALS FOR KIDS AND ADULTS 31 2 External links JAVA CERTIFICATION MOCK EXAMS 4 500 questions with exam simulator SwINGWIKI Open documentation project containing tips tricks and best practices for Java Swing development JAVATIPS Blog project containing best JAVA tips and tricks OO 1 O Q gt G N B hhh O Q gt Q NNN OO WWW EPRACTIZELABS COM WWW CERTIFICATIONACAREER COM JAVA SUN COM DOCS BOOKS JLS THIRD_EDITION HTML J3TOC HTML WWW MINDVIEW NET BOOKS TIJ JAVA SUN COM J2SE 1 5 0 DOCS INDEX HTML WWW ZEUSEDIT COM FORUM VIEWTOPIC PHP T 10 JAVA SUN COM J2SE 1 5 0 DOCS API INDEX HTML JAVA SUN COM DOCS BOOKS TUTORIAL INDEX HTML JAVA SUN COM DEVELOPER ONLINETRAINING NEW2JAVA INDEX HTML WWWwW ALNAJA7 NET PROGRAMMER 393 1TCS 393 HTM CURMUDGEON99 GOOGLEPAGES COM JAVALESSONS COM WWW KIDWARESOFTWARE COM WWW CERTIFICATIONACAREER COM WWW SWINGWIKI ORG WWW AKKIDI COM HHA AHHH RH H H H H H H H H tg rO Rc r r rg rg rg rg rg rg rg m rm r
23. An optional throws clause which declares the exceptions that the constructor may throw This constructor does not declare any exceptions 5 The constructor body which is a Java BLOCK enclosed in This constructor s body contains two statements public Distance int x0 int y0 int xl int yl point0 new java awt Point x0 y0 pointl new Java awt Point x1 yl This constructor accepts four parameters named x0 y0 x1 and yl Each parameter requires a parameter type declaration which in this example is int for all four parameters Java integer values are signed 32 bit twos complement integers The parameters in the parameter list are separated by commas The two assignments in this constructor use Java s new operator to allocate two java awt Point objects The first allocates an object representing the first point x0 y0 and assigns it to the point0 instance variable replacing the null value that the instance variable was initialized to The second statement allocates a second java awt Point instance with x1 y1 and assigns it to the point 1 instance variable This is the constructor for the Distance class Distance implicitly extends from java lang Object Java inserts a call to the super constructor as the first executable statement of the constructor if there is not one explicitly coded The above constructor body is equivalent to the following body with the explicit super constructor call super poin
24. DOWNLOADS LEASE INDIGO SR1 ECLIPSE JAVA INDIGO SR1 LINUX GTK X86_64 TAR GZ6 R 1 TP WIKI ECLIPSE ORG ECLIPSE INSTALLATION T T E T E T 2 3 4 For Mac OS 2 3 5 For Solaris 12 3 History On 23 May 1995 John Gage the director of the Science Office of the Sun Microsystems along with Marc Andreesen co founder and executive vice president at Netscape announced to an audience of SunWorldTM that Java technology wasn t a myth and that it was a reality and that it was going to be incorporated into Netscape Navigator At the time the total number of people working on Java was less than 30 3 This team would shape the future in the next decade and no one had any idea as to what was in store From being the mind of an unmanned vehicle on Mars to the operating environment on most of the con sumer electronics e g cable set top boxes VCRs toasters and also for personal digital assistants PDAs Java has come a long way from its inception Let s see how it all began 3 1 The Green team Figure 2 James Gosling architect and designer of the compiler for the Java technology 1 JAVA TECHNOLOGY THE EARLY YEARS Sun Microsystems Retrieved 9 May 2008 3 JAVA TECHNOLOGY THE EARLY YEARS Sun Microsystems Retrieved 9 May 2008 5 HISTORY OF Java Lindsey Clark S Retrieved 7 MAY HTTP EN WIKIPEDIA ORG WIKI 7 20May 2008 uTTP EN WIKIPEDIA ORG WIKI 2008 13 History Behind closed do
25. EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FKEYWORDSS2F THROW HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVAS20PROGRAMMING Dd Q gt WD 172 Minimize the use of the keyword null in assignment statements This page describes some techniques for preventing NullPointerException It does not describe general techniques for how you should program Java It is of some use to make you more aware of null values and to be more careful about generating them yourself Note that this list is not complete there are no rules for preventing NullPointerException en tirely in Java because the standard libraries have to be used and they can cause NullPointerEx ceptions Also it is possible to observe an uninitialised final field in Java so you can t even treat a final field as being completely trusted during the object s creation A good approach is to learn how to deal with NullPointerExceptions first and become competent with that These suggestions will help you to cause less NullPointerExceptions but they don t replace the need to know about NullPointerExceptions 30 10 Minimize the use of the keyword null in assignment statements This means not doing things like String s null while something if something2 s yep if s null something3 s You can replace this with boolean done false while done amp amp something if something2 done true something3 yep You
26. ExceptionTutorial0l java public class ExceptionTutorial01 public static int divide int a int b return a b public static void main String args System out println divide 4 2 if args length gt 1 int arg0 Integer parselnt args 0 int argl Integer parseInt args 1 System out println divide arg0 argl Output for java ExceptionTutorial01 1 0 2 Exception in thread main java lang ArithmeticException by zero at ExceptionTutorial01 divide ExceptionTutorial01l java 6 at ExceptionTutorial01 main ExceptionTutorial0l java 15 HfL foe Such exceptional code that results in erroneous interpretations at program runtime usually re sults in errors that are called exceptions in Java When the Java interpreter encounters an ex ceptional code it halts execution and displays information about the error that occurs This information is known as a stack trace The stack trace in the above example tells us more about the error such as the thread main where the exception occurred the type of exception java lang ArithmeticException a comprehensible display message by zero and the exact methods and the line numbers where the exception may have occurred 163 Throwing and Catching Exceptions 30 1 Exception arguments In the code above the exception object for java lang ArithmeticException was generated by the Java interpreter itself However there are times when you would
27. Ifthe reference type is a subtype of Throwable throwing a null reference Applications should throw instances of this class to indicate other illegal uses of the null object Object obj null obj toString This statement will throw a NullPointerException The above code shows one of the pitfall of Java and the most common source of bugs No object is created and the compiler does not detect it NullPointerException is one of the most common exceptions thrown in Java 30 8 2 Why do we need nul 1 The reason we need it is because many times we need to create an object reference before the object itself is created Object references cannot exist without a value so we assign the null value to it public Customer getCustomer Customer customer null try customer createCustomer catch Exception ex return customer In the above code we want to create the Customer inside the try block but we also want to return the object reference to the caller so we need to create the object reference outside of the try block because of the scoping rule in Java This is one of the pitfall of Java 30 9 Keyword references 2 STRAY CATCH e THROWS THROW CATEGORY JAVA PROGRAMMING HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FKEYWORDS 2F TRY HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FKEYWORDSS2F CATCH HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FKEYWORDS 2F THROWS HTTP
28. Lets see what is the catching criteria for a catch block to catch the thrown exception A catch block will catch a thrown exception if and only if the thrown exception object is the same as the exception object specified by the catch block the thrown exception object is the subtype of the exception object specified by the catch block try throw new Exception This will be caught below catch Exception ex The thrown object is the same what is specified at the catch block try throw new NullPointerException This will be caught below catch Exception e NullPointerException is subclass of the Exception class There can be more than one catch block for a try block The catching blocks evaluated sequen tially one by one Ifa catch block catch the exception the others will not be evaluated 170 Runtime Exceptions Example try throw new NullPointerException This will be caught below catch Exception e The NullPointerException thrown in the code above is caught here catch NullPointerException e while this code is never executed and the compiler returns an error Because NullPointerException is subclass of the Exception class All NullPointerExceptions will be caught by the first catch block Instead the above code should be rewritten as follows try throw new NullPointerException This will be caught below catch NullPointerException e The abov
29. PASSING upya Walau M eS ies SE Se a eY 88 15 5 cBUINCTIONS S ff ay ae A ai p Q WS AAA 89 15 6 RETURN PARAMETER 64 ss eae S M U e CA a a a 89 15 7 SPECIAL METHOD THE CONSTRUCTOR ee es 91 15 8 STATIG METHODS gt ios sn ho ae Be ee ees Sea eG 92 15 9 EXTERNALLINKS SI goo Gy asa a eck a Aa eee Reais aw RRA 93 16 PRIMITIVE TYPES 95 17 TYPES 97 JZ TU ODATASTYPES IN JAVA 4 S 4 Z SU SS o Se eed Buel ad a da 97 17 2 JAVA AS HYBRID LANGUAGE 0 a eee 98 17 3 EXAMPLES OF TYPES 2 0 ce kas ea q e ee ee ee ee 98 T74 ARRAY TYPES o ie Be Sod Goh GQ haw we R e Ee Pes t ew ee eS 99 17 5 PRIMITIVE DATA TYPES ee ee es 100 17 6 DATA CONVERSION CASTING 101 17 7 AUTOBOXING UNBOXING ee es 101 18 JAVA LANG STRING 103 18 1 JAVAJGANG STRING o S le fishes ee Glee ge bia iS ee ya ES q ug BS 103 18 2 USING STRINGBUFFER STRINGBUILDER TO CONCATENATE STRINGS 104 18 3 COMPARING STRINGS c 2 5 Se eh ee ee u Sus ae BLE uQ 105 184 SPLITTINGA STRING z sy y soe gi AS Beene ad 106 18 5 CREATING SUBSTRINGS 0 a s a aa sas eee 107 18 6 MODIFYING STRING CASES ee 107 18 7 SEEEALSO senan be PER oe a Sh a eS 108 19 ARRAYS 109 19 1 ENTRO TOARRAYS lt a aR A Hae mae ROE Bw 3 aS 109 19 2 ARRAY FUNDAMENTALS 1 0 wu a a ee ee 109 19 3 TWO DIMENSIONAL ARRAYS e 110 19 4 MULTIDIMENSIONAL ARRAY ee 111 20 DATA AND
30. PHP TITLEUSER SAMWILSON 129 HTTP EN w BOOKS ORG W INDEX PHP TITLEUSER SAVH 130 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER SEANJA 131 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLESUSER SHAHIDSIDD 132 HTTP EN w BOOKS ORG W INDEX PHP TITLE USER SHAUNW 133 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER SHENME 134 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER SIGMA_7 135 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER SNARIUS 136 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER SPOCK431 137 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER SPONGEBOB88 138 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER SPOONS21 139 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER STEPHANVANINGEN 140 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLEUSER SUNDAR22IN 141 HTTP EN w BOOKS ORG W INDEX PHP TITLEUSER SUNNYCHAN 142 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER SUPERFLY_JON 143 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLESUSER SWIFT 144 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLEUSER TALKTOATISH 145 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLEUSER TANMINIVAN 146 HTTP EN w BOOKS ORG W INDEX PHP TITLE USER TAROSE TREVOR 190 Externallinks 1 THARENTHEL 1 THEDAvERoss 48 3 THEPHILWELLS 49 2 UNV 0 1 Ucertiry 1 Vinay H152 2 WEBAWARE 53 1 WHITEKNIGHTI 6 WIKIWIZARD 5S5 2 WIKIALT156 1 WIKIMI DHIANN 57 1 WILLEM SouwERn 58 1 WISEEvsEs 59 1 WUR DENE 60 2
31. TYPE int represents a signed 32 bit integer value The allowed data values for int are the integers between 2147483648 to 2147483647 inclusive HTTP EN WIKIBOOKS ORG WIK1 2FNUMERICS20PRIMITIVES HTTP EN WIKIBOOKS ORG WIK1 2FTEXTUALS20PRIMITIVES HTTP EN WIKIBOOKS ORG WIK1 2FOTHER 20PRIMITIVES HTTP EN WIKIBOOKS ORG WIKI PROGRAMMINGS3ASMALLTALK HTTP EN WIKIBOOKS ORG WIKI RUBYS20PROGRAMMING Chapter 16 on page 95 10 Chapter 16 on page 95 CANDO SB 98 Array Types The set of operations that may be performed on int values includes integer arithmetic such as Comparison operations lt gt lt gt assignments bit wise operations such as logical and logical or logical xor negation amp A 7 bit shift operations lt lt gt gt gt gt gt CONVERSIONS to other numeric types and PRO MOTION to other integer types For example to declare a private integer instance field named length one would use the declaration private int length 17 3 2 Example String You use class and interface definition in Java to define new types Class and interface types are associated with object references also sometime refered to as Reference types An object reference has two main attributes Its type associated with a class or an interface e A java object it references that is created by instantiating a class The String class is one such example St
32. The following UML class diagram shows the Map interfaces and their implementations interface RP EP pe czscssscmascmuscssassta AbstractMap qhapaq VO E SortedMap Figure 24 160 External links 29 8 External links e JAVA TUTORIAL ON COLLECTIONS 1 CATEGORY JAVA PROGRAMMING 11 utTtTe JAVA SUN COM DOCS BOOKS TUTORIAL COLLECTIONS INTERFACES COLLECTION HTML 12 HTTP EN WIKIBOOKS ORG WIKI CATEGORYS3AJAVAS20PROGRAMMING 161 Collection Classes 162 30 Throwing and Catching Exceptions Language compilers are adept at pointing out most of the erroneous code in a program however there are some errors that only become apparent when the program is executed Consider the code in Listing 1 1 here the program defines a method divide that does a simple division opera tion taking two integers as parameter arguments and returning the result of their division It can safely be assumed that when the divide 4 2 statement is called it would return the number 2 However consider the next statement where the program relies upon the provided command line arguments to generate a division operation What if the user provides the number zero 0 as the second argument We all know that division by zero is impossible but the compiler couldn t possibly have anticipated the user providing zero as an argument Listing 1 1 A simple division operation exceptions
33. This is a constructor You can pass a Automatically assign an ID to the object and it will give you a newly created Employee object myName name myID nextID Wf nextID Increment the ID counter public String getName return myName myName public int getID return myID public static int getNextID next ID is created return nextID Hi Wil This is a member method that returns the Employee object s name Note how it can access the private field This is another member method This is a static method that returns the that will be assigned if another Employee The following Java code public class EmployeeList public static void main String args System out Employee a Employee b Employee c System System System System out out printin b getID out out println Employee getNextID new Employee John Doe new Employee Jane Smith new Employee Sally Brown println Employee getNextID println a getID a getName b getName println c getID c getName 126 Fundamentals would produce this output Jane Smith 0 3 0 John Doe 1 2 Sally Brown CATEGORY JAVA PROGRAMMING 1 HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVAS20PROGRAMMING 127 Defining Classes 128 23 Creating Objects 23 1
34. WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FAPIS2FJAVA LANG OBJECTS 23HASHCODE 28 29 520METHOD 154 Set or List or Queue The SortedSet interface has additional methods due to the sorted nature of the Set Those are E first returns the first element E last returns the last element SortedSet headSet E toElement returns from the first to the exclusive toElement SortedSet tailSet E fromElement returns from the inclusive fromElement to the end SortedSet subSet E fromElement E toElement returns elements range from fromElement inclusive to toElement exclusive If fromElement and toElement are equal the returned sorted set is empty 29 4 2 List The List has the following implemenations interface Collecti K Collection O Abstract uon AbstractList 3 A gu AbstractSequantialList List interface Queue AttributeList 0 RoleList RoleUnresolvedList Since 1 5 gt Figure 20 java util Vector lt E gt Synchronized use in multiple thread access otherwise use ArrayList java util Stack lt E gt It extends class Vector with five operations that allow a vector to be treated as a stack It represents a last in first out LIFO stack of objects java util ArrayList lt E gt Non synchronized use in single thread environment otherwise use Vector 155 Collection Classes java util LinkedList l
35. WWW ORACLE COM US TECHNOLOGIES JAVA INDEX HTML HOME JAVA NET WWW JAVARANCH COM EN WIKIPEDIA ORG WIKI BASIC EN WIKIPEDIA ORG WIKI PASCALS 20 28PROGRAMMINGS20LANGUAGES29 Mu cm mm m m a YHHHHHHOR 1 O m gt G t tg rO rO g r r About This Book You are an experienced developer and know how to program in other languages like C VISUAL Basic PYTHON or RuBy e You ve heard that Java is great for web applications and web services programming Although this book is generally meant to be for programmers who are beginning to learn pro gramming it can be highly beneficial for intermediate and advanced programmers who may have missed up on some vital information By the end of this book you would be able to solve any complicated problem and tackle it using the best of your learnt skills in Java Once you fin ish you are also encouraged to take upon ambitious programming projects of your own as you certainly would be able to do that as well This book assumes that the reader has no prior knowledge of programming in Java or for that matter any object oriented programming language The book makes it easier to understand development methodology as one reads through the book with practical examples and exercises at the end of each topic and module Although if you are a complete beginner we would suggest that you move slowly through this book and practice each exercise at your pace 2 2 Howcan you p
36. and successfully In the Java language there are three broad categories of data types e PRIMITIVES e OBJECT REFERENCE TYPES e ARRAYS In the following section we will discuss these three categories in detail 17 1 1 Primitives Primitives are the most basic data types available within the Java language These types serve as the building blocks of data manipulation in Java Such types serve only one purpose containing pure simple values of a kind Because these data types are defined into the Java type system by 1 HTTP EN WIKIBOOKS ORG WIKI 23PRIMITIVES 2 HTTP EN WIKIBOOKS ORG WIK1 230BJECT 20REFERENCES20TYPES 3 HTTP EN WIKIBOOKS ORG WIKI S23ARRAYS 97 Types default they come with a number of operations predefined You can not define a new operation for such primitive types In the Java type system there are three further categories of primitives NUMERIC PRIMITIVES These primitive data types hold only numeric data Operations asso ciated with such data types are those of simple arithmetic addition subtraction etc or of comparisons is greater than is equal to etc TEXTUAL PRIMITIVES These primitive data types hold characters which can be alphabets or even numbers but unlike numbers they do not have operations that serve arithmetic pur poses Rather operations associated with such types are those of textual manipulation com paring two words joining characters to make words etc
37. any section of code within curly braces Common blocks include class definitions methods and constructors if else blocks and for 3 HTTP EN WIKIPEDIA ORG WIKI CAMELCASE 114 Scope while and do while loops class BlockScope int classScope valid in all of class BlockScope BlockScope int param param is valid only in this constructor int localVariable 0 valid only in this constructor void someMethod int local 42 valid only in this method if local gt 0 boolean positive true valid only within the if block else positive is not defined here There are three basic kinds of scope for variables in Java local variable declared within a method in a class valid for and occupying storage only for the time that method is executing Every time the method is called a new copy of the variable is used e instance variable declared within a class but outside any method It is valid for and occupies storage for as long as the corresponding object is in memory a program can instantiate mul tiple objects of the class and each one gets its own copy of all instance variables This is the basic data structure rule of Object Oriented programming classes are defined to hold data specific to a class of objects in a given system and each instance holds its own data e static variable declared within a class as static outside any method There is only one
38. appeared first with Java 1 5 It can be used prior to the installation of a Java extension into the JDK or JRE environment It checks if a particular Jar file conflicts with an already installed extension 4 2 6 Software security and cryptography tools The JDK comes with a large number of tools related to the security features of Java Usage of these tools first requires study of the particular security mechanisms The tools are keytool To manage keys and certificates jarsigner To generate and verify digital signatures of JARs Java ARchives policytool To edit policy files kinit To obtain Kerberos v5 tickets 4 HTTP JAVA SUN COM J2SE 1 5 0 DOCS GUIDE APT GETTINGSTARTED HTML 26 Java Development Kit JDK klist To manage Kerberos credential cache and key table ktab To manage entries in a key table 4 2 7 The Java archiver jar short for Java archiver is a tool for creating Java archives or jar files a file with jar as the extension A Java archive is a collection of compiled Java classes and other resources which those classes may require such as text files configuration files images at runtime Internally a jar file is really a ZIP FILE 4 2 8 The Java debugger jdb short for Java debugger is a command line console that provides a DEBUGGING envi ronment for Java programs Although you can use this command line console IDE s normally provide easier to
39. are constantly challenged to develop their skills with a progressive learning curve 1 1 Are you new to programming Figure 1 This stunning image of the sunset on planet Mars wouldn t have been possible without Java If you have chosen Java as your first programming language be assured that Java happens to also be the first choice for educational programs in many universities and colleges Its simple and intuitive SYNTAX that s grammar for programming languages helps beginners feel at ease with complex programming constructs in no time But be aware Java is not just any basic programming language In fact NASA used Java as the driving force quite literally behind its Mars Rover missions Computers mobile phones set top boxes and even digital television sets can all be programmed using the Java programming 1 HTTP EN WIKTIONARY ORG WIKI AKIN 2 HTTP EN WIKTIONARY ORG WIKI SECONDS20NATURE 3 HTTP EN WIKTIONARY ORG WIKI SYNTAX Introduction language Robots air traffic control systems and the self checkout barcode scanners in your favourite supermarkets are all being programmed in Java 1 2 Programming with Java By now you might truly be able to grasp the power of the Java programming language there is still lots more you can do with Java Not every programmer gets to program applications that take unmanned vehicles onto other planets Software that we encounter in our daily life is somewhat humble in th
40. as opposed to being loaded from a series of class files Large portion of the runtime libraries will now be shared among multiple JVM instances from HTTP JAVA SUN COM FEATURES 2003 05 BLOCH_QA HTML72 3 5 6 Mustang version 1 6 0 Java SE 6 Released on 11 December 2006 30 What s New in Java SE 6 e Web Services First class support for writing XML web service client applications Scripting You can now mix in JavaScript technology source code useful for prototyping Also useful when you have teams with a variety of skill sets More advanced developers can plug in their own scripting engines and mix their favorite scripting language in with Java code as they see fit 26 Chapter 21 on page 117 27 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FPRIMITIVES20TYPES 23AUTOBOXINGS2FUNBOXING 28 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FANNOTATION 29 HTTP JAVA SUN COM FEATURES 2003 05 BLOCH_QA HTML 30 JAVA PLATFORM STANDARD EDITION 6 31 Sun Microsystems Retrieved 9 May 2008 19 History Database No more need to find and configure your own JDBC database when developing a database application Developers will also get the updated JDBC 4 0 a well used API with many important improvements such as special support for XML as an SQL datatype and bet ter integration of Binary Large OBjects BLOBs and Character Large OBjects CLOBs into the APIs More Desktop APIs GUI developers get a large number of new
41. compilers 3 5 3 Kestrel Java 1 3 3 5 4 Merlin Java 1 4 Java 1 4 has improved programmer productivity by expanding language features and available APIs e Assertion Regular Expression 22 HTTP EN WIKIPEDIA ORG WIKI SOLARISS2Z0OPERATINGS2Z0ENVIRONMENT 23 HTTP EN WIKIPEDIA ORG WIKI MICROSOFTS2OWINDOWS 24 HTTP EN WIKIPEDIA ORG WIKI MAC 200S 25 HTTP EN WIKIPEDIA ORG WIKI LINUX 18 Versions e XML processing Cryptography and Secure Socket Layer SSL Non blocking I O NIO Logging 8 5 5 Tiger version 1 5 0 Java SE 5 Released in September 2004 Major changes include GENERICS Provides compile time type safety for collections and eliminates the drudgery of casting e AUTOBOXING UNBOXING Eliminates the drudgery of manual conversion between primi tive types such as int and wrapper types such as Integer e Enhanced for Shorten the for loop with Collections use e Static imports Lets you import all the static part of a class ANNOTATION Metadata Enabling tools to generate code and deployment descriptors from annotations in the source code This leads to a declarative programming style where the programmer says what should be done and tools emit the code to do it Annotations can be inspected through source parsing or by using the additional reflection APIs added in Java 5 JVM Improvements Most of the run time library is now mapped into memory as a memory image
42. copy of such a variable no matter how many objects are instantiated from that class CATEGORY JAVA PROGRAMMING 4 uttTp EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVA 20PROGRAMMING 115 Data and Variables 116 21 Generics Generics were added to the Java language syntax in version 1 5 This means that code using Generics will not compile with Java 1 4 and less Java was long criticized for the need to explicitly type cast an element when it was taken out of a container collection class There was no way to enforce that a collection class contains only one type of object e g to forbid at compile time that an Integer object is added toaCollection that should only contain St rings This is now possible since Java 1 5 In the first couple of years of Java evolution Java did not have a real competitor This has changed by the appearance of Microsoft Cs With Generics Java is better suited to compete against C Similar constructs to Java Generics exist in other languages see W GENERIC PROGRAMMING for more information 21 1 What are Generics Generics are so called because this language feature allows methods to be written generically with no foreknowledge of the type on which they will eventually be called upon to carry out their behaviors A better name might have been type parameter argument Because it is basically that to pass a Type as a parameter to a class at creation time When an object is created parameters c
43. d Double 65 55 Other boolean 1 booleanb Boolean true void Void The types short int long float and double are usually used in arithmetic operations byte and char can also be used but less commonly The character type char is used for text processing The type byte is commonly used in binary file input output operations String objects representing literal character strings in Java in the java lang package java lang String is not a primitive type but instead is a special class built into the Java lan guage For further info see String 100 Autoboxing unboxing 17 6 Data Conversion Casting Data conversion casting can happen between two primitive types There are two kinds Implicit casting operation is not required the magnitude of the numeric value is always preserved However precision may be lost when converting from integer to floating point types e Explicit casting operation required the magnitude of the numeric value may not be pre served Example for implicit casting int i 65 long i int is converted to long casting is not needed Example for explicit casting long 1 656666L int i int l long is converted to int casting is needed The following table shows the conversions between primitive types it shows the casting oper ation for explicit conversions from from from from from from from from byte char short int long float double
44. for running and developing Java code thus it is divided into two essential software the Java Runtime Environment JRE which is needed to run Java applications and applets and the Java Development Kit JDK which is needed to develop those Java applications and ap plets However as a developer you would only require the JDK which comes equipped with a JRE as well Given below are installation instruction for the JDK for various operating systems 2 3 2 For Windows Download instructions Some Windows based systems come built in with the JRE however for the purposes of writing Java code by following the tutorials in this book you would require the JDK nevertheless To acquire the latest JDK version 7 you can manually DOWNLOAD THE JAVA SOFTWARE from the Oracle website 12 utTte EN WIKIBOOKS ORG WIKI S2FAUTHORSS20 26 20CONTRIBUTORS 13 HTTP EN WIKIBOOKS ORG WIKI BE 20BOLD 14 HTTP EN WIKIBOOKS ORG WIKI 2FCONVENTIONS 15 Chapter 4 on page 21 16 HTTP WWW ORACLE COM TECHNETWORK JAVA JAVASE DOWNLOADS INDEX HTML About This Book For the convenience of our readers the following table presents direct links to the latest JDK for the Windows operating system Operating system Setup Installer License Windows x86 DOWNLOAD ORACLE BINARY CODE LI CENSE AGREEMENT Windows x64 DOWNLOAD ORACLE BINARY CODE LI CENSE AGREEMENT You must follow the instructions for the setup installer wizard step by
45. have duplicates in it You may wonder how duplicates are detected when we are adding an object to the Set We have to see if that object exists in the Set or not It is not enough to check the object references the objects values have to be checked as well To do that fortunately each java object has the boolean equal Object obj 3 method avail able inherited from Object You need to override it That method will be called by the Set imple mentation to compare the two objects to see if they are equal or not There is a problem though What if I put two different type of objects to the Set I put an Ap ple and an Orange They can not be compared Calling the equal method would cause a ClassCastException There are two solutions to this Solution one Override the int hashCode method and return the same values for the same type of objects and return different values for different type of objects The equal method is used to compare objects only with the same value of hashCode So before an object is added the Set implementation needs to e find all the objects in the Set that has the same hashCode as the candidate object hashCode and for those call the equal methods passing in the candidate object if any of them returns true the object is not added to the Set Solution two Create a super class for the Apple and Orange let s call it Fruit class Put Fruits in the Set You need to do the followi
46. how programs are actually made As we have already dis cussed A program is a set of instructions very simple tasks provided to a computer These instructions are called statements in Java Statements can be anything from a single line of code to a complex mathematical equation This section helps you understand what statements are and how they work 10 1 What exactly are statements Statements are a single instruction in a program a single unit of code Consider the following line Listing 1 1 A simple assignment statement int age 24 This line is a simple instruction that tells the system to initialize a variable and set its value as 24 Within that simple definition we talked about initialization of a variable and setting its value This all might sound a bit too technical but it will make sense as you read ahead 10 2 Where do you find statements Java in the same style as C and C places statements within functions or methods The func tion in turn is placed within a class declaration If the above statement was the only one in the program it would look similar to this public class MyProgram public static void main String args int age 24 The class declaration and function declaration will be described in the upcoming chapters 10 3 Variables Christmases are usually exciting birthdays too And the one thing that makes them exciting are you get presents Think of a present you ve ever got
47. immutable objects Once a String is created it can not be modified takes up memory until garbage collected Be careful of writing a method like this public String convertToString Collection lt String gt coll Sirenas Siem O s for String oneElem coll loops through every element in coll See gt Su Cavemen e W up return str On the operation a new String object is created at each iteration Suppose coll contains the elements Foo Bar Bam Baz The method creates five Strings Foo Foo Bar Foo Bar Bam and Foo Bar Bam Baz even though only last one is actually useful Instead use STRINGBUFFER as shown below where only one STRINGBUILDER object is created 3 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FAPIS2FJAVA LANG STRINGBUFFER 4 HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMINGS2FAPIS2FJAVA LANG STRINGBUILDER 104 Comparing Strings To avoid unnecessary memory use like this use the STRINGBUFFER or STRINGBUILDERS class They provide similar functionality to Strings but store their data in a mutable way Also be cause object creation is time consuming using StringBuffer or StringBuilder produces much faster code public String convertToString Collection lt String gt coll StringBuilder buf new StringBuilder for String oneElem coll loops through every element in coll buf append oneElem buf append r
48. in Listing 1 1 as under 165 Throwing and Catching Exceptions Listing 1 7 Catching division by zero errors int result 0 try result a b catch ArithmeticException ex result 0 return result Using the code in Listing 1 7 the program would now not abruptly terminate but continue its ex ecution whilst returning zero for a division by zero operation I know it s not the correct answer but hey it saved your program from terminating abnormally 30 4 Exception classes in the JCL The box 1 1 below talks about the various exception classes within the java lang package of the JCL 166 Exception classes in the JCL Box 1 1 The Java exception classes e Throwable The Throwable class is the superclass of all errors and exceptions in the Java lan guage Only objects that are instances of this class or one of its subclasses are thrown by the Java Virtual Machine or can be thrown by the Java throw state ment A throwable contains a snapshot of the execution stack of its thread at the time it was created It can also contain a mes sage string that gives more information about the error Finally it can contain a cause another throwable that caused this throwable to get thrown The cause facility is new in release 1 4 It is also known as the chained exception facility as the cause can itself have a cause and so on leading to a chain of exceptions each caused by anot
49. interface defines a standard way of looping through the elements The following code is a possible implementation of addA11 method of the LinkList import java util Collection import java util Iterator public String addA11 Collection coll int sizeBefore _linkList size Iterator iter coll iterator while iter hasNext _linkList add iter next if sizeBefore gt _linkList size return true else return false The above code just iterates through the passed in collection and adds the elements to the link list You do not have to do that since that is already defined What you might need to code for is to loop through a Customer collection import java util Collection import java util Iterator import java yourcompany Customer public String printCustomerNames Collection customerColl StringBuffer buf new StringBuffer Iterator iter customerColl iterator while iter hasNext Customer cust Customer iter next buf append cust getName buf append n return buf toString 150 Set or List or Queue Notice two things The above code will work for all type of collections e We have to know the type of objects inside the collection because we call a method on it 29 3 2 Map O Map O SortedMap Figure 17 Figure 1 Map Interfaces A map sometimes also called an Associated Array or a Di
50. lang String or simply String 2 Distance 3 java awt Point For more information see chapter JAVA PROGRAMMING CLASSES OBJECTS AND TYPES Primitive Types In addition to object or reference types Java supports PRIMITIVE TYPES The primitive types are used to represent Boolean character and numeric values This program uses only one primitive type explicitly int which represents 32 bit signed integer values The program also implicitly uses double which is the return type of the distance method of java awt Point double values are 64 bit IEEE floating point values The main method uses integer values 0 1 2 and 3 to access elements of the command line arguments The Distance constructor s four parameters also have the type int Also the int Value method has a return type of int This means a call to that method such as int Value args 0 is an expression of type int This helps explain why the main method cannot call new Distance args 0 args 1 args 2 args 3 this is an error Since the type of the args array element is String and our constructor s parameters must be int such a call would result in an error because Java cannot automatically convert values of type String into int values Java s primitive types are boolean byte char short int long float and double each of which are also Java language keywords Array Types Java supports ARRAYS which are aggregate types which h
51. linking or runtime binding In order for the Method Over riding to work the method call that is going to be called can not be determined at compilation time It will be decided at runtime and will be looked up in a table obj has MyClass or SubOfMyClass 1 MyClass obj new SubOfMyClass 2 3 MyClass obj new MyClass 4 5 obj myMethod During compilation it is not known what reference the In the above example obj reference has the type MyClass on both line 1 and line 3 However the obj reference points two different objects On line 1 it references SubOfMyClass object on line 3 it references MyClass object So on line 5 which method will be called method define in MyClass or the method that defined in its subclasses Because the obj reference can point to object and all its sub object and that will be known only at runtime a table needs to be kept with all the possible method address to be called Also another rule is that when you do an override the visibility of the new method that overrides the super class method can not be reduced The visibility can be increased however So if the super class method visibility is public the override method can not be package or private In the case of the exception the override method may throw can be the same as the super class or it can be one of that exception inherited class So the common rule is that the override method must throw the s
52. lockingQueue T q A ER OPriontyqueue e java util PriorityQueue lt E gt J yQ Smals O G orders elements according to an a ArrayBlockingQueue SynchronousQueue order priority specified at construc tion time null element is not allowed Figure 21 java util concurrent ArrayBlockingQueue lt E gt orders elements FIFO syncronized thread safe java util concurrent SynchronousQueue lt E gt each put must wait for a take and vice versa does not have any internal capacity not even a capacity of one an element is only present when you try to take it you cannot add an element using any method unless another thread is trying to remove it 157 Collection Classes 29 5 Map Classes The Map interface has the following implementations interface Map AbstractMap interface CuncurrentHashMap SortedMap Since 1 5 gt Figure 22 java util TreeMap lt E gt guarantees that the map will be in ascending key order sorted according to the natural order for the key s class not synchronized java util HashMap lt E gt is roughly equivalent to Hashtable except that it is unsynchronized and permits nulls java util concurrent ConcurrentHashMap same Hashtable plus retrieval operations includ ing get generally do not block so may overlap with update operations including put and re move java util Hashtable lt E gt Synchronized null can not be u
53. lots of insert and remove consider using the LinkedList The name LinkList implies a special data structure where the elements nodes are connected by pointers To remove an element from the link list the pointers need to be rearranged Head Node Node 2 Node n point point First gt Data to next gt Data to next gt Data Head Node Node 2 Node n Size re res point I point Firstl gt Data to next Data to next Data null elem Ar Pra i 156 Set or List or Queue _elem_ 29 4 3 Queue The Queue interface adds the following operations to the Collection interface E element Retrieves but does not remove the head of this queue This method differs from the peek method only in that it throws an exception if this queue is empty boolean offer E o Inserts the specified element into this queue if possible E peek Retrieves but does not remove the head of this queue returning null if this queue is empty E poll Retrieves and removes the head of this queue or null if this queue is empty E remove Retrieves and removes the head of this queue This method differs from the poll method in that it throws an exception if this queue is empty interface Collection AbstractCollection interface AbstractQueue Queue o f A aa
54. mapping key value pairs 29 3 1 Collection Use the Collection interface if you need to keep related usually the same type of objects to gether in a collection where you can Search for a particular element List the elements e Maintain and or change the order of the elements by using the collection basic operations Add Remove Update Access the elements by an index number The advantages of using the Collection interface are Gives a generic usage as we talked about above it is easy to switch implementation e It makes it easy to convert one type of collection to an other The Collection interface defines the following basic operations boolean add E o using Element type E 2 Chapter 21 on page 117 149 Collection Classes e boolean addAll Collection c boolean remove Object o e boolean removeAll Collection c e boolean retainAll Collection c The methods above return true if the collection has changed due to the operation Note that in addAll 0 we can add any type of collection This is the beauty of using the Collection interface You can have a LinkedList and just call the addA11 1ist method passing in a list You can pass in a Vector an ArrayList a HashSet a TreeSet a YourImpOfCollection All those different type of collections will be magically converted to a LinkList Lets have a closer look at this magic The conversion is easy because the Collection
55. might also consider replacing null with in the first example but default values bring about bugs caused by default values being left in place A NullPointerException is actually better as it allows the runtime to tell you about the bug rather than just continue with a default value 30 11 Minimize the use of the new Typelint syntax for creating arrays of objects An array created using new Object 10 has 10 null pointers That s 10 more than we want so use collections instead or explicitly fill the array at initialisation with 173 Throwing and Catching Exceptions Object objects blah 5 new File usr bin Or Object objects objects new Object blah 5 new File usr bin 30 12 Check all references obtained from untrusted methods Many methods that can return a reference can return a null reference Make sure you check these For example File file new File etc File files file listFiles if files null stuff File listFiles can return null if etc is not a directory You can decide to trust some methods not to return null if you like but that s an assumption you re making Some methods that don t specify that they might return null actually do instead of throwing an exception 30 13 Comparing string variable with a string literal When you compare a variable with a string literal always put the string literal first For exam
56. programmers With Generics Java strict type checking can be extended to container objects Using Generics with container classes gives an impression that a new container type is created with each different type parameter Before Generics Collection collCustomer new Arraylist collCustomer add new Customer Collection coll0bject collCustomer No problem both collObject and collCustomer have the same type With generics Collection lt Customer gt collCustomer new ArrayList lt Customer gt collCustomer add new Customer Collection lt Object gt collObject collCustomer Compilation Error Both collObject and collCustomer have the same type BUT it is against the Generic rule that is collCustomer can contain only Customer objects and collObject can contain only Object object So there is an additional check to the normal type checking the type of the parameter type has to be matched too 21 3 Note for C programmers Java Generics are similar to C Templates in that both were added for the same reason The syntax of Java Generic and C Template are also similar There are some differences however The C template can be seen as a kind of macro that generates code before compilation The generated code depends on how the Template class is referenced The amount of code generated depends on how many different types of classes are created from the Template C Template
57. step with the default set tings to ensure that Java is properly installed on your system Once the setup is completed it is highly recommended to restart your Windows operating system If you kept the default settings for the setup installer wizard you JDK should now be installed at C Program Files Java jdk1 7 0_01 You would require the location to your bin folder at a later time this is located at C Program Files Java jdk1 7 0_01 bin Updating environment variables Optional In order for you to start using the JDK compiler utility with the Command Prompt you would need to set the environment variables that points to the bin folder of your recently installed JDK Follow the steps below to permanently include your Java platform to your environment variables 1 For Windows XP click Start gt Control Panel System For Windows 2000 click Start gt Settings gt Control Panel gt System For Window Vista or Windows 7 click Start gt Control Panel gt System and Maintenance gt System Alternatively you can also press to open the Run dialog With the dialog open type the following command at the prompt rund1132 shel132 d11 Control_RunDLL sysdm cpl 2 Navigate to the Advanced tab on the top and select Environment Variables 3 Under System variables select the vari able named Path and click Edit 17 HTTP DOWNLOAD ORACLE COM OTN PUB JAVA JDK 7U1 B08 JDK 7U1 WINDOWS 1586 EXE 18 HTTP WWW ORACLE CO
58. the common face of the euro coins belongs to the European Commission Authorised is reproduction in a format without relief drawings paintings films provided they are not detrimental to the image of the euro LFK Lizenz Freie Kunst http artlibre org licence lal de CFR Copyright free use EPL Eclipse Public License http www eclipse org org documents epl v10 php 193 List of Figures 1 NASA PD 2 PETER CAMPBELL GFDL 3 USER ARUNREGINALD 8 GFDL 4 ARUN REGINALD cc by sa 3 0 5 ARUN REGINALD cc by sa 3 0 6 ARUN REGINALD cc by sa 3 0 7 ARUN REGINALD cc by sa 3 0 8 ARUN REGINALD cc by sa 3 0 9 10 11 12 13 14 15 ARUN REGINALD GFDL 16 ARUN REGINALD GFDL 17 Original version created by B USER ERVINN SVG version cc by sa 2 5 created by MYSELF 18 Original version created by B USER ERVINN SVG version cc by sa 2 5 created by MYSELF 19 20 21 22 23 24 25 ARUNREGINALD GFDL 167 HTTP EN w BOOKS ORG W USERS3APETERS20CAMPBELL 168 HTTP EN w BOOKS ORG W USERS3AARUNREGINALD 169 HTTP EN w BOOKS ORG W USERS3AARUNREGINALD 170 HTTP EN w BOOKS ORG W USERS3AARUNREGINALD 171 HTTP EN w BOOKS ORG W USERS3AARUNREGINALD 172 HTTP EN w BOOKS ORG W USERS3AARUNREGINALD 173 HTTP EN w BOOKS ORG W USERS3AARUNREGINALD 174 ETTP EN w BOOKS ORG W USERS3AARUNREGINALD 175 H
59. the console public void printDistance System out println Distance between point0 and pointl is point0 distance point1 This INSTANCE METHOD executes within the context of an implicit Distance object The instance field references point0 and point1 refer to instance fields of that implicit object You can also use the special variable this to explicitly reference the current object Within an instance method Java binds the name this to the object on which the method is executing and the type of this is that of the current class The body of the printDistance method could also be coded as System out println Distance between this point0 and this pointi is this point0 distance this point1 to make the instance field references more explicit This method both computes the distance and prints it in one statement The distance is computed with point0 distance point1 distance is an instance method of the java awt Point class of which point0 and point1 are instances The method operates on point0 binding this to the object that point0 refers to during the execution of the method 21 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FINSTANCES20METHODS 56 Detailed Program Structure and Overview and accepting another Point as a parameter Actually it is slightly more complicated than that but we ll explain later The result of the distance m
60. the syntax elements of Java may be found at SYNTAX We will provide here only enough description of the syntax to grasp the above program Java programs consist of a sequence of tokens There are different kinds of tokens For example there are word tokens such as class and public which represent KEYWORDS special words with reserved meaning in Java Other words non keywords such as Distance point0 x1 and printDistance are identifiers Identifiers have many different uses in Java but primarily they are used as names Java also has tokens to represent numbers such as 1 and 3 these are known as LITERALS STRING LITERALS such as Distance between consist of zero or more char acters embedded in double quotes and OPERATORS such as and are used to express basic computation such as addition or String concatenation or assignment There are also left and right braces and which enclose BLocks8 The body of a class is one such block Some to kens are punctuation such as periods and commas and semicolons You use WHITESPACE such as spaces tabs and newlines to separate tokens For example whitespace is required be tween keywords and identifiers publicstatic is a single identifier with twelve characters not two Java keywords 8 2 2 Declarations and Definitions Sequences of tokens are used to construct the next building blocks of Java classes declarations and definitions A class declaration provides the name and vi
61. tricks to play like the ever popular yet newly incorporated SwingWorker utility to help you with threading in GUI apps JTable sorting and filtering and a new facility for quick splash screens to quiet impatient users Monitoring and Management The really big deal here is that you dont need do anything spe cial to the startup to be able to attach on demand with any of the monitoring and management tools in the Java SE platform Compiler Access Really aimed at people who create tools for Java development and for frame works like JavaServer Pages JSP or Personal Home Page construction kit PHP engines that need to generate a bunch of classes on demand the compiler API opens up programmatic access to javac for in process compilation of dynamically generated Java code The compiler API is not directly intended for the everyday developer but for those of you deafened by your screaming inner geek roll up your sleeves and give it a try And the rest of us will happily benefit from the tools and the improved Java frameworks that use this Pluggable Annotations allows programmer to write annotation processor so that it can analyse your code semantically before javac compiles For example you could write an annotation processor that verifies whether your program obeys naming conventions Desktop Deployment At long last Java SE 6 unifies the Java Plug in technology and Java Web Start engines which just makes sense Installation of the Java We
62. type of a variable matches the type of data stored in that variable If you declare a variable to hold a String for instance you cannot assign an integer value to that variable Some languages such as C define an interpretation of such a statement and use that interpretation without any warning others such as PL I define a conversion for almost all such statements and perform the conversion to complete the assignment Strong typing is intended to prevent mistakes made by unwittingly assigning the wrong kind of value to a variable and catching those mistakes when the program is compiled rather than waiting to find it when the program is running 20 2 Case Conventions Java is case sensitive A method called mymethod is completely separate from a method called myMethod Be careful By convention most identifiers that includes more than one word uses CAMEL CASE Classes begin with a capital letter methods and variables do not Constructors have to start with a capital because they must have the same name as the class Package names use lowercase and do not use camel case Thus package org wikibooks samplecode class CaseConventions int variable int multipleWordVariable CaseConventions String id void method void longMethodName 20 3 Scope Java uses block scope which means that a variable is un defined and becomes useless at the end of the block in which it is defined A block is
63. use debugging environments 4 2 9 Documenting code with Java As programs grow large and complex programmers need ways to track changes and to under stand the code better at each step of its evolution For decades programmer have been employ ing the use of special programming constructs called comments regions that help declare user definitions for a code snippet within the source code But comments are prone to be verbose and incomprehensible let alone be difficult to read in applications having hundreds of lines of code e javadoc Java provides the user with a way to easily publish documentation about the code using a special commenting system and the javadoc tool The javadoc tool generates doc umentation about the application programming interface APD of a set of user created Java classes javadoc reads source file comments from the java source files and generates HTML documents that are easier to read and understand without looking at the code itself e javap Where Javadoc provide a detailed view into the API and documentation of a Java class the javap tool prints information regarding members constructors methods and variables in a class In other words it lists the class API and or the compiled instructions of the class javap is a formatting disassembler for Java bytecode 4 2 10 The native2ascii tool native2ascii is an important though underappreciated tool for writing properties files files containing con
64. variable has package visibility the variable can be referenced only in the same package through the class it is defined in Ifa variable has private visibility the variable can be accessed only in the class it is defined in If a method is defined in a public class and it has public visibility the method can be called anywhere in the application through the class it is defined in If a method has package visibility the method can be called only in the same package through the class it is defined in If a method has private visibility the method can be called only in the class it is defined in 1 1 HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3A 84 15 Methods 15 1 Method Definition A method is an operation on a particular object An object is an instance of a class When we define a class we define its member variables and its methods For each method we need to give a name we need to define its input parameters and we need to define its return type We also need to set its visibility private package or public If the method throws an Exception that needs to be declared as well The syntax of method definition is class MyClass public ReturnType methodName ParamOneType paraml ParamTwoType param2 throws ExceptionName ReturnType retType return retType We can declare that the method does not return anything using the void java keyword For example private void methodName String paraml Strin
65. 0LITERALS 25 HTTP EN WIKIBOOKS ORG W JAVAS20PROGRAM GS2FLITERALSS2FSTRINGS 20LITERALS 26 HTTP EN WIKIBOOKS ORG W JAVAS20PROGRAM GS2FLITERALSS2FNULL 27 HTTP EN W BOOKS ORG W CATEGORYS3AJAVA S20PROGRAMMING 64 Whitespaces 9 4 Whitespaces Whitespace in Java is used to separate the tokens in a Java source file Whitespace is required in some places such as between ACCESS MODIFIERS28 TYPE NAMES and Identifiers and is used to improve readability elsewhere Wherever whitespace is required in Java one or more whitespace characters may be used Wher ever whitespace is optional in Java zero or more whitespace characters may be used Java whitespace consists of the space character 0x20 e the tab character hex 0x09 the form feed character hex 0x0c the line separators characters newline hex 0x0a or carriage return hex 0x0d characters Line separators are special whitespace characters in that they also terminate line comments whereas normal whitespace does not Other Unicode space characters including vertical tab are not allowed as whitespace in Java 9 5 Required Whitespace Below is the declaration of an abstract method taken from a Java class public abstract Distance distanceTo Destination dest Whitespace is required between public and abstract between abstract and Distance between Distance and distanceTo and between Destination and dest However the follow
66. 10 on page 67 13 Chapter 9 4 on page 65 14 Chapter 9 4 on page 65 15 uTTp EN WIKIBOOKS ORG WIKI JAVA 20PROGRAMMINGS2FSYNTAXS2FCOMMENTS 16 HTTP EN WIKIPEDIA ORG WIKI ASCII 17 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FSYNTAXS2FUNICODES20ESCAPES 20SEQUENCES 62 Literals double Nu03C0 Math PI The following demonstrate the use of Unicode escape sequences in other Java syntax Declare Strings pi and quote which contain u03C0 and Nu0027 respectively String pi string String quote string Note that a Unicode escape sequence functions just like any other character in the source code E g u0022 double quote needs to be quoted in a string just like Declare Strings doubleQuotel and doubleQuote2 which both contain double quote String doubleQuotel string String doubleQuote2 string u0022 doesn t work since doesn t work See UNICODE ESCAPE SEQUENCES for full details CATEGORY JAVA PROGRAMMING 9 2 Literals Java Literals are syntactic representations of boolean character numeric or string data Literals provide a means of expressing specific values in your program For example in the following statement an integer variable named count is declared and assigned an integer value The literal 0 represents natually enough the value zero int count 0 The following method call passes a String literal string the boolea
67. 9 1 LesatH 1 LMDELGADO 1 LukE101 1 Lu s VIT RIO CARGNINI 6 MALEIST34 2 MATRIXFRoG 5 14 MATTYLAWS 72 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER IMAGINATIONAC 73 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER ITSBORIN 74 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER J36MILES 75 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER JACKPOTTE 76 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER JAMES SUTHERLAND 77 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER JGUK 78 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER JIMMYATIC 79 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER JK33 80 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER JOMEGAT 81 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER JONATHAN_WEBLEY 82 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER JPKOTTA 83 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER JSONSTEIN 84 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER KAATIEP 85 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER KEJIA 86 HrTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER KENJ0418 87 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER KRIAK 88 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER KRISCHIK 89 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER LCAWTE 90 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER LESATH 91 uTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER LMDELGADO 92 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER LUKELOL 93 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER LUSC3 ADS_VITSC3SB3RIO_ CARGNINI 94 HTTP EN WIKIBOOKS ORG W INDEX PHP T
68. AMMATION 20JAVAS2FTABLEAUX ORG WIKI PROGRAMMEREN 20IN 520JAVAS2FARRAYS WIKIBOOKS WIKIBOOKS WIKIBOOKS WIKIBOOKS ORG WIKI JAVA_STANDARD 3A 20DATENSTRUKTUREN ORG WIKI JAVAS2FVETORES JAVA ARRAYS 145 Arrays 146 29 Collection Classes Collections are a group of objects bound together by a common characteristic Java has various built in classes to support the collection of objects either of the same type or general The most basic construct to work with is the array of which you will come to know of in a SECTION later in this chapter 29 1 Introduction to Collections The most basic collection interface is called Collection This interface gives the user a generic usage of a collection import java util Collection Interface import java util ArrayList Implementation import java util HashSet Implementation Collection colll new ArrayList Collection coll2 new HashSet lt Use colll amp coll2 gt In the above there are two collections The usage of the collections are the same the implemen tations are different If the existing collection implementations do not meet your needs you can write your version of the implementation Your version of the implementation just needs to implement the same java util Collection interface then you can switch to using your implementation and the code that is using the collection does not need to be changed import java util Collection i
69. AMMINGS2FKEYWORDSS2F TRANSIENT 132 Creating object receiving from a remote source Normally a minor change to the class can cause the serialization to fail You can still allow the class to be loaded by defining the serialization version id private static final long serialVersionUID 42L CATEGORY JAVA PROGRAMMING 4 HTTP EN WIKIBOOKS ORG WIKI CATEGORYS3AJAVAS20PROGRAMMING 133 Creating Objects 134 24 Interfaces 24 1 Interfaces Java does not allow you to create a subclass from two classes There is no multiple inheritance The major benefit of that is that all java objects can have acommon ancestor That class is called Object All java classes can be up casted to Object Example class MyObject When you type the above code it actually means the following class MyObject extends Object The compiler adds extends Object if not specified So it can be guaranteed that certain methods are available in all java classes This makes the language simpler To mimic multiple inheritance java offers interfaces which are similar to abstract classes In interfaces all methods are abstract by default without the abstract key word Interfaces have no implementation and no variables but constant values can be defined in interfaces however a single class can implement as many interfaces as required public interface MyInterface public static final String CON
70. APPLETS 6 HTTP EN W PEDIA ORG W JAVAS20APPLET 7 HTTP EN W PEDIA ORG W PROTOCOLS20 28COMPUTING 29 8 HTTP EN W PEDIA ORG W PROTOCOLS20 28COMPUTING 29 9 HTTP EN W PEDIA ORG W JAVAS20REMOTES20METHODS S20INVOCATION 10 HTTP EN Ww BOOKS ORG W JAVAS20PROGRAMMINGS2FCLIENTS20SERVER 11 HTTP EN 0w PEDIA ORG W WEBS20BROWSER 45 Execution Having a standard client it makes sense to create a unified standard back end service as well and that is the APPLICATION SERVER Web Application is an application that is running in the APPLICATION SERVER and it can be accessed and used by the BROWSER client There are three main area of interest in Web Applications those are The WEB BRowsER This is the container of rendering HTML text and running client scripts The HTTP proroco Text data are sent back and forth between Browser and the Server The WEB SERVER to serve static content APPLICATION SERVER to serve dynamic content and host EJB 0s Wikipedia also has an article about WEB APPLICATION 7 2 J2EE code execution As the focus was shifting from reaching GUI clients to thin client applications with Java version 2 Sun introduced J2EE Java 2 Extended Edition J2EE added e COMPONENTS BASE ARCHITECTURE Servlet JSP EJB Containers With J2EE the following Java component executions are possible Client Computer Application Server Browser HTTPServlet lt html
71. BOOKS ORG W INDEX PHP TITLE USER DEVOURERO9 38 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER DICKDICKDICK 39 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER DIRK_H C3 BCNNIGER 40 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER DIRK_GENTLY 41 HTTR EN w BOOKS ORG W INDEX PHP TITLE USER DJB 42 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER DMONEGO 43 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER DRAWDE83 44 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER ELI_VERBUYST 45 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER ELLMIST 46 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER ERVINN 186 Externallinks 14 EXABYTE 6 EXPLANATOR 3 FELIPEOCHOA0918 9 3 FisHpIPO 1 FKEREKD 1 FLATIPAC 12 FORAGE 2 FRATERM 2 FREDMARANHAO 1 FRIGOTONIS 11 FTIERCEL 1 FUNNYMAN3595 2 GoLLoXp59 2 Groxus 1 Grunny 1 Guanaco 5 GUANGPU HUANG 1 HMPERSON184 2 Hacinpaz 1 HERMIONE198056 1 HETHRIR 7 1 HErTHRIRBoOT 6 HIGHLAND 1 HRopuLF 2 IMAcwin95 47 TTP EN W BOOKS ORG W INDEX PH 48 TTP EN W BOOKS ORG W INDEX PH 49 TTP EN W BOOKS ORG W INDEX PH 50 TTP EN W BOOKS ORG W INDEX PH 51 TTP EN W BOOKS ORG W INDEX PH 52 TTP EN W BOOKS ORG W INDEX PH 53 TTP EN W BOOKS ORG W INDEX PH 54 TTP EN W BOOKS ORG W INDEX PH 55 TTP EN W BOOKS ORG W INDEX PH 56 TTP EN W BOOKS ORG W INDEX PH 57 TTP EN W BOOKS ORG W I
72. EOLOL A collection of introductory lessons to Java JAVA EXERCISES AT LEOLOL A collection of Java exercises with sample solutions Newsgroups news comp lang java comp lang java GOOGLE S WEB INTERFACE 41 42 43 44 45 46 47 48 49 50 51 52 COMPUTER FREEONLINEBOOKSTORE ORG SHOWBOOK PHP SUBCATEGORYID 17 WWW MESHPLEX ORG WIKI JAVA INTRODUCTION_TO_JAVA WWW UCERTIFY COM VENDORS SUN HTML WWW KODEJAVA ORG GPWIKI ORG WWW WIKIJAVA ORG WWW EBOOKSLAB INFO DOWNLOAD FREE JAVA EBOOKS JAVA SUN COM DOCS CODECONV HTML CODECONVTOC DOC HTML WWW LEOLOL COM DRUPAL TUTORIALS JAVA LESSONS WWW LEOLOL COM DRUPAL TUTORIALS JAVA EXERCISES GROUPS GOOGLE COM GROUPS GROUP COMP LANG JAVA H H H HHH HH H H H H H DO ee Ago tQ e PQ POO 49 aa 179 EBOOKS MZWRITER NET E BOOKS SHARE_EBOOK 310 220 SUN CERTIFIED DEVELOPER FOR JAVA WEB Links 180 32 License 181 License 182 33 GNU Eree Documentation License 1 REDIRECT WIKIBOOKS GNU FREE DOCUMENTATION LICENSE DE JAVA2 DEUTSCH J SE 1 HTTP EN WIKIBOOKS ORG WIKI GNU S20FREES2ODOCUMENTATIONS20OLICENSE 2 HTTP DE WIKIBOOKS ORG WIKI JAVA 3 HTTP DE WIKIBOOKS ORG WIKI JAVA S20STANDARD 183 GNU Free Documentation License 184 34 Authors Edits User 2 1WHEELI 1 A B 102 3 AVERMA 1 ADRiLey 1 ADDPS4CAT 35 ADRIGNOLA 3 ALAINR345 1 ALEXANDER ORLOV 4 ANONYMOUS DISSIDE
73. ETORES JAVA ARRAYS 111 Arrays 112 20 Data and Variables A variable in Java can store two kinds of variables e JAVA PRIMITIVE TYPE VALUES e areference to a JAVA OBJECT Java s primitive types are e integers whole numbers declared as byte short int or long only int need be of interest to a beginner floating point numbers decimal numbers declared as float or double only float need be of interest at first e characters declared as char representing one character like A or boolean holding only true or false as values In addition the Java language has special features for its String class and strings can be treated very much like primitives for many purposes As in most languages a variable is declared to be a particular type of data the syntax for a declaration is variabletype variablename To store a value in a variable a program statement like variablename data And can reference the variable and use the data stored in it by its name For example to create an int primitive type value named year that stores 2007 year 2007 To access the data in year use the variable in place of the number System out println year Produces 1 Chapter 16 on page 95 2 Chapter 23 on page 129 113 Data and Variables 2007 20 1 Strong Typing Variables in Java are strongly typed which means that the compiler checks the
74. G WIKI JAVAS20PROGRAMMINGS 2FJAVAS20PROGRAMMINGS 20ENVIRONMENTS23THES20BYTECODE 4 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS 2FJAVAS20PROGRAMMINGS 20ENVIRONMENTS23THES20JITS20COMPILER 35 Compilation 6 2 Automatic Compilation of Dependent Classes In Java if you have used any reference to any other java object then the class for that object will be automatically compiled if that was not compiled already These automatic compilations are nested and this continues until all classes are compiled that are needed to run the program It is usually enough to compile only the high level class since all the dependent classes will be automatically compiled javac MainClass java However you can t rely on this feature if your program is using reflection to create objects or you are compiling for servlets or a jar package In these cases you should list these classes for explicit compilation javac MainClass java ServletOne java The best way is to use a build tool to build your application The build tool would check all the needed dependencies and compile only the needed class for the build The ANT tool is the best and the most popular build tool currently available Using ANT you would build your application from the command line by typing ant build xml The xml file contains all the information needed to build the application Note In rare cases your code may a
75. GHLIGHTING and CODE COMPLETION These features are a vital part of the exer cise of writing code Nevertheless there are a variety of different open source editors available as alternatives to Notepad that support these features For the purposes of the tutorials in this book the most recommended editor is the Notepad a free and open source fully integrated text editor that sup ports syntax highlighting and code completion You need to DOWNLOAD THE LATEST VERSION OF NOTEPAD in order to start writing code with the editor 21 HTTP EN WIKIPEDIA ORG WIKI SYNTAXS20HIGHLIGHTING 22 uTTe EN WIKIPEDIA ORG WIKI CODE S20COMPLETION 23 HTTP NOTEPAD PLUS PLUS ORG About This Book Note Amongst others there are many editors available online that are specifically designed to code Java applications Such editors have countless other features that facilitate programming with Java e g DEBUGGING and application design interfaces etc Text editors that have compre hensive features and utilities to facilitate programmers are called Integrated Development En vironments or IDEs Java programmers often recommend the two most widely used IDEs for Java programming needs these are e NetBeans DOWNLOAD HERE and READ THE INSTALLATION INSTRUCTIONS HEREC Eclipse Download 32 BIT HERE and 64 BIT HERE You can READ THE INSTALLATION IN STRUCTIONS HERF a H b H c H d H R e H R f H T T
76. I JAVALANGUAGE WWW NETBEANS ORG WWW ECLIPSE ORG WWW ZEUSEDIT COM JAVA HTML JAVA SUN COM JAVA SUN COM DOCS WHITE LANGENV WWW ROSEINDIA NET JAVA WWWwW JAVAPASSION COM JAVAINTRO WWW BEANSHELL ORG JAVA SUN COM DOCS BOOKS JLS THIRD_EDITION HTML J3TOC HTML JAVA SUN COM DOCS BOOKS VMSPEC 2ND EDITION HTML VMSPECTOC DOC HTML JAVA SUN COM DOCS BOOKS VMSPEC 2ND EDITION JVMS CLARIFY HTML WWW IJDISTRO COM WIKI JAVA NET BIN VIEW JAVAPEDIA WWW MINDVIEW NET BOOKS TIJ JAVAGAMEDEVELOPMENT NET WWW JAVABEAT NET WWW APL JHU EDU HALL JAVA FAQS AND TUTORIALS HTML FINDSHELL COM WWW LANDOFCODE COM JAVA r H H H p ip ip ip ip ipin nn n dn n H H H HHPH tg tO TQ mt hg TT mt pg bg md td a doma mt bg 7 ma ma md a 178 Externallinks ONLINE JAVA TUTORIAL FULL JAVA TUTORIAL A collection of free premium programming tutorials JAVA CERTIFICATION PRACTICE TESTS AND ARTICLES KODE JAVA LEARN JAVA PROGRAMMING BY EXAMPLES GAMES PROGRAMMING WIKI Java tutorials and lessons based on game programming WIKIJAvaA Examples and tutorials in Java DOWNLOAD FREE JAVA EBOOKS FROM 83 EBOOKS COLLECTION Free Java Ebooks to down load from ebookslab info DOWNLOAD FREE SUN CERTIFIED DEVELOPER FOR JAVA WEB SERVICES Free Java Ebooks to download from ebooks mzwriter net CODE CONVENTIONS FOR THE JAVA PROGRAMMING LANGUAGE At SUN JAVA LESSONS AT L
77. IBOOKS ORG WIKI CATEGORY 3AJAVAS20PROGRAMMING 139 Destroying Objects 140 27 Overloading Methods and Constructors Iftwo methods ofa class whether both declared in the same class or both inherited by a class or one declared and one inherited have the same name but different signatures then the method name is said to be overloaded This fact causes no difficulty and never of itself results in a compile time error There is no required relationship between the return types or between the throws clauses of two methods with the same name but different signatures Methods are overridden on a signature by signature basis If for example a class declares two public methods with the same name and a subclass overrides one of them the subclass still inherits the other method In this respect the Java programming language differs from C When a method is invoked the number of actual arguments and the compile time types of the arguments are used at compile time to determine the signature of the method that will be in voked If the method that is to be invoked is an instance method the actual method to be invoked will be determined at run time using dynamic method lookup CATEGORY JAVA PROGRAMMING 1 HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVAS20PROGRAMMING 141 Overloading Methods and Constructors 142 28 Arrays 28 1 Intro to Arrays An array is similar to a table of data keyed by number In Java an array
78. IKI PIZZA 20 28P ROGRAMMINGS20 LANGUAGE S29 20 HTTP EN WIKIPEDIA ORG WIKI GENERICS20JAVA S20 28PROGRAMMINGS2Z0LANGUAGE S29 30 Similar concepts NETREXX 21 HTTP EN WIKIPEDIA ORG WIKI NETREXX 31 The Java Platform 32 5 Getting Started 1 REDIRECT JAVA PROGRAMMING GETTING STARTED 5 1 Getting Started With Java it is as easy as pie to get started However if you encounter difficulty in getting started with Java you may find useful help and tips in this chapter Our authors have tried their best to include as much useful content as possible in this chapter for your reference But nevertheless if problems persist please consult the authors using the DISCUSSION page above This section is a quick start to using Java installing Java software compiling and running pro grams and some small sample programs to illustrate the basics of getting started with Java CATEGORY JAVA PROGRAMMING 1 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FGETTING 20STARTED 2 HTTP EN WIKIBOOKS ORG WIKI TALK S3AJAVAS20PROGRAMMINGS2FGETTINGS20STARTED 3 HTTP EN WIKIBOOKS ORG WIKI CATEGORYS3AJAVAS20PROGRAMMING 33 Getting Started 34 6 Compilation We have already discussed COMPILATION BASICS Here s a recap of the concepts we d seen earlier and some additional details 6 1 Compiling to bytecode In Java programs are not compiled into executable files they are compiled into BYTECODE as discuss
79. ITLE USER MALFIST 95 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER MATRIXFROG 96 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER MATTYLAWS 188 Externallinks 1 MAxBowsHER 1 Messi 3 METABOHEMIAN 1 Muayes 1 MIKE LIFEGUARD 63 Miko 2 MKN 1 Ms2GER 3 MsHOoNLE 095 1 Mstenta 2 N3137310 1 OLDGEEk 1 Pappu 22 PANIC2K4110 1 Pengo 3 PHILIP 2 PrrEL113 2 PowerorD 15 QUITEUNUSUALI S RDEv 6 RAKESH KPN117 RALPHCOOK 8 10 Rappo 9 RAVICHANDAR84 2 RayKippy 97 HTTP EN W BOOKS ORG W INDEX P 98 HTTP EN W BOOKS ORG W INDEX P 99 TTP EN W BOOKS ORG W INDEX P 100 HTTP EN W BOOKS ORG W INDEX P 101 HTTP EN W BOOKS ORG W INDEX P 102 HTTP EN W BOOKS ORG W INDEX P 103 HTTP EN W BOOKS ORG W INDEX P 104 HTTP EN W BOOKS ORG W INDEX P 105 HTTP EN W BOOKS ORG W INDEX P 106 HTTP EN W BOOKS ORG W INDEX P 107 HTTP EN W BOOKS ORG W INDEX P 108 HTTP EN W BOOKS ORG W INDEX P 109 HTTP EN W BOOKS ORG W INDEX P 110 HTTP EN W BOOKS ORG W INDEX P 111 HTTP EN 0W BOOKS ORG W INDEX P 112 HTTP EN W BOOKS ORG W INDEX P 113 HTTP EN W BOOKS ORG W INDEX P 114 HTTP EN W BOOKS ORG W INDEX P 115 HTTP EN W BOOKS ORG W INDEX P 116 HTTP EN W BOOKS ORG W INDEX P 117 HTTP EN 0W BOOKS ORG W INDEX P 118 HTTP EN W BOOKS ORG W INDEX P 119 HTTP EN W BOOKS ORG W INDEX P 120 HTTP EN W BOOKS ORG W INDEX P 121 HTT
80. If that happens in different servers then the same class must be available on both servers Same class means that exactly the same version of the class must be available otherwise the object won t be able to be re created This is a maintenance problem to those applications where java serialization is used to persist object or sent the object through the network When a class is modified there could be a problem re creating those objects that were serialized using an earlier version of the class Java has built in support for serialization using the Serializable interface however a class must first implement the Serializable interface By default a class will have all of its fields serialized when converted into a data stream with TRANSIENT fields being skipped If additional handling is required beyond the default of writing all fields you need to provide an implementation for two methods private void writeObject java io ObjectOutputStream out throws IOException private void readObject java io ObjectInputStream in throws IOException ClassNotFoundException private void readObjectNoData throws ObjectStreamException If the object needs to write or provide a replacement object during serialization it needs to im plement the following two methods with any access specifier Object writeReplace throws ObjectStreamException Object readResolve throws ObjectStreamException 3 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGR
81. Introduction Before a Java object can be created the class byte code must be loaded from the file system with class extension to memory This process of locating the byte code for a given class name and converting that code into a Java Cass class instance is known as class loading There is one CLass created for each type of Java class All objects in java programs are created on heap memory An object is created based on its class You can consider a class as a blueprint template or a description how to create an object When an object is created memory is allocated to hold the object properties An object reference point ing to that memory location is also created To use the object in the future that object reference has to be stored as a local variable or as an object member variable The Java Virtual Machine JVM keeps track of the usage of object references If there are no more reference to the object the object can not be used any more and becomes garbage After a while the heap memory will be full of unused objects The JVM collects those garbage objects and frees the memory they allocated so the memory can be reused again when a new object is created See below a simple example Create an object MyObject obj new MyObject Use the object obj printMyValues The obj contains the object reference pointing to an object created from the MyObject class The obj object reference is in s
82. Java Programming Print version Wikibooks org February 15 2012 Contents 1 INTRODUCTION 1 1 ARE YOU NEW TO PROGRAMMING eee ee eee ee es 1 2 PROGRAMMING WITH JAVA 0 ee ss sas ss s ee 2 1 WHO SHOULD READ THIS BOOK J o 2 2 HOW CAN YOU PARTICIPATE se e a a ia a e ea a ee 2 3 NECESSARY PREREQUISITES ee ee 3 HISTORY 3 1 THE GREEN TEAM z Z a mat Soe aa a Seow PAG Ara ba A ed 3 2 RESHAPING THOUGHT a ere a s ka j e 2 gus s us us ee S 0 Bee ee a OG 3 3 THE DEMISE OF AN IDEA BIRTH OF ANOTHER 3 4 RECENT HISTORY ane u 5 A la ne S Sisa Pie ran Sa oh ee tio 3 5 VERSIONS 5 amp u S ats as S Q n ua AOA ta BR Pe eS he ed oe Ses Mae 3 6 CITATIONS 6 sedis la ah RA ee a at ee ded A te oe Q SL R Be 4 THEJAVA PLATFORM 4 1 JAVA RUNTIME ENVIRONMENT JRE 4 2 JAVA DEVELOPMENT KIT JDK 4 3 SIMILAR CONCEPTS eee ee es 5 GETTING STARTED 5 1 GETTING STARTED 24 002 uQ A N We PA ee Bs das ee 6 COMPILATION 6 1 COMPILING TO BYTECODE 0 eee ee ee ee ee a 6 2 AUTOMATIC COMPILATION OF DEPENDENT CLASSES 6 3 PACKAGES SUBDIRECTORIES AND RESOURCES 2 0 000 eee 6 4 FILENAME CASE ay au i am tay ae ce yankee Ate Swe Rend Nc 6 5 GOMPILER OPTIONS aroser ar dresden ta aoe cta 6 0 ADDITIONAL TOOLS G V sacca elk a q aa 6 7 JBUILDER daia ce e kuspa us q he els id aie ot eds
83. M TECHNETWORK JAVA JAVASEBUSINESS DOCUMENTATION JAVA SE BCL LICENSE 430205 HTML 19 HTTP DOWNLOAD ORACLE COM OTN PUB JAVA JDK 7U1 B08 JDK 7U1 WINDOWS X64 EXE 20 HTTP WWWwW ORACLE COM TECHNETWORK JAVA JAVASEBUSINESS DOCUMENTATION JAVA SE BCL LICENSE 430205 HTML Necessary prerequisites 4 In the Edit System Variable dialog go the the Variable value field This field is a list of directory paths separated by semi colons 5 To add a new path append the location of your JDK bin folder separated by a semi colon 6 Click OK on every opened dialog to save changes and get past to where you started Start writing code Once you have successfully installed the JDK on your system you are ready to program code in the Java programming language However to write code you would need a decent text editor Windows comes with a default text editor by default Notepad In order to use notepad to write code in Java you need to follow the steps below 1 Click Start gt All Programs gt Accessories gt Notepad to invoke the application Y x Alternatively you can also press Win R to open the Run dialog With the dialog open type the following command at the prompt notepad 2 Once the Notepad application has fired up you can use the editor to write code for the Java programming language The problem with Notepad however is that it does not support developer friendly features such as SYNTAX HI
84. Method You can write a non object oriented program by using only static methods in java Because java evolved from the C programming language static methods are left over from a non object ori ented language You write static methods the same way you do normal methods the only difference is that you cannot reference any member variables or any object methods Static methods can reference only static variables and call only other static methods However you can create an object and use it inside a static method public class MyObject public String memberVar static private String memberStaticVar static public String myStaticMethod l memberVar Value gt ERROR Cannot reference member var memberStaticVar Value This is okay static vars can be used Create an object MyObject obj new MyObject obj memberVar Value This is okay since an object is created return I am a static method 15 9 External links e ABSTRACT METHODS INTERACTIVE JAVA LESSON 1 HTTP JAVALESSONS COM CGI BIN FUN JAVA TUTORIALS MAIN CGI SES A0789 amp CODE ABS amp SUB FUN 93 Methods 94 16 Primitive Types 1 redirect JAVA PROGRAMMING TYPES 1 Chapter 17 on page 97 95 Primitive Types 96 17 Types Data Types or simply Types in Java are a way of telling what certain data is It is seen as a way of declaring allowed values for certain data
85. NDEX PH 58 TTP EN W BOOKS ORG W INDEX PH 59 TTP EN W BOOKS ORG W INDEX PH 60 TTP EN W BOOKS ORG W INDEX PH 61 TTP EN W BOOKS ORG W INDEX PH 62 TTP EN W BOOKS ORG W INDEX PH 63 TTP EN W BOOKS ORG W INDEX PH 64 TTP EN W BOOKS ORG W INDEX PH 65 TTP EN W BOOKS ORG W INDEX PH 66 TTP EN W BOOKS ORG W INDEX PH 67 TTP EN W BOOKS ORG W INDEX PH 68 TTP EN W BOOKS ORG W INDEX PH 69 TTP EN W BOOKS ORG W INDEX PH 70 TTP EN W BOOKS ORG W INDEX PH 71 TTP EN W BOOKS ORG W INDEX PH P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE N N DPPH HH DP PDH HH DPD DH HF HF DP DDH ABA EXABYTE EXPLANATOR FELIPEOCHOAO918 FISHPI FKEREKI FLATIPAC FORAGE FRATERM FREDMARANHAO FRIGOTONI FTIERCEL FUN GOLLOXP GRO GRU GUA GUA HMPERSON1 HAGINDAZ HERMIONE1980 HETHRIR HETHRIRBOT HIGHLAND HROSC3SBOULF IMACWIN95 YMAN3595 US NY ACO GPU HUANG 187 Authors 2 IMAGINATIONAC 1 ITrsBoRIN 3 1 J36MILES 7 JACKPOTTE 1 JAMES SUTHERLAND 33 Jeuk 1 Jimmyatic 1 Jx33 2 7 JOMEGAT 2 JONATHAN WEBLEY 1 Jexorra 8 JsoNSTEIN83 2 KaATIEP84 1 KEJIABS 1 KENJO418 1 KRIAK 4 KRiscHIK88 1 LcAwrE8
86. NT 2 ANTIDRUGUE 0 1 APHONIK 6 ARLEN2212 1 ARSENALFAN 225 ARUNREGINALD 4 7 ASHMAILIT 2 AZ156815 1 B3r 1 BRUTE 2 Bastie 1 BENO10002 2 BENPAGE267 1 TTP EN W BOOKS ORG W INDEX PHP T 2 TTP EN W BOOKS ORG W INDEX PHP T 3 TTP EN W BOOKS ORG W INDEX PHP T 4 TTP EN W BOOKS ORG W INDEX PHP T 5 TTP EN W BOOKS ORG W INDEX PHP T 6 TTP EN W BOOKS ORG W INDEX PHP T 7 TTP EN W BOOKS ORG W INDEX PHP T 8 TTP EN W BOOKS ORG W INDEX PHP T 9 TTP EN W BOOKS ORG W INDEX PHP T 10 HTTP EN W BOOKS ORG W INDEX PHP T 11 TTP EN W BOOKS ORG W INDEX PHP T 12 TTP EN W BOOKS ORG W INDEX PHP T 13 TTP EN W BOOKS ORG W INDEX PHP T 14 HTTR EN W BOOKS ORG W INDEX PHP T 15 TTP EN W BOOKS ORG W INDEX PHP T 16 HTTP EN W BOOKS ORG W INDEX PHP T 17 HTTP EN W BOOKS ORG W INDEX PHP T 18 HTTP EN W BOOKS ORG W INDEX PHP T 19 TTP EN W BOOKS ORG W INDEX PHP T 20 HTTP EN W BOOKS ORG W INDEX PHP T 21 TTP EN W BOOKS ORG W INDEX PHP T H H H pp ip ipi ip ip p p a nn n nH p HH Fl P El El Fl EFJ Pi El Pi E P PF EF PF Fi Pi F PJ P gt Pi SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE N N DPPH HH DPPH HH FP DO VO HH DD LWHEEL tA By 10 AVERMA ADRILEY ADDPS4CAT ADRIGNOLA ALAINR345 ALEXANDER ORLOV ANONYMOUS_DISSIDENT ANTIDRUGUE AP AR ARSENALFAN ARUNREGINALD TAS Az1568 B3T BRUTE
87. OSBBSDOSBESDOS 0 BO SB2SE7 81 AB_SE4S5B8 87 191 Authors 192 List of Figures GFDL Gnu Free Documentation License http www gnu org licenses fdl html cc by sa 3 0 Creative Commons Attribution ShareAlike 3 0 License http creativecommons org licenses by sa 3 0 cc by sa 2 5 Creative Commons Attribution ShareAlike 2 5 License http creativecommons org licenses by sa 2 5 cc by sa 2 0 Creative Commons Attribution ShareAlike 2 0 License http creativecommons org licenses by sa 2 0 cc by sa 1 0 Creative Commons Attribution ShareAlike 1 0 License http creativecommons org licenses by sa 1 0 cc by 2 0 Creative Commons Attribution 2 0 License http creativecommons org licenses by 2 0 cc by 2 0 Creative Commons Attribution 2 0 License http creativecommons org licenses by 2 0 deed en cc by 2 5 Creative Commons Attribution 2 5 License http creativecommons org licenses by 2 5 deed en cc by 3 0 Creative Commons Attribution 3 0 License http creativecommons org licenses by 3 0 deed en GPL GNU General Public License http www gnu org licenses gpl 2 0 txt PD This image is in the public domain ATTR The copyright holder of this file allows anyone to use it for any purpose provided that the copyright holder is properly attributed Redistribution derivative work commer cial use and all other use is permitted EURO This is the common reverse face of a euro coin The copyright on the design of
88. P EN W BOOKS ORG W INDEX P P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE SE N N DPPH HH DD DH HF HPP DH HF HF DP DP NNNUNN AXBOWSHER ESSI ETABOHEMIAN HAYES IKE LIFEGUARD IKM KN S2GER SHONLE STENTA 31373 OLDGEEK PADDU PAN PENGO PHI PITEL POWEROID QUITEUNUSUAL RDEV RAKESH_KPN RALPHCOOK RAPPO RAVICHANDAR84 RAYKIDDY C2K4 LIP 189 Authors 7 RECENT RUNES122 2 REMI 11 RICKY CLARKSON 4 1 ROBERT HoRNING125 3 SBJoHNNY126 1 SAILWINHEIN 6 SamwiLsoN 8 1 SavH12 1 SEANJA 30 4 SHaAHIDSIDD 31 2 SHAUNW 32 1 SHENME 68 SIGMA 7134 1 Snarius 1 Spock431 8 SPONGEBOB88 16 SpooN 38 1 STEPHANVANINGEN 9 4 SUNDAR22IN140 3 SUNNYCHAN 7 SUPERFLY JON142 1 SWIFT 1 TALKTOATISH144 2 TANMINIvVAN 4 1 TAROSE TREVORI46 122 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER RECENT_RUNES 123 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER REMI 124 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER RICKY_CLARKSON 125 HTTP EN W BOOKS ORG W INDEX PHP TITLE USER ROBERT_HORNING 126 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLE USER SBJOHNNY 127 HTTP EN WIKIBOOKS ORG W INDEX PHP TITLEUSER SAILWINHEIN 128 HTTP EN WIKIBOOKS ORG W INDEX
89. S 2 2 eee ee ee 10 33 VARIABLE S coeno d te e Sek A e P eS Ge eS A ed 10 4 DATA TYPES a aoe e ag eed Base S yi Gece a Die eG ans ak 10 5 WHOLE NUMBERS AND FLOATING POINT NUMBERS 000002 ee 10 6 ASSIGNMENT STATEMENTS 2 0 0 eee ee eee CLASSES OBJECTS AND TYPES 11 1 OBJECTSAND CLASSES a a u q Ca w 4 9 ee eee 11 2 INSTANTIATION AND CONSTRUCTORS ee eee eee RTS A A at ad a eee Ba end lt 11 4 MULTIPLE CLASSES IN A JAVA FILE ee 115 EXTERNAL LINKS 25 5 in 40d e a ae EE Bas PACKAGES 12 1 JAVA PACKAGE NAME SPACE ee 12 2 WILDCARD IMPORTS 0 0 ee ee 12 3 IMPORTING PACKAGES FROM JARFILES ee 12 4 CLASS LOADING NAMESPACE oohoo ee ee NESTED CLASSES 13 1 NEST A CLASS INSIDE A CLASS a 13 2 NEST A CLASS INSIDE A METHOD e eee eee ee ee eee 13 3 ANONYMOUS CLASSES so u She q E eA eS es G why eee ACCESS MODIFIERS VAT GAGCCESS MODIFIERS ua e a N A dS Soest a O W k aah E ee la METHODS 15 1 METHOD DEFINITION a sa sas sos as s as as as nis ss s 15 2 METHOD OVERLOADING 6 cee ee ee ee es 43 43 46 49 51 51 52 60 61 62 63 64 65 65 66 67 67 67 67 68 68 69 71 71 71 72 73 74 75 75 76 76 77 79 79 80 80 83 83 Contents 15 37 METHOD OVERRIDING e ie n aa a WEE De ee Ea 87 15 4 PARAMETER
90. S20PROGRAMMING 2FOPERATORS Chapter 9 3 on page 64 CO OO 1 O Q G N 61 Syntax There are also left and right braces and which enclose BLocks 9 The body of a class is one such block STATEMENTS A Block contains one or more Java STATEMENT S 1 the smallest building block of Java separated by semicolons A statement is Separators Some tokens are punctuation such as periods and commas and semicolons 13 WHITESPACE You use WHITESPACE such as spaces tabs and newlines to separate tokens For example whitespace is required between keywords and identifiers publicstatic is a single identifier with twelve characters not two Java keywords COMMENTS Comments are not part of the executing code Comments are used to document the code 9 1 Unicode Most Java program text consists of ASCII characters but any Unicode character can be used as part of identifier names in comments and in character and string literals UNICODE ESCAPE SEQUENCES may also be used to express a Unicode character For example z which is the Greek Lowercase Letter pi is a valid Java identifier double zx Math PI and in a string literal String pi string tt may also be represented in Java as the Unicode escape sequence u03C0 Thus the following is a valid but not very readable declaration and assignment 10 Chapter 9 3 on page 64 11 Chapter 10 on page 67 12 Chapter
91. STANT This value can not be changed public String methodOne This method must be implemented by the class that implements this interface public class MyObject implements MyInterface Implement MyInterface interface public String methodOne 135 Interfaces 24 2 Externallinks e INTERFACES INTERACTIVE JAVA LESSON INTERFACES 2 INTERACTIVE JAVA LESSON CATEGORY JAVA PROGRAMMING FR PROGRAMMATION JAVA INTERFACES 1 HTTP JAVALESSONS COM CGI BIN FUN JAVA TUTORIALS MAIN CG1I SES A07898 CODE IFL amp SUB FUN 2 HTTP JAVALESSONS COM CGI BIN FUN JAVA TUTORIALS MAIN CGI SES A0789 amp CODE IF2 amp SUB FUN 3 HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVA S20PROGRAMMING 4 HTTP FR WIKIBOOKS ORG WIKI PROGRAMMATIONS20JAVA 2FINTERFACES 136 25 Using Static Members 25 1 What does static mean When you declare a e method or member variable static it is independent of any particular but rather it is shared among all instances of a class To access a static method or member variable no instance needs to be created The static keyword is used to declare a method or member variable static 25 2 What can it be used for Static variables can be used as data sharing amongst objects of the same class For example to implement a counter that stores the number of objects created at a given time can be defined as SO public AClass static private int counter
92. TTP EN w BOOKS ORG W USERS3AARUNREGINALD 176 HTTP EN w BOOKS ORG W USERS3AERVINN 177 HTTP EN w BOOKS ORG W USERS3AMIK 178 HTTP EN W BOOKS ORG W USERS3AERVINN 179 HTTP EN W BOOKS ORG W USERS3AMIK 180 HTTP EN W BOOKS ORG W USERS3AARUNREGINALD 194
93. This special object is bound to this method and used only for returning values so do not use a public class The best way is to use a nested class see example 89 Methods below public class MyObject Nested object is for return values from getPersonInfoById method public static class Return0bj private int age private String name public void setAge int val this age val public int getAge return age public void setName String val name val public String getName return name End of nested class defination Method using the nested class to return values public Return0bj getPersonInfoByld int ID int age String name Get the name and age based on the ID from the database ReturnObj ret new ReturnObj ret setAge age ret setName name return ret In the above example the getPersonInfoByld method returns an object reference that contains both values the name and age See below how you may use that object MyObject obj new MyObject MyObject ReturnObj person obj getPersonInfoByld 102 System out println Person Name person getName System out println Person Age person getAge 90 Special method the Constructor 15 7 Special method the Constructor There is a special method for each class that will be executed each time an object
94. VARIABLES 113 20 1 STRONG TYPING s sacos pu meaa g sala qopuy nuq saus US uw HEE RAR N 114 20 2 CASE CONVENTIONS o oo su csan p a a kuu ER wee ee Pe eS 114 20 3 SCOPE ZQ ole 3208 Se BO Ln ta u ek et is a ts U bd Bb ha BS ls HLS 114 21 GENERICS 117 21 1 WHAT AREGENERICS k a p S a 3 s l e Q WS usq s U a 117 21 2 INTRODUCTION amp 6 p llm A Ba ay BR ge l N Q Aa Ne i Re Be en ess 118 21 3 NOTE FOR C PROGRAMMERS es 119 214 CLASS ELSA A Re Es a es Wes Rae hk Be ie 120 21 5 VARIABLE ARGUMENT sue w v0 a s da we Qa 121 21 6 WILDCARD TYPES orice ne ea Bee a es Boe eR ae a a Be ee 122 22 DEFINING CLASSES 125 221 FUNDAMENTALS p a sa aed iD ae Roar aS Ree WO EO eo 125 Contents 23 CREATING OBJECTS 129 231 INTRODUCTION sL S sos e e Balen eo EAN et Sul ua we eA U a h Sau ee 129 23 2 CREATING OBJECT WITH THE new KEYWORD 2 002 eee eee 129 23 3 CREATING OBJECT BY CLONING AN OBJECT 130 23 4 CREATING OBJECT RECEIVING FROM A REMOTE SOURCE 132 24 INTERFACES 135 24 INTERFACES xA y a a E A id aus 135 2472 EXTERNAL INKS 000 a it A cio 136 25 USING STATIC MEMBERS 137 25 1 WHAT DOES STATIC MEAN 0 e 137 25 2 WHAT CAN IT BE USED FOR ee eee 137 25 3 DANGER OF STATIC VARIABLES 138 25 4 EXTERNAL LINKS gt e si AA ale ae US SA ee ae ee a aes baw ee 138 26 DESTROYING OBJECTS 139 26 1 FINALIZE o une Sed doe u
95. W ORACLE COM TEC JAVA SE BCL LICENSE 43020 28 HTTP DOWNLOAD ORACLE CO 29 uTTP DOWNLOAD ORACLE CO 30 HTTP WWW ORACLE COM TEC HNETWORK JAVA JAVASEBUSINESS DOCUME JAVA SE BCL LICENSE 430205 HTML LINUX 1586 RPM LINUX I586 TAR TATION LINUX X64 RPM LINUX X64 TAR GZ TATION 11 About This Book Note Amongst others there are many editors available online that are specifically designed to code Java applications Such editors have countless other features that facilitate programming with Java e g DEBUGGING and application design interfaces etc Text editors that have compre hensive features and utilities to facilitate programmers are called Integrated Development En vironments or IDEs Java programmers often recommend the two most widely used IDEs for Java programming needs these are e NetBeans DOWNLOAD HERE and READ THE INSTALLATION INSTRUCTIONS HEREC Eclipse Download 32 BIT HERE and 64 BIT HERE You can READ THE INSTALLATION IN STRUCTIONS HERF A HTTP EN WIKIPEDIA ORG WIKI DEBUGGING b HTTP DOWNLOAD NETBEANS ORG NETBEANS 7 0 1 FINAL BUNDLES NETBEANS 7 0 1 ML LINUX SH rp NETBEANS ORG COMMUNITY RELEASES 70 INSTALL HTML ta ae TP WWW ECLIPSE ORG DOWNLOADS DOWNLOAD PHP FILE TECHNOLOGY EPP DOWNLOADS d H R e H R f H LEASE INDIGO SR1 ECLIPSE JAVA INDIGO SR1 LINUX GTK TAR GZ amp R 1 rP WWW ECLIPSE ORG DOWNLOADS DOWNLOAD PHP FILE TECHNOLOGY EPP
96. WUTZOFANT1 81 2 YMS 62 6 YuuKI MAYyUKI 63 9 ZEROONE 84 1 Boxoaummp lpyma 1 KAADER 147 EN W BOOKS ORG W INDEX PH 148 EN W BOOKS ORG W INDEX PH 149 EN W BOOKS ORG W INDEX PH 150 EN W BOOKS ORG W INDEX PH 151 EN W BOOKS ORG W INDEX PH 152 EN W BOOKS ORG W INDEX PH 153 EN W BOOKS ORG W INDEX PH 154 EN W BOOKS ORG W INDEX PH 155 EN W BOOKS ORG W INDEX PH 156 EN W BOOKS ORG W INDEX PH 157 EN W BOOKS ORG W INDEX PH 158 EN W BOOKS ORG W INDEX PH 159 EN W BOOKS ORG W INDEX PH 160 EN W BOOKS ORG W INDEX PH 161 EN W BOOKS ORG W INDEX PH 162 EN W BOOKS ORG W INDEX PH 163 EN W BOOKS ORG W INDEX PH 164 EN W BOOKS ORG W INDEX PH 165 EN W BOOKS ORG W INDEX PH SB8SDO0SBC D0SB8 D1 80_ D0 93 166 EN W BOOKS ORG W INDEX PH 3 T o D o P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T P T D1 P T E9S9CS8ASE9S95 SB7 WUR DE WUTZOF ott sll J J D DDD oo pany HOH HHH H HHH HHH RHR HK H dH H HA UA AARP P PF Fl PF EF Pi Fl P FE PI P P P E Ei THAREN THEDAV THEPHI UNV UCERTI VINAY_ WEBAWA HITE KIW KIAL KIM LLEM SEEY W W W W W W THEL EROSS LWELLS FY H RE IGHT ZARD T DHIANN __SOUWER ES NE ANT YUUKI_MAYUKI ZEROO 5D0592 83 D1 88 D SE7SA0 E SDOSBESD
97. When one declares an array the data type is changed to include square brackets While you can instead place these square brackets next to the variable name this is not recommended To create an array you will need to use the new operator to have it create the Array with the specified number of elements Declares an array named datal and has it assigned to an array with 25 11 Chapter 16 on page 95 12 Chapter 16 on page 95 99 Types elements private int datal new int 25 Declares an array named data2 that contains these three elements private int data2 new int 1 99 2 Arrays are described in more detail in the Arrays section 17 5 Primitive Data Types The Java primitive data types contain pure values no operations It is basically data types similar to what other non object oriented languages have There are arrays of primitive types in Java but because they are not objects primitive values can not be put in a collection For this reason object wrappers are defined in JDK java lang package for all the primitive types Size bits Example Minimum Max Wrapper Value imum Class Value Integer types byte 8 byte b 128 127 Byte 65 char 16 char c 0 216 1 Character X char c 65 short 16 short s 215 215 1 Short 65 int 32 int i 65 qa 231 1 Integer long 64 longl z983 263 1 Long 65L Floating point types float 32 float f Float 65f double 64 double
98. _cacheTemplate new HashTable Clone Customer object for performance reason public Customer createCustomerObject See if a template object exists in our cache Customer template _cacheTemplate get Customer if template null Create template template new Customer _cacheTemplate put Customer template return template clone Now lets see how to make the Customer object cloneable First the Customer class needs to implement the Cloneable Interface Override and make the clone method public as that is protected in the Object class e Call the super clone method at the beginning of your clone method e Override the clone method in all the subclasses of Customer public class Customer implements Cloneable public Object clone throws CloneNotSupportedException Object obj super clone return obj In the above example we used cloning for speed up object creation An other use of cloning could be to take a snapshot of an object that can change in time Lets say we want to store Customer objects in a collection but we want to disassociate them from the live objects So before adding the object we clone them so if the original object changes from that point forward the added object wont Also lets say that the Customer object has a reference to an Activity object that contains the customer activities Now we are faci
99. a archives are executable Therefore the java tool would only be able to execute files that are executable Non executable class files and Java archives are simply called class libraries 4 1 2 Do you have a JRE Most computers come with pre installed copy of the JRE If your computer doesn t have a JRE then the above commands would not work You can always check what version of the JRE is in stalled on the computer by writing the following command in Command Prompt on Windows and Terminal on Linux or Mac OS java version 4 1 3 Java Virtual Machine JVM Quite possibly the most important part of the JRE is the Java Virtual Machine JVM The JVM acts like a virtual processor enabling Java applications to be run on the local system It s main purpose is to interpret read translate the received byte code and make it appear as native code The olden Java architecture used this process of interpretation to execute Java byte code Even though the process of interpretation brought the WORA principle to diverse machines it had a drawback it consumed a lot of time and clocked the system processor intensively to load an application Figure 2 AJVM interpreter translates the byte code line by line to make it appear as if a native application is being executed Java Runtime Environment xx Local JVM Interpreter Machine Java Byte code Figure 5 Just in Time Compilation The new versions of the JRE since ve
100. a q q p dow e Se ete VS Sw ES BA As hd 139 27 OVERLOADING METHODS AND CONSTRUCTORS 141 28 ARRAYS 143 28 1 INTRO TO ARRAYS sei ta dues ce wh FO ies Siege Sow ee Ae Sep WE a Geese ete 143 28 2 ARRAY FUNDAMENTALS 0 s ee ee ee 143 28 3 TWO DIMENSIONAL ARRAYS et 144 28 4 MULTIDIMENSIONAL ARRAY ee 145 29 COLLECTION CLASSES 147 29 1 INTRODUCTION TO COLLECTIONS eee ee ee ee eee 147 29 2 GENERIGS L eek dn 2k A A a a BS SE ee es 148 293 GOELECTION OR MAP os 5 ecco s na epee ea AO an ai 149 29 4 SET OR LIST OR QUEUB ii ek W a Ra ee ew ees 151 295 IMAP GEASSES o L el amp a wae ee ete BL aod OR eh Sus a UW GR G ses 158 29 6 THREAD SAFE COLLECTIONS 2 000 eee ee ee eee 159 29 7 CLASSES DIAGRAM UML ee eee 159 29 8 EXTERNAL LINKS 3 33 ne Ga en a a a SO OS EE ee ee OER 161 30 THROWING AND CATCHING EXCEPTIONS 163 30 1 EXCEPTION ARGUMENTS eee e 164 30 2 CATCHING AN EXCEPTION 165 30 3 EXCEPTION HANDLERS 1 ce e 165 30 4 EXCEPTION CLASSES INTHEJCL 166 305 CATCH CLAUSES 4 A A ee 168 30 6 EXAMPLE OF HANDLING EXCEPTIONS 169 30 7 APPLICATION EXCEPTIONS 1 v s kuy k Sua ee gue w A Y Sa q s S A 170 30 8 RUNTIME EXCEPTIONS S us U es 171 30 9 KEYWORD REFERENCES ee 172 30 10 MINIMIZE THE USE OF THE KEYWORD NULL IN ASSIGNMENT STATEMENTS
101. ame exception or itis any of its subclasses 87 Methods NOTE A common mistake to think that if we can override methods we could also override member variables This is not the case as member variables are not overriden 1 MyClass obj new SubOfMyClass 2 3 MyClass obj new MyClass 4 5 String var obj myMemberVar The myMemberVar is defined in the MyClass object In the above example it does not count what object the obj reference points to because it was declared MyClass type on both line 1 and line 3 the variable in the MyClass object will be referenced In real examples we rarely use public variables but if you do keep in mind that Java does not support variable overriding 15 4 Parameter Passing We can pass in all the primitive data types or any object references to a method An object cannot be passed to a method only its references All parameters those are primitive types and object references are passed by value In other words if you change the passed in parameter values inside the method that will have no effect on the original variable that was passed in When you pass in an object reference to a method and then you change that inside the method that will have no effect on the original object reference However if you modify the object itself that will stay after the method returns Think about the object reference as a pointer to an object If you change the object the
102. an be passed to the created object through the con structor Now with Generics we can also pass in Types The type place holders will be replaced with the specified type before the object is created Type parameter arguments can be set for a class When an object is created from that class the type parameter argument will be replaced with the actual Type public class Person lt T gt private Person lt T gt person Create an Employee person Person lt Employee gt emplPerson new Person lt Employee gt Create a Customer person Person lt Customer gt custPerson mew Person lt Customer gt 1 HTTP EN WIKIPEDIA ORG WIKI GENERICS20PROGRAMMING 117 Generics for a method Just like class declarations method declarations can be generic that is parameterized by one or more type parameters public static lt T gt void assign Person lt T gt person T obj person setPerson obj use of generics is optional For backwards compatibility with pre Generics code it is okay to use generic classes without the generics type specification thing lt T gt In such a case when you retrieve an object reference from a generic object you will have to manually typecast it from type Object to the correct type The compiler should also warn about unsafe operations 21 2 Introduction Java is a strongly typed language That s one of the reasons why it is s
103. and values are being assigned to the data members In general there are four constructor types default non default copy and cloning A default constructor will build the most basic instance Generally this means assigning all the members values like null zero or an empty string Nothing would stop you however from your default sports car color from being red but this is generally bad programming style Another programmer would be confused if your basic car came out red instead of say colorless A non default constructor is designed to create an object instance with prescribed values for most if not all of the object s members The car is red goes from 0 60 in 12 seconds tops out at 190mph etc A copy constructor is not included in the Java language however one can easily create a con structor that do the same as a copy constructor It s important to understand what it is As the name implies a copy constructor creates a new instance to be a duplicate of an already existing one In Java this can be also accomplished by creating the instance with the default constructor and then using the assignment operator to equivocate them This is not possible in all languages though so just keep the terminology under your belt Java has the concepts of cloning object and the end results are similar to copy constructor Cloning an object is faster than creation with the new keyword because all the object memory is copied at once to de
104. architecture be able to build tools that integrate seamlessly with the environ ment and other tools For more information see ECLIPSE 9 HTTP EN WIKIBOOKS ORG WIKI BORLAND 10 HTTP WWW ECLIPSE ORG 38 NetBeans e PLUGINCENTRAL 6 10 NetBeans The NetBeans IDE is a free open source Integrated Development Environment for software de velopers The IDE runs on many platforms including Windows Linux Solaris and the MacOS It is easy to install and use straight out of the box The NetBeans IDE provides developers with all the tools they need to create professional cross platform desktop enterprise web and mobile applications More info can be found at http www netbeans org products ide 6 11 BlueJ BlueJ is an IDE that includes templates and will compile and run the applications for you BlueJ is often used by classes because it is not necessary to set classpaths BlueJ has it s own sets of Library s and you can add your own under preferences That sets the classpath for all compilations that come out of it to include those you have added and the Blue libraries BlueJ offers an interesting GUI for the creation of packages and programs Classes are repre sented as boxes with arrows running between them to represent inheritance implementation or if one is constructed in another BlueJ adds all those classes the project into the classpath at compile time BLUEJ Homesite 6 12 Kawa Kawa was develo
105. articipate Content is constantly being updated and enhanced in this book as is the nature of wiki based content This book is therefore in a constant state of evolution Any Wikibooks users can partic ipate in helping this book to a better standard as both a reader or an author contributor 2 2 1 As areader If you are interested in reading the content present in this book we encourage you to e share comments about the technical accuracy content or organisation of this book by telling the authors in the Discussion section for each page You can find the link Discussion on each page in this book leading you to appropriate sections for discussion leave a signature when providing feedback writing comments or giving suggestion on the Discussion pages This can be achieved by appending to your messages Do not add your signatures to the Book pages they are only meant for the Discussion pages review pages at the bottom of every page of this book you are likely to find controls that help you review and give feedback for pages Content on each page should be rated according to the four Wikibooks quality metrics Reliability Completeness Neutrality and Presentation share news about the Java Programming Wikibook with your family and friends and let them know about this comprehensive Java guide online become a contributing author if you think that you have information that could fill in some missing gaps in this book
106. as well So for all the purposes of this book you would only require the JDK In this section we would explore in further detail what these two software components of the Java platform do 4 1 Java Runtime Environment JRE Any piece of code written in the Java programming language can be run on any operating system platform or architecture in fact it can be run on any device that supports the Java platform Before Java this amount of ubiquity was very hard to achieve If a software was written for a Unix based system it was impossible to run the same application on a Windows system in this case the application was native only to Unix based systems A major milestone in the development of the Java programming language was to develop a special runtime environment that would execute any Java application independent of the com puter s operating system platform or architecture The Java Runtime Environment JRE sits on top of the machine s operating system platform and architecture If and when a Java application is run the JRE acts as a liaison between the underlying platform and that application It interprets the Java application to run in accordance with the underlying platform such that upon running the application it looks and behaves like a native application The part of the JRE that accomplishes this complex liaison agreement is called the Java Virtual Machine JVM We will discuss the JVM in detail later Figure
107. at respect Software programs and applications written in Java however cover vast area of the computing ecosphere Here are just some examples of the ubiquitous nature of Java applications in real life e OPENOFFICE ORG a desktop office management suite that rivals the Microsoft Office suite has been written in Java The popular building game MINECRAFT is also written in Java Online browser based games like RUNESCAPE a 3D massively multi player online role playing game MMORPG run on graphics routines 3D rendering and networking capabilities pow ered by the Java programming language Two of the world s renowned digital video recorders TIVO and BSkyB s Sky use built in live television recording software to record rewind and play your favourite television shows These applications make extensive use of the Java programming language The above mentioned application illustrates the reach and ubiquity of Java applications Here s another fact almost 80 of mobile phone vendors adopt Java as their primary platform for the development of applications The second most widely used mobile based operating system AN DROID uses Java as one of its key application platforms developers are encouraged to develop application for Android in the Java programming language 1 2 1 What can Java not do Well to be honest there is nothing that Java can not do You can program just about anything in Java This book would h
108. atic inner class An inner class can be declared static A static inner class has no enclosing instance and therefore cannot access instance variables and methods of the outer class You do not specify an instance when creating a static inner class This is equivalent to the inner classes in C 13 2 Nest a class inside a method These inner classes also called local classes cannot have access modifiers like local variables since the class is private to the method The inner class can be only abstract or final public class OuterClass public void method class InnerClass In addition to instance variables of the enclosing class local classes can also access local vari ables of the enclosing method but only ones that are declared final This is because the local class instance might outlive the invocation of the method and so needs its own copy of the vari able To avoid problems with having two different copies of a mutable variable with the same name in the same scope it is required to be final so it cannot be changed 13 3 Anonymous Classes In Java a class definition and its instantiation can be combined into a single step By doing that the class does not require a name Those classes are called anonymous classes An anonymous class can be defined and instantiated in contexts where a reference can be used and it is a nested class to an existing class Anonymous class is a special case of the
109. ava byte code the NET platform compiles the code into CIL thus making J different from the Java programming language Furthermore because J implements the NET Base Class Libraries BCL instead of the Java Class Libraries J is nothing more than a non standard extension of the Java program ming language Due to the lack of interest from developers Microsoft had to withdraw their support for J and focused on a similar programming language C To compare Java with C read COMPARISON OF C AND JAVA 8 HTTP EN W PEDIA ORG W WRITES2Z0ONCES2CS20RUNS20ANYWHERE 9 HTTP EN W PEDIA ORG W NETS20FRAMEWORK 10 HTTP EN W PEDIA ORG W MONOS20 28SOFTWARE S29 11 HTTP EN W PEDIA ORG W COMMONS20LANGUAGES20RUNTIME 12 HTTP EN W PEDIA ORG W COMMONS20INTERMEDIATES20LANGUAGE 13 HTTP EN Ww PEDIA ORG W IS20SHARP 14 HTTP EN Ww PEDIA ORG W IS20PLUSS20PLUS 15 HTTP EN Ww PEDIA ORG W COMPARISONS200F 20C520SHARPS20AND 20JAVA 29 The Java Platform 4 3 2 Third party compilers targeting the JVM The word Java by itself usually refers to the Java programming language which was designed for use with the Java platform Programming languages are typically outside of the scope of the phrase platform However Oracle does not encourage the use of any other languages with the platform and lists the Java programming language as a core part of the Java 2 platform The language an
110. ava had to reivent itself developers into the development for Had gained acceptance in the Internet market one the main reasons for the success of the completely not just the brand but the the focus on both the brand and the Java technology Internal workings of Its compiler and the platform JVM to render it faster than alternatives Figure 3 Development of Java over the years From version 1 0 to version 1 7 Java has displayed a steady growth 3 5 1 Initial Release versions 1 0 and 1 1 Introduced in 1996 for the SoLARIS22 WINDOws Mac OS Classic and Linux Java was initially released as the Java Development Kit 1 0 JDK 1 0 This included the Java runtime the virtual machine and the class libraries and the development tools e g the Javac compiler Later Sun also provided a runtime only package called the Java Runtime Environment JRE The first name stuck however so usually people refer to a particular version of Java by its JDK version e g JDK 1 0 3 5 2 Java 2 version 1 2 Introduced in 1998 as a quick fix to the former versions version 1 2 was the start of a new be ginning for Java The JDKs of version 1 2 and later versions are often called Java 2 as well For example the official name of JDK 1 4 is The Java TM 2 Platform Standard Edition version 1 4 Major changes include e Rewrite the event handling Add Event Listeners e Change Thread synchronizations e Introduction of the JIT Just in time
111. ava util Hashtable Map map new Hashtable map put key value All collections need to have the same basic operations Those are e Adding element s to the collection e Removing element s from the collection e Obtaining the number of elements in the collection e Listing the contents of the collection Iterating through the collection Before selecting a particular collection implementation ask the following question e Can my collection contain the same elements i e are duplicates allowed e Can my collection contain the null element e Should the collection maintain the order of the elements Is the order important in any way e How do you want to access an element By index key or just with an iterator Does the collection need to be synchronized From a performance perspective which one needs to be faster updates or reads From a usage perspective which operation will be more frequent updates or reads Once you know your needs you can select an exsisting implementation But first decide if you need a Collection or a Map 29 2 Generics Since JDK version 1 5 an enhancement to the type system of the Java language has been added It is called Generics Most often Generics will be used with the collection classes parameterized type lt E gt All collection implementations since 1 5 now have one parameterized type lt E gt added The E refers to an Element type When a c
112. ave a fixed element type which can be any Java type and an integral size This program uses only one array String args This indicates that args has an array type and that the element type is String The Java VM con structs and initializes the array that is passed to the main method See ARRAYS for more details on how to create arrays and access their size The elements of arrays are accessed with integer indices The first element of an array is always element 0 This program accesses the first four elements of the args array explicitly with the indices 0 1 2 and 3 This program does not perform any input validation such as verifying that the user passed at least four arguments to the program We will fix that later 26 Chapter 11 on page 71 27 Chapter 16 on page 95 28 Chapter 28 on page 143 29 Chapter 28 on page 143 59 Understanding a Java Program void vOID is not a type in Java it represents the absence of a type Methods which do not return values are declared as void methods This class defines two void methods public static void main String args public void printDistance 8 3 Comments in Java programs See HERE for more information on that important topic 30 HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMINGS2FKEYWORDS S2FVOID 31 HTTP EN WIKIBOOKS ORG WIKI JAVA_PROGRAMMINGS2FUNDERSTANDING_A_JAVA_PROGRAMS 2FJAVADOC_AND_OTHER_COMMENTS 60 9 Syntax Java der
113. bStart application got a much needed makeover Security Java SE 6 has simplified the job of its security administrators by providing various new ways to access platform native security services such as native Public Key Infrastructure PKI and cryptographic services on Microsoft Windows for secure authentication and com munication Java Generic Security Services Java GSS and Kerberos services for authentica tion and access to LDAP servers for authenticating users The lities Quality Compatibility Stability Bug fixes 3 5 7 Dolphin version 1 7 0 Java SE 7 Anticipated for 2010 3 6 Citations 20 4 The Java Platform As one of the necessary prerequisites for using this book to program in the Java programming language we asked you download and install the Java platform The Java platform is the name given to the computing platform from Oracle that helps users to run and deuelop Java applica tions The platform does not just enable a user to run and develop Java application but also features a wide variety of tools that can help developers work efficiently with the Java program ming language The platform consists of two essential software the Java Runtime Environment JRE which is needed to run Java applications and applets and the Java Development Kit JDK which is needed to develop those Java applications and ap plets If you have installed the JDK you should know that it comes equipped with a JRE
114. be public private protected or package default public class OuterClass private String outerInstanceVar public class InnerClass public void printVars System out println Print Outer Class Instance Var outerInstanceVar The inner class has access to the enclosing class instance s variables and methods even private ones as seen above This makes it very different from the nested class in C which are equiva lent to the static inner classes see below An inner object has a reference to the outer object The nested object can only be created with a reference to the outer object See below public void test Inner OuterClass outer new OuterClass OuterClass InnerClass inner outer mew InnerClass When in a non static method of the outer class you can directly use new InnerClass since the class instance is implied to be this 79 Nested Classes You can directly access the reference to the outer object from within an inner class with the syntax OuterClass this although this is usually unnecessary because you already have access to its fields and methods Inner classes compile to separate class bytecode files usually with the name of the enclosing class followed by a followed by the name of the inner class So for example the above inner class would typically be compiled to a file named OuterClass InnerClass class 13 1 1 St
115. boolean to byte byte byte byte byte byte byte N A to char char char char char char N A to short short short short short N A short to int int int int N A tolong long long N A to float float N A to dou N A ble to N A N A N A N A N A N A N A boolean 17 7 Autoboxing unboxing Autoboxing unboxing Autoboxing and unboxing language features since Java 1 5 make the programmers life much easier when it comes to working with the primitive wrapper types Consider this code frag ment int age 23 Integer ageObject new Integer age Primitive wrapper objects were Javas way of allowing one to treat primitive data types as though they were objects Consequently one was expected to wrap one s primitive data type with the corresponding primitive wrapper object as shown above Autoboxing Since Java 1 5 one may write as below and the compiler will automatically create the wrap object The extra step of wrapping the primitive is no longer required It has been automati cally boxed up on your behalf 101 Types Points to ponder Keep in mind that the compiler still creates the missing wrapper code so one doesn t really gain anything performance wise Consider this feature a programmer convenience not a per formance booster int age 23 Integer ageObject age Unboxing Uses the same process in reverse Study the following code for a moment The if statement require
116. cept numbers with decimal points numbers with decimal points are called floating points 10 5 Whole numbers and floating point numbers The data types that one can use for whole numbers are byte short int and long but when it comes to floating point numbers we use float or double Now that we know that we can modify the code in Listing 1 2 as Listing 1 3 The correct way to assign a type to floating point variables double age 10 5 Why not float you say Well there are several reasons why not 10 5 can be anything a float or a double but by a certain rule it is given a certain type This can be explained further by looking at the table below Data type Values accepted Declaration byte Any number between 128 byte b 123 and 127 short Any number between short s 12345 32 768 and 32 767 68 Assignment statements Data type Values accepted Declaration int Any number between int i 1234567 2 147 483 648 and 2 147 483 647 long Any number between long l 1234567890L 9 223 372 036 854 775 808 and 9 223 372 036 854 775 807 float Extremely large numbers float f 123 4567f beyond the scope of dis cussion here double Extremely large numbers double d 1234 56789 beyond the scope of dis cussion here The only dif ference between double and float is the addition of an f as a suffix after the float value The above table only list the number data types We will look at the others a
117. ception you have two provide two exception handling catch blocks one for each exception Refer to code listing 1 6 for an example 30 5 2 The catch any clause There are times when too many blocks are just verbose especially when all exceptions are handled in a similar manner What you need here is a catch any clause where all excep tions are handled in a single catch block Because all exceptions in Java are the sub class of java lang Exception class you can have a single catch block that catches an exception of type Exception only Hence the compiler is fooled into thinking that this block can handle any exception Using this the code in Listing 1 6 can be rewritten as follows Listing 1 8 The catch any clause try Z Yagua catch Exception ex Exception handling code for ANY exception Note You can also use the java lang Throwable class here since Throwable is the parent class for the application specific Exception classes However this is discouraged in Java programming circles This is because Throwable happens to also be the parent class for the non application specific Error classes which are not meant to be handled explicitly as they are catered for by the JVM itself 30 5 3 The multi catch clause As of JDK 7 Java added another convenient feature in their exception handling routines the multi catch clause This is a combination of the previous two catch clauses and let s you handle 168 Examp
118. complex string parsing In addition the class libraries provide an abstract interface to tasks that would normally depend heavily on the hardware and operating system Tasks such as network access and file access are often heav ily dependent on the native capabilities of the platform The Java java net and java io libraries implement the required native code internally then provide a standard interface for the Java ap plications to perform those tasks Finally some underlying platforms may not support all of the features a Java application expects In these cases the class libraries can either emulate those features using whatever is available or provide a consistent way to check for the presence of a specific feature 4 3 Similar concepts The success of the Java platform and the concepts of the WRITE ONCE RUN ANYWHERE principle has led to the development of similar frameworks and platforms Most notable of these is the Microsoft s NET FRAMEWORK and its open source equivalent Mono 4 3 1 The NET framework The NET framework borrows many of the concepts and innovations of Java their alternative for the JVM is called the COMMON LANGUAGE RUNTIME CLR while their alternative for the byte code is the COMMON INTERMEDIATE LANGUAGE CIL In fact the NET platform had an implementation of a Java like language called VISUAL J 3 formerly known as J 14 J is normally not supported with the JVM because instead of compiling it in J
119. cope inside the After the the object becomes garbage Object references can be passed in to methods object references can be returned from methods 23 2 Creating object with the new keyword 99 of new objects are created using the new keyword 1 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FAPI 2FJAVA LANG CLASS 2 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FAPIS2FJAVA LANG CLASS 129 Creating Objects Create an MyObject for the first time the application started MyObject obj new MyObject When an object from the MyObject class is created for the first time The JVM searches the file system for the definition of the class that is the Java byte code The file has the extention of class The CLASSPATH environment variable contains locations where Java classes are stored The JVM is looking for the MyObject class file Depending on which package the class belongs to the package name will be translated to a directory path When the MyObject class file is found the JVM s class loader loads the class in memory and creates a Class object The JVM stores the code in memory allocates memory for the static vari ables and executes any static initialize block Memory is not allocated for the object member variables at this point memory will be allocated for them when an instance of the class an ob ject is created There is no limit on how many objects from the same class ca
120. ctionary can be thought of as an array where the index need not be an integer Use the Map interface if you need to keep related objects together in a Map where you can e Access an element by a key object Map one object to other java util Map lt K V gt maps keys to values A map cannot contain duplicate keys each key can map to at most one value The Map interface provides three collection views which allow a map s contents to be viewed as a set of keys collection of values or set of key value mappings The key is usually a non mutable object The value object however can be a mutable object java util SortedMap lt K V gt same as the Map interface plus the keys in the Map are sorted 29 4 Set or List or Queue O Collection Set List Queue O o o o SortedSet O BlockingQueue J Figure 18 Figure 2 151 Collection Classes There is no direct implementation for the java util Collection interface The Collection in terface has five sub interfaces Those are java util Set lt E gt contains unique elements so duplicates not allowed It is similar to a mathematical Set java util List lt E gt elements are put in the list in a certain order and can be accessed by an index Duplicates are allowed the same element can be added to a list java util SortedSet lt E gt same as the Set interface plus the elements in the SortedSet are sorted java util Queue lt E gt
121. d Whenever it looks as if a String object was modified a new String was actually created and the old one was thrown away The Java language provides special support for the string concatenation operator and for conversion of other objects to strings For example Sh ne ste Eues Pp a u SCcCond partu Is the same as String str First part second part Integers will also be converted to String after the operator String str Age 25 1 Chapter 17 on page 97 2 Chapter 11 on page 71 103 java lang String Each Java object has the String toString inherited from the Object class This method pro vides a way to convert objects into Strings Most classes override the default behavior to provide more specific and more useful data in the returned St ring The String class provides a nice set of methods for string manipulation Since String objects are immutable all methods return a new St ring object For example name name trim The trim method returns a copy of the string with leading and trailing whitespace removed Note that the following would do nothing useful name trim wrong This would create a new trimmed string and then throw it away Study the String class and its methods carefully Strings are ubiquitous in Java it will serve you well to know how to manipulate them skillfully 18 2 Using StringBuffer StringBuilder to concatenate strings Remember that String objects are
122. d runtime are therefore commonly considered a single unit There are cases where you might want to program using a different language say PYTHON and yet be able to generate Java byte code instead of the Python compiled code to be run with the JVM Many third party programming language vendors provide compilers that can compile code written in their language to Java byte code For instance Python developers can use JYTHON compilers to compile Python code to the Java byte code format as illustrated below Figure 5 Third party JVM targeted compilation for non Java source compilation to Java byte code Illustrated example shows Python source being compiled to both Python compiled code and Java byte code SomeApplication py SomeApplication pyc Python Compiler ait PY Python Python Compiled Source Code SomeApplication class y Jython Compiler Java Byte code Figure 8 Of late JVM targeted third party programming and scripting languages have seen tremendous growth Some of these languages are also used to extend the functionalities of the Java language itself A few examples include the following e Groovy e PIZZA19 GJ Generic Java later officially incorporated into Java SE 5 16 HTTP EN WIKIPEDIA ORG WIKI PYTHON 20 28PROGRAMMING 20LANGUAGES29 17 HTTP EN WIKIPEDIA ORG WIKI JYTHON 18 HTTP EN WIKIPEDIA ORG WIKI GROOVY 20 28PROGRAMMINGS20LANGUAGES29 19 HuTTP EN WIKIPEDIA ORG W
123. default ualue for that datatype This is false for booleans 0 for all numeric primitive types and null for all ref erence types So for example the previous note created an array of DataType references all of which are initialized to null To access an item use the syntax variable i where is the index To set an item use the syntax variable i data To find the length of an array use the syntax variable length 19 3 Two Dimensional Arrays A two dimensional array is represented by an array of arrays Because an array is also an object like any other object having the Object as the super class it can be used to create an array where the element of the array is also an array In this way an array with any number of dimensions can be created Here are examples of two dimensional arrays with initializer blocks String twoDimArray a b c d e CEN gt wh nin Wa by ke 1 m n o int twoDimIntArray 0 1 2 3 4 10 11 12 13 14 20 21 22 23 24 Note that the above twoDimArray is equivalent to the following more verbose code String twoDimArray new String 3 for int i 0 i lt twoDimArray length i twoDimArray i new String 5 for int j 0 j lt twoDimArray i length j twoDimArray 1 3 i j In the above example we defined an array which has three elements each element contains an array having 5 elements We could creat
124. duction it has some room for improvement and later in the module we will show some of these improvements But let s not get too far ahead of ourselves 8 1 The Distance Class Intent Source and Use This class is named Distance so using your favorite editor or Java IDE first create a file named Distance java then copy the source below and paste it into the file and save the file public class Distance private java awt Point point0 pointl public Distance int x0 int y0 int xl int yl point0 new java awt Point x0 y0 pointl new java awt Point xl yl public void printDistance System out println Distance between point0 and pointl is point0 distance point1 public static void main String args Distance dist new Distance intValue args 0 intValue args 1 intValue args 2 intValue args 3 dist printDistance private static int intValue String data return Integer parselnt data At this point you may wish to review the source to see how much you might be able to un derstand While perhaps not being the most literate of programming languages someone with understanding of other procedural languages such as C or other OO languages such as C or C will be able to understand most if not all of the sample program 51 Understanding a Java Program Once you save the file COMPILE the program javac Distance java
125. e 18 5 Creating substrings It may also be sometimes useful to create substrings or strings using the order of letters from an existing string This can be done in two methods The first method involves creating a substring out of the characters of a string from a given index to the end For example SLI ei coffee String substr str substring 3 In this example substr would return fee As previously discussed the index of the first char acter in a string is 0 By counting from there it is apparent that the character in index 3 is the second f in coffee This is known as the beginIndex All characters from the beginIndex until the end of the string will be copied into the new substring The second method involves a user defined beginIndex and endIndex For example String str supporting SELIG Subst str substrings 7 5 The string returned by substr would be port Please note that the endIndex is not inclusive This means that the last character will be of the index endIndex 1 Therefore in this example every character from index 3 to index 6 inclusive was copied into the substring Note Substring is considered to be one word This is why the method name does not seem to follow the common syntax of Java It is easy to mistake the method substr for subStr which does not exist and would return with a syntax error on compilation Just remember that this style only applies to methods or other elemen
126. e NullPointerException will be caught here catch Exception e while other exception are caught here 30 8 Runtime Exceptions The java lang RuntimeExcept ion exception class is inherited from Java lang Exception It is a special exception class because catching this exception class or its subclasses are not en forced by the Java compiler runtime exception Runtime exceptions are usually caused by data errors like arithmetic overflow divide by zero Runtime exceptions are not business related exceptions In a well debugged code runtime exceptions should not occur Runtime exceptions should only be used in the case that the ex ception could be thrown by and only by something hard coded into the program These should not be able to be triggered by the software s user s 30 8 1 NullPointerException NullPointerException is a RuntimeException In Java a special null can be assigned to an ob ject reference NullPointerException is thrown when an application attempts to use an object reference having the null value These include Calling an instance method on the object referred by a null reference e Accessing or modifying an instance field of the object referred by a null reference 171 Throwing and Catching Exceptions Ifthe reference type is an array type taking the length of a null reference Ifthe reference type is an array type accessing or modifying the slots of a null reference
127. e another in a progressive manner a guidebook for efficient programming with the Java programming language and a comprehensive manual resource for the advanced programmer This book is intended to be used in conjunction with various other online resources such as e the JAVA PLATFORM API DOCUMENTATION the OFFICIAL JAVA WEBSITE and e active Java communities online such as JAVA NET and JAVARANCH etc 2 1 Who should read this book Every thing you would need to know to write computer programs would be explained in this book By the time you finish reading you will find yourself proficient enough to tackle just about anything in Java and programs written using it This book serves as the first few stepping stones of many you would need to cross the unfriendly waters of computer programming We have put a lot of emphasis in structuring this book in a way that lets you start programming from scratch with Java as your preferred language of choice This book is designed for you if any one of the following is true You are relatively new to programming and have heard how easy it is to learn Java You had some BASIC or PASCAL in school and have a grasp of basic programming and logic You already know and have been introduced to programming in earlier versions of Java TP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMINGS2FAUTHORS S20 26 CONTRIBUTORS DOWNLOAD ORACLE COM JAVASE 7 DOCS API OVERVIEW SUMMARY HTML
128. e got a significant amount of code it s usually better to organize it in a way that makes sense It would look more complex and incomprehensible written as it is in Listing 1 6 Now that we have looked into the anatomy of a simple assignment statement we can look back at what we ve achieved We know that e A statement is a unit of code in programming e If we are assigning a variable a value the statement is called an assignment statement e An assignment statement include three parts a data type variable name also called an iden tifier and the value ofa variable We will look more into the nature of identifiers and values in the section titled IDENTIFIERS LITERALS AND EXPRESSIONS later Now before we more on to the next topic you need to try and understand what the code below does Listing 1 7 Multiple assignment statements with expressions int firstNumber 10 int secondNumber 20 int result firstNumber secondNumber The first two statements are pretty much similar to those in Listing 1 5 but with different variable names The third however is a bit interesting We ve already talked of variables as being similar to gift boxes Think of your computer s memory as a shelf where you put all those boxes Whenever you need a box or variable you call its identifier that s the name of the variable So calling the variable identifier firstNumber gives you the number 10 calling secondNumber would give you 20 hence wh
129. e the array having the 5 elements first and use that one in the initialize block String oneDimArray 00 01 02 03 04 3 String twoDimArray oneDimArray proT it 12 EEES an nano 20 2107 AZ 23 1241 Fi Since they are arrays of array references these multi dimensional arrays can be jagged i e subarrays can have different lengths or the subarray reference can even be null Consider String weirdTwoDimArray 10 11 12 null 20 219 22 23 2A be Note that the length of a two dimensional array is the number of one dimensional arrays it contains In the above example weirdTwoDimArray length is 3 whereas weirdTwoDimAr ray 2 length is 5 110 Multidimensional Array 19 4 Multidimensional Array Going further any number of dimensional array can be defined or lt elementType gt lt elementType gt lt arrayName gt lt arrayName gt DE JAVA_STANDARD FR PROGRAMMATION JAVA TABLEAUX NL PROGRAMMEREN IN JAVA ARRAYs PT JAVA VETORES DATENSTRUKTUREN ES PROGRAMACI N EN HT TIP HTTP HTTP aor WN HTTP SLE Ses FR NL HTTP DE PT WIKIBOOKS ORG WIKI PROGRAMACISF3N 20ENS20JAVAS2FARRAYS WIKIBOOKS WIKIBOOKS ORG WIKI PROGRAMMEREN 20IN 520JAVAS2FARRAYS WIKIBOOKS WIKIBOOKS ORG WIKI JAVA_STANDARD 3A 20DATENSTRUKTUREN ORG WIKI PROGRAMMATIONS20JAVAS2FTABLEAUX ORG WIKI JAVAS2FV
130. ed to the user became very important and usually needs the help of a graphic artist To help programmers and graphic artists to work together the separation between data code and how it is presented became crucial The view User Interface Logic contains the logic that is necessary to construct the presen tation This could be handled by JSP technology The servlet acts as the controller and contains the logic that is necessary to process user events and to select an appropriate response The business logic model actually accomplishes the goal of the interaction This might be a query or an update to a database This could be handled by EJB technology For more information about MVC please see MVC 7 3 Jini After J2EE Sun had a vision about the next step of network computing That is Jin1 The main idea is that in a network environment there would be many independent services and con 32 33 HTTP EN WIKIPEDIA ORG WIKI MODEL VIEW CONTROLLER HTTP EN WIKIPEDIA ORG WIKI JINI 49 Execution sumers Jini would allow these services consumers to interact dynamically with each other in a robust way The basic features of Jini are No user intervention is needed when services are brought on or offline In contrast to EJBs where the client program has to know the server and port number where the EJB is deployed in Jini the client is supposed to find to discover the service in the network Self hea
131. ed EARLIER which the JVM then executes at runtime Java source code is compiled into bytecode when we use the javac compiler The bytecode gets saved on the disk with the file extension class When the program is to be run the bytecode is converted using the JUST IN TIME JIT COMPILER The result is machine code which is then fed to the memory and is executed So Java has four step compilation compiler will check the syntactical error create byte code create a blank class file merge that byte code to that blank class file The Java classes Byte Codes are compiled to machine code and loaded into memory by the JVM when needed the first time This is different than other languages like C C where the whole program had to be compiled to machine code and linked to create an executable file before the program could start JIT compilers compile byte code once and the compiled machine code are re used again and again to speed up execution Early Java compilers compiled the byte code to machine code each time it was used but more modern compilers cache this machine code for reuse on the machine Even then java s JIT compiling was still faster than an interpreter language where code is compiled from high level language instead of from byte code each time it was used 1 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FJAVAS20PROGRAMMINGS 20ENVIRONMENT 2 HTTP EN WIKIPEDIA ORG WIKI BYTECODE 3 HTTP EN WIKIBOOKS OR
132. ed the obj reference to a new object that is no effect whatsoever on the passed in object 15 5 Functions In java functions methods really are just like in C except that they must be declared inside a class and objects are passed by reference automatically You cannot create pointers to a function but Java has events which really are function pointers under the hood for when you need that type of functionality int a_function double d return int d 15 6 Return Parameter So as we can see a method may or may not return a value If the method does not return a value we use the void java keyword Same as the parameter passing the method can return a primitive type or an object reference So a method can return only one value What if you want to return more than one value from a method You can always pass in an object reference to the method and let the method modify the object properties The modified values can be considered as an output value from the method However better option and cleaner if you create an Object array inside the method assign the return values and return the array to the caller You could have a problem however if you want to mix primitive data types and object references as the output values from the method There is a better approach Defines special return object with the needed return values Create that object inside the method assign the values and return the reference to this object
133. en the first class object is instantiated or created This means that the method executes in a static or non object con text there is no implicit separate instance available when the static methods run from various objects and the special variable this is not available As such static methods cannot access instance methods or instance fields such as printDistance or point0 directly The main method can only invoke the instance method printDistance method via an instance refer ence such as dist 8 2 3 Data Types Most declarations have a data type Java has several categories of data types reference types primitive types array types and a special type void Reference Types A reference type is a Java data type which is defined by a Java class or interface Reference types derive this name because such values refer to an object or contain a reference to an object The idea is similar to pointers in other languages like C Java represents sequences of character data or STRING2 with the reference type java lang String which is most commonly referred to as String String literals such as Distance between are constants whose type is String 23 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FKEYWORDSS2FPRIVATE 24 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FSTATICS 20METHODS 25 Chapter 18 on page 103 58 Detailed Program Structure and Overview This program uses three separate reference types 1 java
134. en you add the two up the answer should be 30 That s what the value of the last variable result would be The part of the third statement where you add the numbers i e firstNumber secondNumber is called an expression and the expression is what decides what the value is to be Ifit s just a plain value nothing fancy then it s called a literal With the information you have just attained you can actually write a decent Java program that can sum up values To learn more continue reading 1 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FIDENTIFIERSS2C 20LITERALS 20ANDS20EXPRESSIONS 70 11 Classes Objects and Types 11 1 Objects and Classes An object is composed of members and methods The members also called data members characteristics attributes or properties describe the object The methods generally describe the actions associated with a particular object Think of an object as a noun its members as adjectives describing that noun and its methods as the verbs that can be performed by or on that noun For example a sports car is an object Some of its members might be its height weight acceler ation and speed An object s members just hold data about that object Some of the methods of the sports car could be drive park race etc The methods really don t mean much unless associated with the sports car and the same goes for the members The blueprint that lets us build our sports car object is cal
135. erased from memory and prohibits the user from manually destroying objects 26 1 finalize When an object is garbage collected the programmer may want to manually perform cleanup such as closing any open input output streams To accomplish this the finalize method is used Note that finalize should never be manually called except to call a super class finalize method from a derived class finalize method Also we can not rely on when the finalize method will be called If the java application exits before the object is garbage collected the finalize method may never be called protected void finalize throws Throwable try doCleanup Perform some cleanup If it fails for some reason it is ignored finally super finalize Call finalize on the parent object The garbage collector thread runs in a lower priority than the other threads If the application creates objects faster than the garbage collector can claim back memory the program can run out of memory The finalize method is required only if there are resources beyond the direct control of the Java Virtual Machine that need to be cleaned up In particular there is no need to explicitly close an OutputStream since the OutputStream will close itself when it gets finalized Instead the finalize method is used to release either native or remote resources controlled by the class CATEGORY JAVA PROGRAMMING 1 HTTP EN WIK
136. et was that the language they were aim ing for had to be developed for an EMBEDDED SYSTEM with limited resources An embedded system is a computer system scaled to a minimalistic interface demanding only a few functions from its design For such a system C or any successor would seem too large as all the lan guages at the time demanded a larger footprint than what was desired And other than this the language lacked some other important features as well The team thus had to think in a different way to go about solving all these problems Co founder of Sun Microsystems Bill Joy envisioned a language combining the power of Mesa and C in a paper he wrote for the engineers at Sun named Further Gathering ideas Gosling began work on enhancing C and named it C a pun on the evolutionary structure of the language s name The and meant putting in and taking out stuff He soon abandoned the name and called it Oak after the tree that stood outside his office 7 HTTP EN WIKIPEDIA ORG WIKI 1990 HTTP EN WIKIPEDIA ORG WIKI NEXT JAVA TECHNOLOGY THE EARLY YEARS Sun Microsystems Retrieved 9 May 2008 11 Java TECHNOLOGY THE EARLY YEARS 12 Sun Microsystems Retrieved 9 May 2008 13 HTTP EN WIKIPEDIA ORG WIKI EMBEDDEDS20SYSTEM 14 JAVA TECHNOLOGY THE EARLY YEARS 5 Sun Microsystems Retrieved 9 May 2008 o 14 Reshaping thought Table 1 GT FP JP Who s who of the Java tech nology lt ref na
137. ethod is a double precision floating point number This method uses the syntax Distance between this point0 and this pointl is this point0 distance this point1 to construct a String to pass to the System out println This expression is a series of STRING CONCATENATION methods which concatenates Strings or the String representation of primitive types such as doubles or objects and returns a long string For example the result of this expression for the points 0 3 and 4 0 is the String Distance between java awt Point x 0 y 3 and java awt Point x 4 y 0 is 5 0 which the method then prints to System out In order to print we invoke the println This is an instance method from java io PrintStream which is the type of the static field out in the class java lang System The Java VM binds System out to the standard output stream when it starts a program The main method The main method is the main entry point which Java invokes when you start a Java program from the command line The command java Distance 0 3 4 0 instructs Java to locate the Distance class put the four command line arguments into an array of String values then pass those arguments the public static main String method of the class We will introduce arrays shortly Any Java class that you want to invoke from the command line or desktop shortcut must have a main method with this signature
138. eturn buf toString StringBuilder was introduced in Java 5 Unlike StringBuffer StringBuilder isn t thread safe so you cant use it in more than one thread see the chapter on CONCURRENCY However because it doesn t have to worry about synchronization St ringBuilders are faster 18 3 Comparing Strings Comparing strings is not as easy as it may first seem We cannot just use a simple equality state ment such as if myString Hello World Can t Use this System out println Match Found To test for equality use the equals Object method inherited by every class and defined by String to return true if and only if the object passed in is a String containing the exact same data String greeting Hello World if greeting equals Hello World true HE oo 5 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FAPI S2FJAVA LANG STRINGBUFFER 6 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FAPIS2FJAVA LANG STRINGBUILDER T HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMING S2FCONCURRENTPROGRAMMING 105 java lang String if greeting equals hello world false HE axe To order St ring objects use the compareTo method which can be accessed wherever we use a String datatype Let s take a look at an example String myString Hello World ENTES if myString compareTo Hello World 0 System out println Match Fo
139. exception This is called the try block because you try your various method calls there The try block is an ordinary scope preceded by the keyword try Listing 1 5 A basic try block try Code that might generate exceptions If you were checking for errors carefully in a programming language that didn t support excep tion handling you d have to surround every method call with setup and error testing code even if you call the same method several times With exception handling you put everything in a try block and capture all the exceptions in one place This means your code is much easier to write and read because the goal of the code is not confused with the error checking 30 3 Exception handlers Of course the thrown exception must end up some place This place is the exception handler and there s one for every exception type you want to catch Exception handlers immediately follow the try block and are denoted by the keyword catch Listing 1 6 Exception handling with catch blocks try Suppose the code here throws two exceptions NullPointerException and NumberFormatException then each is handled in a separate catch block catch NullPointerException ex Exception handling code for the NullPointerException catch NumberFormatException ex Exception handling code for the NumberFormatException etc Using the syntax in Listing 1 6 we can write the potentially problematic division code
140. figuration data or resource bundles files containing language translations of text 5 HTTP EN WIKIPEDIA ORG WIK1 Z1P 20 28FILES20FORMATS29 6 HTTP EN WIKIPEDIA ORG WIKI DEBUGGING 27 The Java Platform Such files can contain only ASCII and Latin 1 characters but international programmers need a full range of character sets Text using these characters can appear in properties files and re source bundles only if the non ASCII and non Latin 1 characters are converted into Unicode escape sequences uXXXX notation The task of writing such escape sequences is handled by native2ascii You can write the interna tional text in an editor using the appropriate character encoding then use native2ascii to gen erate the necessary ASCII text with embedded Unicode escape sequences Despite the name native2ascii can also convert from ASCII to native so it is useful for converting an existing prop erties file or resource bundle back to some other encoding native2ascii makes most sense when integrated into a build system to automate the conversion 4 2 11 Remote Method Invocation RMI tools 4 2 12 Java IDL and RMI IIOP Tools 4 2 13 Deployment amp Web Start Tools 4 2 14 Browser Plug In Tools 4 2 15 Monitoring and Management Tools Troubleshooting Tools With Java 1 5 a set of monitoring and management tools have been added to the JDK in addition to a set of troubleshooting tools The monitoring and management tools are
141. following table presents direct links to the latest JDK for the Linux operating system Operating system RPM Tarball License Linux x86 DOWNLOAD DOWNLOAD ORACLE BINARY CODE LICENSE AGREEMENT Linux x64 DOWNLOAD DOWNLOAD ORACLE BINARY CODE LICENSE AGREEMENT Start writing code The most widely available text editor on Gnome desktops is the Gedit application while on the KDE desktops one can find Kate However unlike Notepad on Windows both these editors support syntax highlighting and code completion and therefore are sufficient for our purposes However if you require a robust and standalone text editor like the Notepad editor on Win dows you would require the use of the minimalistic editor loaded with features SciTE Follow the instructions below if you wish to install SciTE For Ubuntu go to Application gt Acces sories gt Terminal Alternatively you can press to open the Run Application window At the prompt type xterm or gnome terminal to open the Terminal window At the prompt write the following sudo apt get install scite You should now be able to use the Scite editor for your programming needs OTN PUB JAVA JDK 7U1 B08 JDK 7Uu1 OTN PUB JAVA JDK 7U1 B08 JDK 7U1 HNETWORK JAVA JAVASEBUSINESS DOCUME 5 HTML OTN PUB JAVA JDK 7U1 B08 JDK 7U1 OTN PUB JAVA JDK 7U1 B08 JDK 7U1 1 2 3 25 HTTP DOWNLOAD ORACLE CO 26 HTTP DOWNLOAD ORACLE CO GZ 27 HTTP WW
142. g param2 return When the method returns nothing the return keyword at the end of the method is optional The return keyword can be used anywhere in the method when the executation flow reach the return keyword the method execution is stopped and the execution flow returns to the caller method 15 2 Method Overloading For the same class we can define two methods with the same name However the parameter types and or the number of parameters must be different for those two methods In the java terminology this is called method overloading It is useful to use method overloading when we need to do something different based on a parameter type For example we may have the operation runAroundThe We can define two methods with the same name but different input 85 Methods parameter type public void runAroundThe Building block public void runAroundThe Park park Related terminology is the method signature In java the method signature contains method name and the input parameter types The java compiler takes the signature for each method and makes sure that each method signature is unique for a class For example the following two method definitions are valid public void logit String param Error err public void logit Error err String param Because the type order is different If both input parameters were type String that would be a p
143. generally kept in the root di rectory of the java project Ant parses this file and executes the tasks therein Below we give an example build xml file Ant tool is written in Java and is open source so it can be extended if there is a task you d like to be done during the build that is not in the pre defined tasks list It is very easy to hook your ant task code to the other tasks your code only needs to be in the classpath and the Ant tool will load it at runtime For more information about writing your own Ant tasks please see the project website at http ant apache org Example build xml file xml Source lt xml version 1 0 gt lt project name ExampleApp basedir default main gt lt property name source dir value source gt lt property name libraries dir value libraries gt lt property name build dir value build gt lt property name classes dir value build dir classes gt lt property name dist dir value build dir dist gt lt property name main class value com example ExampleApp gt lt path id classpath gt lt fileset dir libraries dir includes jar gt lt path gt lt target name clean gt lt delete dir build dir gt lt target gt 16 HTTP EN WIKIBOOKS ORG WIKI PROGRAMMINGS3AAPACHES20ANT 40 Ant lt target name compile gt lt mkdir dir classes dir gt lt javac srcdir S
144. grams often contain non code files such as images and properties files These are re ferred to generally as resources and stored in directories local to the classes in which they re used For example if the class com example ExampleApp uses the icon png file this file could be stored as com example resources icon png These resources present a problem when a program is complied because javac does not copy them to wherever the class files are being complied to see above it is up to the programmer to move the resource files and directories See also the section on how to automate this using ant below 6 4 Filename Case The Java source file name must be the same as the public class name the file contains There can be only one public class defined per file The Java class name is case sensitive as is the source file name The naming convention for the class name is for it to start with a capital letter 6 5 Compiler Options 6 5 1 Debugging and Symbolic Information 6 6 Additional Tools 6 6 1 IDEs This section contains a little about the different IDEs available and their strengths and weak nesses 8 Chapter 6 13 on page 40 37 Compilation 6 7 JBuilder JBuilder is a IDE with proprietary source code sold by BORLAND One of the advantages in integration with together a modeling tool 6 8 JCreator There s info at http www apcomputerscience com ide jcreator index htm 6 9 Eclipse Eclipse
145. gt doPost request response lt a href http gt Process form request bata base te Render HTML response lt form action http gt PRE ee om doGet request response lt html gt Render HTML page 1 Figure 11 Figure 3 Servlet Execution Java Servlet code 12 HTTP EN 0W PEDIA ORG WIKI APPLICATIONS20SERVER 13 HTTP EN W PEDIA ORG WIKI APPLICATIONS20SERVER 14 HTTP EN W PEDIA ORG WIKI WEBS20BROWSER 15 HTTP EN W PEDIA ORG W WEBS20BROWSER 16 HTTP EN 0W PEDIA ORG W HYPERTEXTS20TRANSFERS20PROTOCOL 17 HTTP EN WIKIPEDIA ORG WIKI PROTOCOLS 20 28COMPUTINGS 29 18 HTTP EN WIKIPEDIA ORG WIKI WEB 20SERVER 19 HTTP EN 0W PEDIA ORG WIKI APPLICATIONS20SERVER 20 HuTTP EN W PEDIA ORG WIKI ENTERPRISES20JAVABEAN 21 HTTP EN W PEDIA ORG WIKI WEBS20APPLICATION 22 HTTP EN WIKIPEDIA ORG W SOFTWARES20COMPONENTRY 46 J2EE code execution Figure 3 Java got its popularity with server side programming more specifically with J2EE23 servlets Servlets are running in a simple J2EE framework to handle client HTTP requests They are meant to replace CGI PROGRAMMING for web pages rendering dynamic content The servlet is running in a so called SERVLET CONTAINER WEB CONTAINER The servlets re sponsibility is to e Handle the request by doing the business logic computation e Connecting to a database if needed Create HTML to present to the user through the br
146. her Error An Error indicates serious problems that a reasonable application should not try to catch Most such errors are abnor mal conditions Exception The class Exception and its subclasses are a form of Throwable that indicates conditions that a reasonable application might want to catch Also this is the class that a programmer may want to extend when adding business logic exceptions Run timeException RuntimeException is the superclass of those exceptions that can be thrown during the normal operation of the Java Virtual Machine A method is not required to declare in its throws clause any subclasses of RuntimeExcep tion that might be thrown during the execution of the method but not caught Throwable RuntimeException java lang Figure 25 Figure 1 1 The exception classes and their inheritance model in the JCL 167 Throwing and Catching Exceptions 30 5 Catch clauses There are three distinct types of catch clauses used by Java programmers We have already ex plored the first rule in the code listings above Given below are all three clauses we will explain all three in greater detail in the next sections 1 The catch list clause 2 The catch any clause 3 The multi catch clause 30 5 1 The catch list clause Using the catch list clause you provide one handle for every one type of exception So for in stance if a guarded block generates a NullPointerException and an ArithmeticEx
147. hould be used to render the presentation of the static and dynamic data Depending on the complexity of the data 100 separation is not easy to achieve Using custom 23 HTTP EN WIKIPEDIA ORG WIKI JAVA 20EE 24 HTTP EN WIKIPEDIA ORG WIKI HYPERTEXTS2ZOTRANSFERS2ZOPROTOCOL 25 HTTP EN WIKIPEDIA ORG WIKI COMMONS20GATEWAYS20 INTERFACE 26 HTTP EN WIKIPEDIA ORG WIKI WEBS20CONTAINER 27 HTTP EN WIKIPEDIA ORG WIKI JAVAS20SERVLET 47 Execution tags however may help to get closer to 100 This is advocated also in MVC architecture see below Client Computer Server Computer Browser 7 Application Server HttpServlet lt html gt 3 2 JSP doPost request response Curis lt a href http gt rea co ii n Process form request lt Embedded Java gt lt form action http gt 3 o doGet request response PS lt form gt r lt form gt lt html gt Process GET request lt html gt EJB Server 1 ApplicationServer2 EJB class extends EJBObject I Data base Figure 13 Figure 5 EJB Execution EJB code Figure 5 In the 1990s with the client server computing a trend started that is to move away from Mainfraim computing That resulted in many small separate applications in a Compa ny Enterprise Many times the same data was used in different applications A new philosophy Enterprise Computing was created to address these issues The idea was to crea
148. id main String args int numNames getInt Number of names String names new String numNames for int i 0 i lt names length i names i getString Enter name i 1 for int i 0 i lt names length i System out println names i public static int getInt String prompt System out print prompt int integer input nextInt input nextLine Get rid of this line so that getString won t read it return integer public static String getString String prompt System out print prompt return input nextLine Copy the code and compile it The program will ask you to enter some names then reprints the names in order It demonstrates three major aspects of arrays how to define an array how to set data and how to access it The code String names new String numNames tells Java to create an array of size numNames that will store Strings To set data use names x data where x is the index to access Note that all Java arrays start at 0 and go to array size 1 Thus if you dimension an array to size 10 the highest index is 9 19 2 Array Fundamentals To create an array use the syntax DataType variable new DataType ArraySize Alterna tively if you know the data beforehand you can write DataType variable item 1 item 2 1tem n 109 Arrays All elements of the array will be automatically initialized with the
149. ime e g the Re flection API therefore code that uses such functions are not necessarily fully compiled to native code Was JVM the first virtual machine Java was not the first virtual machine based platform though itis by far the most successful and well known Previous uses for virtual machine technology primarily involved EMULATORS to aid development for not yet developed hardware or operating systems but the JVM was designed to be implemented entirely in software while making it easy to efficiently port an implementation to hardware of all kinds 3 HTTP EN WIKIPEDIA ORG WIKI EMULATORS 24 Java Development Kit JDK 4 2 Java Development Kit JDK The JRE takes care of running the Java code on multiple platforms however as developers we are interested in writing pure code in Java which can then be converted into Java byte code for mass deployment As developers we do not need to write Java byte code rather we write the code in the Java programming language which is quite similar to writing C or C code Upon downloading the JDK a developer ensures that their system has the appropriate JRE and additional tools to help with the development of applications in the Java programming language Java code can be found in files with the extension java These files are called Java source files In order to convert the Java code in these source files to Java byte code you need to use the Java compiler toolinstalled with yo
150. ing is not legal publicabstractDistance distanceTo Destination dest because whitespace is required between keywords and identifiers The following is lexically valid publicabstractDistance distanceTo Destination dest but means something completely different it declares a method which has the return type publicabstractDistance It is unlikely that this type exists so the above would result in a se mantic error 28 Chapter 14 on page 83 29 Chapter 17 on page 97 65 Syntax 9 6 Indentation Java ignores all whitespace in front of a statement As this these two code snippets are identical for the compiler public static void main String args printMessage void printMessage System out println Hello World public static void main String args printMessage void printMessage System out println Hello World However the first one s style with whitespace is preferred as the readability is higher The method body is easier to distinguish from the head even at a higher reading speed CATEGORY JAVA PROGRAMMING CATEGORY JAVA PROGRAMMING 30 HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVAS20PROGRAMMING 31 HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVAS20PROGRAMMING 66 10 Statements Now that we have the Java platform on our systems and have run the first program successfully we are geared towards understanding
151. intended for monitoring and managing the virtual machine and the execution environment They allow for example monitoring memory usage during the execution of a Java program The troubleshooting tools provide rather esoteric insight into aspects of the virtual machine Interestingly the Java debugger is not categorized as a troubleshooting tool All the monitoring and management and troubleshooting tools are currently marked as experi mental which does not affect jdb So they might disappear in future JDKs 4 2 16 Java class libraries JCL In most modern operating systems a large body of reusable code is provided to simplify the pro grammer s job This code is typically provided as a set of DYNAMICALLY LOADABLE LIBRARIES that applications can call at runtime Because the Java platform is not dependent on any spe cific operating system applications cannot rely on any of the existing libraries Instead the Java platform provides a comprehensive set of standard class libraries containing much of the same reusable functions commonly found in modern operating systems 7 HTTP EN WIKIPEDIA ORG WIKI LIBRARYS20 28COMPUTERS2Z0SCIENCES29 23DYNAMIC 20LINKING 28 Similar concepts The Java class libraries serve three purposes within the Java platform Like other standard code libraries they provide the programmer with a well known set of functions to perform common tasks such as maintaining lists of items or performing
152. ion and definitions of the class s members This class contains exactly six members which we will explain in turn 1 Two field declarations named point0 and point1 2 Aconstructor declaration 3 Three method declarations Example Instance Fields The declaration private java awt Point point0 pointl declares two INSTANCE FIELDS Instance fields represent named values that are allocated when ever an instance of the class is constructed When a Java program creates a Distance instance that instance will contain space for point0 and point1 When another Distance object is cre ated it will contain space for its own point0 and point1 values The value of point 0 in the first Distance object can vary independently of the value of point 0 in the second Distance object This declaration consists of 1 The PRIVATE access modifier which means these instance fields are not visible to other classes 2 The type of the instance fields In this case the type is java awt Point This is the class Point in the java awt package 3 The names of the instance fields in a comma separated list These two fields could also have been declared with two separate but more verbose declarations private java awt Point point0 private java awt Point pointl Since the types of these fields is a reference type i e a field that refers to or can hold a reference to an object value Java will implicitly initialize the va
153. is a free IDE plus a developer tool framework that can be extended for a particular de velopment need IBM was behind this free software development and it replaced IBM Visual Age tool The idea was to create a standard look and feel that can be extended The extendibility has distinguished Eclipse from other IDE tools Eclipse also meant to compete with Microsoft Vi sual Studio tools Microsoft tools give a standard way of developing code in the Microsoft world Eclipse gives a similar standard way of developing code in the Java world with big success so far With the online error checking only coding can be speed up by at least 50 coding does not include programming The goal for Eclipse are twofold e Give a standard IDE for developing code e Give a starting point and the same look and feel for all other more sophisticated tools built on Eclipse IBM s WSAD and later IBM Rational Software Development Platform are built on Eclipse Standard Eclipse features Standard window management perspectives views browsers explorers As you type error checking immediate error indications As you type help window type or lt ctrl gt space Automatic build changed source code automatically compiled Built in debugger full featured GUI debugger Source code generation getters and setters Searches for implementation for references Code refactoring global reference update Plug in based
154. is an object like all other objects Look at the following program import java util x public class ArrayExample static Scanner input new Scanner System in public static void main String args int numNames getInt Number of names String names new String numNames for int i 0 i lt names length i names i getString Enter name i 1 for int i 0 i lt names length i System out println names i public static int getInt String prompt System out print prompt int integer input nextInt input nextLine Get rid of this line so that getString won t read it return integer public static String getString String prompt System out print prompt return input nextLine Copy the code and compile it The program will ask you to enter some names then reprints the names in order It demonstrates three major aspects of arrays how to define an array how to set data and how to access it The code String names new String numNames tells Java to create an array of size numNames that will store Strings To set data use names x data where x is the index to access Note that all Java arrays start at 0 and go to array size 1 Thus if you dimension an array to size 10 the highest index is 9 28 2 Array Fundamentals To create an array use the syntax DataType variable new DataType ArraySize Alterna tivel
155. is created from that class That is the Constructor Constructor does not have a return value and its name is the same as the class name The Constructor can be overloaded you can define more than one constructor with different parameters For example public class MyClass private String memberField MyClass Constructor there is no input parameter a public MyClass MyClass Constructor there is one input parameter a public MyClass String paraml memberField paraml In the above example we defined two constructors one with no input parameter and one with one input parameter You may ask which constructor will be called Its depends how the object is created with the new keyword See below MyClass objl mew MyClass The constructor with no input parameter will be called MyClass obj2 new MyClass Init Value The constructor with one input param will be called In the above example we created two objects from the same class or we can also say that obj1 and obj2 both have the same type The difference between the two is that in the first one the memberVar field is not initialized in the second one that is initialized to Init Value obj1 and obj2 contains the reference to the object Each class must have a constructor If we do not define one the compiler will create a default so called empty constructor automatically public class MyClas
156. ives much of its syntax from the C programming language basic assignment statement syntax expressions control flow statements and blocks etc will be very familiar to C program mers Unicode Java source code are built by Unicode characters Tokens Java programs consist of a sequence of different kinds of tokens For example there are word tokens such as class and public which are KEYWORDS KEYWORDS Those are special words with reserved meaning in Java Those words can not be used by the programers to name identifiers Identifiers Other words non keywords are identifiers Identifiers have many different uses in Java but primarily they are used as names class names method names and variable names LITERALS Java also has tokens to represent numbers such as 1 and 3 these are known as LITERALS String LITERALS such as http en wikibooks org Java_Programming consist of zero or more characters embedded in double quotes OPERATORS And operators such as and are used to express basic computation such as addition or String concatenation or assignment BLocks HTTP EN WIKIBOOKS ORG WIKI PROGRAMMINGS3AC HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMING 2FKEYWORDS HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMING 2FKEYWORDS Chapter 9 2 on page 63 Chapter 9 2 on page 63 Chapter 9 2 on page 63 HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMING 2FOPERATORS HTTP EN WIKIBOOKS ORG WIKI JAVA
157. le thread environments when you make sure that only one thread updating the collection at any given time Anew Java JDK package was introduced at Java 1 5 thatis java util concurrent This package supplies a few Collection implementations designed for use in multi threaded environments The following table list all the synchronized collection classes synchronized non synchronized java util Vector java util ArrayList List java util Stack java util LinkList java util concurrent CopyOnWriteArrayList java util TreeSet Set java util HashSet java util LinkHashSet java util concurrent CopyOnWriteArraySet java util TreeMap java util Hashtable java util HashMap Map java util concurrent ConcurrentHashMap java util LinkedHashMap java util IdentityHashMap java util EnumMap 29 7 Classes Diagram UML The following UML class diagram shows the Collection interfaces and their implementations 159 Collection Classes interface e Np zz Ea E AE E E E apak asa AbstractCollection sas O Abstractset lll u 4 interface SortedSet CopyOnWriteArraySet 1 interface O AbstractList AbstractSequantialList list ly j js interface Queue a T s E Vemma O AbstractQueue interface BlockingQueue ArrayBlockingQueue SynchronousQueue OP Since 15 gt E Figure 23
158. le of handling exceptions exceptions in a single handler while also maintaining their types So instead of being boxed into a parent Exception super class they retain their individual types Listing 1 9 The multi catch clause try Z Gee catch NullPointerException NumberFormatException ex Exception handling code specifically for the NullPointerException and the NumberFormatException 30 6 Example of handling exceptions Let s examine the following code public void methodA throws SomeException met hodbody public void methodB throws CustomException AnotherException Met hodbody public void methodC methodB methodA In the code sample methodC is invalid Because methodA and methodB pass or throw excep tions methodC must be prepared to handle them This can be handled in two ways a try catch block which will handle the exception within the method and a throws clause which would in turn throw the exception to the caller to handle The above example will cause a compilation error as Java is very strict about exception handling So the programmer forced to handle any possible error condition at some point A method can do two things with an exception Ask the calling method to handle it by the throws declaration Or handle the exception inside the method by the try catch block To construct a throws declaration add throws ExceptionName additional exceptions can be added wi
159. led a class A class doesn t tell us how fast our sports car goes or what color it is but it does tell us that our sports car will have a member representing speed and color and that they will be say a number and a word or hex color code respectively The class also lays out the methods for us telling the car how to park and drive but these methods can t take any action with just the blueprint they need an object to have an effect In Java a class is located in a file similar to its own name If you want to have a class called SportsCar its source file needs to be Sport sCar java The class is created by placing the follow ing in the source file public class SportsCar Insert your code here The class doesn t do anything yet as you will need to add methods and member variables first 11 2 Instantiation and Constructors In order to get from class to object we build our object by instantiation Instantiation sim ply means to create an instance of a class Instance and object are very similar terms and are sometimes interchangeable but remember that an instance refers to a specific object which was created from a class This instantiation is brought about by one of the class s methods called a constructor As its name implies a constructor builds the object based on the blueprint Behind the scenes this 71 Classes Objects and Types means that computer memory is being allocated for the instance
160. ling by adapting when services consumers of services come and go Services peri odically need to renew a lease to indicate that they are still available e Consumers of JINI services do not need prior knowledge of the service s implementation The implementation is downloaded dynamically and run on the consumer JVM without config uration and user intervention For example the end user may be presented with a slightly different user interface depending upon which service is being used at the time The imple mentation of the user interface code would be provided by the service being used A minimal Jini network environment consists of One or more services A lookup service keeping a list of registered services One or more consumers Jini is not widely used at the current writing 2006 There are two possible reasons for it One is Jini a bit complicated to understand and to set it up The other reason is that Microsoft pulled out from Java which caused the industry to turn to the use of proprietary solutions CATEGORY JAVA PROGRAMMING 34 HTTP EN WIKIBOOKS ORG WIKI CATEGORY 3AJAVAS20PROGRAMMING 50 8 Understanding a Java Program This article presents a small Java program which can be run from the console It computes the distance between two points on a plane You need not understand the structure and meaning of the program just yet we will get to that soon Also because the program is intended as a simple intro
161. local class to a method above and hence they also can use final local variables of the enclosing method Anonymous classes are most useful to subclass and upcast to an Adapter Class or to an interface 80 Anonymous Classes public interface ActionListener public void click ActionListener clk mew ActionListener public void click implementation of the click event return In the above example the class that implements the ActionListener is anonymous The class is defined where it is instantiated The above code is harder to read than if the class explicitly defined so why use it If many imple mentations are needed for an interface and those classes are used only in one particular place using anonymous class makes sense The following example uses anonymous class to implement an action listener import java awt import java awt event import java io Serializable class MyApp implements Serializable BigObject That ShouldNotBeSerializedWithAButton bigOne Button aButton new Button MyApp aButton addActionListener new ActionListener public void actionPerformed ActionEvent e System out printin Hello There The following example does the same thing but it names the class that implements the action listener import java awt import java awt event import java io Serializable class MyApp implements Serializable
162. lues of point0 and point1 to null when a Distance instance is created The null value means that a reference value does not refer to an object The special Java literal nu11 is used to represent the null value in a program While you can explicitly assign null values in a declaration as in private java awt Point point0 private java awt Point pointl null null ou it is not necessary and most programmers omit such default assignments 15 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FINSTANCES20FIELDS 16 HTTP EN WIKIBOOKS ORG WIKI JAVA 20PROGRAMMINGS2FKEYWORDSS2FPRIVATE 54 Detailed Program Structure and Overview Example Constructor A CONSTRUCTOR is a special method in a class which is used to construct an instance of the class The constructor can perform initialization for the object beyond that which the Java VM does automatically For example Java will automatically initialize the fields point0 and point1l to null Below is the constructor for this class It consists of five parts 1 The optional ACCESS MODIFIER s 8 In this case the constructor is declared public 2 The constructor name which must match the class name exactly Distance in this case 3 The constructor PARAMETERS 9 The parameter list is required Even if a constructor does not have any parameters you must specify the empty list The parameter list declares the type and name of each of the method s parameters 4
163. m which is half way through your code instead of the point where you imported JFrame along with everything else in javax swing If you change this to import javax swing JFraim the error will be at the import instead of within your code Furthermore if you import javax swing and import java util and javax swing Queue is later added in a future version of Java your code that uses Queue java util will fail to compile This particular example is fairly unlikely but if you are working with non Sun libraries it may be more likely to happen 12 3 Importing packages from jar files If you are importing library packages or classes that reside ina jar file you must ensure that the file is in the current classpath both at compile and execution time Apart from this require ment importing these packages and classes is the same as if they were in their full expanded directory structure 76 Class Loading Name Space Example To compile and run a class from a projects top directory that contains the two directories source and libraries you could use the following command javac classpath libraries lib jar source MainClass java And then to run it similarly java classpath libraries lib jar source MainClass The above is simplified and demands that MainClass be in the default package or a package called source which isn t very desirable 12 4 Class Loading Name Space A fully qualified class na
164. m its package package only from only from only from N A N A de default its package its package its package fault is public private N A only from only from N A N A its class its class 83 Access Modifiers Points to ponder Note that Interface method visibility is puB11C by default You do not need to specify the access modifier it will default to P uB r r c For clarity it is considered a good practice to put the PUBLIC keyword The same way all member variables defined in the Interface by default will become static FINAL once inherited in a class d HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMING 2FKEYWORDSS2FPUBLIC HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMING 2FKEYWORDSS2FPUBLIC HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMING 2FKEYWORDSS2FPUBLIC HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMING 2FKEYWORDS S2FSTATIC HTTP EN WIKIBOOKS ORG WIKI JAVA S20PROGRAMMING 2FKEYWORDSS2FFINAL An Fs If a class has public visibility the class can be referenced by anywhere in the program If a class has package visibility the class can be referenced only in the package where the class is defined If a class has private visibility it can happen only if the class is defined nested in an other class the class can be accessed only in the outer class If a variable is defined in a public class and it has public visibility the variable can be reference anywhere in the application through the class it is defined in If a
165. m member The demise of an idea birth of another 3 3 The demise of an idea birth of another By now the work on Oak had been significant but come the year 1993 people saw the demise of set top boxes interactive TV and the PDAs A failure that completely ushered the inventors thoughts to be reinvented Only a miracle could make the project a success now And such a miracle awaited anticipation NATIONAL CENTER FOR SUPERCOMPUTING APPLICATIONS NCSA had just unveiled its new commercial web browser for the internet the previous year The focus of the team now diverted towards where they thought the next wave of computing would be the internet The team then divulged into the realms of creating the same embeddable technology to be used in the web browser space calling it AN APPLET a small application The team now needed a proper identity and they decided on naming the new technology they created Java ushering a new gen eration of products for the internet boom A by product of the project was a cartoon named Duke created by Joe Parlang which became its identity then Finally at the SunWorldTM conference Andreesen unveiled the new technology to the masses Riding along with the explosion of interest and publicity in the Internet Java quickly received widespread recognition and expectations grew for it to become the dominant software for browser and consumer applications 3 4 Recent history Initially Java wa
166. mArray i length j twoDimArray 1 3 i j In the above example we defined an array which has three elements each element contains an array having 5 elements We could create the array having the 5 elements first and use that one in the initialize block String oneDimArray 00 01 02 03 04 3 String twoDimArray oneDimArray proT it 12 EEES an nano 20 2107 AZ 23 1241 Fi Since they are arrays of array references these multi dimensional arrays can be jagged i e subarrays can have different lengths or the subarray reference can even be null Consider String weirdTwoDimArray 10 11 12 null 20 219 22 23 2A be Note that the length of a two dimensional array is the number of one dimensional arrays it contains In the above example weirdTwoDimArray length is 3 whereas weirdTwoDimAr ray 2 length is 5 144 Multidimensional Array 28 4 Multidimensional Array Going further any number of dimensional array can be defined or lt elementType gt lt elementType gt lt arrayName gt lt arrayName gt DE JAVA_STANDARD FR PROGRAMMATION JAVA TABLEAUX NL PROGRAMMEREN IN JAVA ARRAYs PT JAVA VETORES DATENSTRUKTUREN ES PROGRAMACI N EN HT TIP HTTP HTTP aor WN HTTP SLE Ses FR NL HTTP DE PT WIKIBOOKS ORG WIKI PROGRAMACISF3N 20ENS20JAVAS2FARRAYS ORG WIKI PROGR
167. me CITEREFEarlyYearsSun1 gt Has worked for GT Green Team FP FirstPer son and JP Java Prod ucts Group Name Lisa Friendly V Yes V Yes John Gage James Gosling v Yes V Yes V Yes Bill Joy Jonni Kanerva V Yes Tim Lindholm V Yes V Yes Details FirstPerson employee and member of the Java Prod ucts Group Science Office Director Sun Microsys tems Lead engineer and key ar chitect of the Java technol ogy Co founder and VP Sun Microsys tems Princi pal designer of the UC Berkeley ver sion of the UNIX OS Java Products Group em ployee author of The Java FAQ1 FirstPerson employee and member Java Products Group 15 History 16 Table 1 GT FP JP Who s who of the Java tech nology lt ref name CITEREFEarlyYearsSun1 gt Has worked for GT Green Team FP FirstPer son and JP Java Prod ucts Group Name Scott McNealy Patrick V Yes V Yes Naughton George Paolini Kim Polese V Yes Lisa Poulson Wayne Rosing v Yes Eric Schmidt Mike Sheri V Yes dan Details Chairman President and CEO of Sun Microsystems Green Team member FirstPerson co founder Corporate Marketing Director Sun s Java Software Divi sion FirstPerson product mar keting Original di rector of public re lations for Java technol ogy Burson Marsteller FirstPerson President Former Sun Microsystems Chief Tech nology Officer Green Tea
168. me collision is likely to happen To avoid this a Java class can be put in a name space This name space in Java is called the package The Java package needs to be unique across Vendors to avoid name collisions For that reason Vendors usually use their domain name in reverse order That is guaranteed to be unique For example a company called Your Company Inc would use a package name something like this com yourcompany yourapplicationname yourmodule YourClass To put a class in a package the package keyword is used at the top of each class file For Example package com yourcompany yourapplication yourmodule When we want to reference a Java class that is defined outside of the current package name space we have to specify which package name space that class is in So we could reference that class with something like com yourcompany yourapplication yourmodule YourClass To avoid having to type in the package name each time when we want to reference an outside class we can declare which package the class belongs to by using the import Java keyword at the top of the file For Example import com yourcompany yourapplication yourmodule YourClass Then we can refer to that class by just using the class name YourClass In rare cases it can happen that you need to reference two classes having the same name in different packages In those cases you can not use the import keyword for both classes One of
169. me consists of the package name plus the class name For example the fully qualified class name of HashMap is java util HashMap Sometime is can happen that two classes have the same name but they can not belong to the same package otherwise it would be the same class It can be said that the two classes with the same name are in different name spaces In the above example the HashMap class is in the java util name space Let be two Customer classes with different name spaces in different packages e com bluecompany Customer com redcompany Customer when we need to use both classes in the same program file we can use the import keyword only for one of the classes For the other we need to use the fully qualified name The runtime identity of a class in Java 2 is defined by the fully qualified class name and its defin ing class loader This means that the same class loaded by two different class loaders is seen by the Virtual Machine as two completely different types FR PROGRAMMATION JAVA EXTENSIONS IT JAVA PACKAGE 1 HTTP FR WIKIBOOKS ORG WIKI PROGRAMMATIONS20JAVAS2FEXTENSIONS 2 HTTP IT WIKIBOOKS ORG WIKI JAVAS2FPACKAGE 77 Packages 78 13 Nested Classes In Java you can define a class inside an other class A class can be nested e inside another class e or inside a method 13 1 Nest a class inside a class When a class is declared inside another class the nested class access modifier can
170. mport com yourcomp util YourCollectionImpl Collection colll Collection coll2 new YourCollectionImpl new YourCollectionImpl lt Use colll amp coll2 gt The Java JDK collection implementations are quite powerful and good so it is unlikely that you will need to write your own All collections contain object references Because of that if the object is changed after it was put in the collection the object that is in the collection also changes The object is not really in the collection only the object reference is It is not guaranteed that the objects inside the collections won t change This is an issue only if you put an actively used object in the collection 1 Chapter 28 on page 143 147 Collection Classes In that case when you are adding an object that could change any time you need to make a copy or clone of the object A new object will be created and its reference will be put in the collection In that case there will be no object references outside of the collection so the objects inside the collection can only be changed if we take out an object reference from the collection Aside from the java util Collection interface the Java JDK has the java util Map interface as well This defines key value mappings Implementations of the Map interface do not contain collections of objects Instead they contain collections of key gt value mappings import java util Map import j
171. n be created Code and static variables are stored only once no matter how many objects are created Memory is allocated for the object member variables when the object is created Thus the size of an object is determined not by its code s size but by the memory it needs for its member variables to be stored 23 3 Creating object by cloning an object Cloning is not automatically available to classes There is some help though as all Java objects inherit the protected Object clone method This base method would allocate the memory and do the bit by bit copying of the object s states You may ask why we need this clone method Couldn t I create a constructor and just passing in the same object and do the copying variable by variable Lets see public class MyObject private int memberVar MyObject MyObject obj this memberVar obj memberVar You might think that accessing the private memberVar variable of obj would fail but as this is in the same class this code is legal The clone method copies the whole object s memory in one operation This is much faster than using the new keyword Object creation with the new keyword is expensive so if you need to create lots of objects with the same type performance will be better if you create one object and clone new ones from it See below a factory method that will return a new object using cloning 130 Creating object by cloning an object HashTable
172. n literal true and the special null value null to the method parse 18 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FSYNTAXS2FUNICODES20ESCAPES 20SEQUENCES 19 HTTP EN WIKIBOOKS ORG WIKI CATEGORYS3AJAVAS20PROGRAMMING 63 Syntax List items parse string true null BOOLEAN LITERALS 2 NUMERIC LITERALS e CHARACTER LITERALS e INTEGER LITERALS e FLOATING POINT LITERALS STRING LITERALS null 9 3 Blocks Java has a concept called block that is enclosed between the and characters called curly braces A block executed as a single statetement and can be used where a single statetement is accepted After a block is executed all local variables defined inside the block is discarded go out of scope This is a block Blocks can be nested This is a nested block CATEGORY JAVA PROGRAMMING 20 HTTP EN WIKIBOOKS ORG W JAVAS20PROGRAM GS2FLITERALSS2FBOOLEANS 20LITERALS 21 HTTP EN W BOOKS ORG W JAVAS20PROGRAM GS2FLITERALSS2FNUMERICS 20LITERALS 22 HTTP EN W BOOKS ORG W JAVAS20PROGRAM GS2FLITERALSS2FNUMERICS ZOLITERALSS2FCHARACTERSZ0LITERALS 23 HTTP EN W BOOKS ORG W JAVAS20PROGRAM GS2FLITERALSS2FNUMERICS ZOLITERALSS2ZFINTEGERS2Z0LITERALS 24 HTTP EN W BOOKS ORG W JAVAS20PROGRAM GS2FLITERALSS2FNUMERICS 20LITERALS 2FFLOATINGS20POINTS2
173. need to explicitly create your own exceptions As with any object in Java you always create exceptions on the heap using new which allocates storage and calls a constructor There are two constructors in all standard exceptions 1 The default constructor and 2 A constructor taking a string argument so that you can place pertinent information in the exception Listing 1 2 Instance of an exception object with the default constructor new Exception Listing 1 3 Instance of an Exception object by passing string in constructor new Exception Something unexpected happened This string can later be extracted using various methods as you ll see The keyword throw produces a number of interesting results After creating an exception object with new you give the resulting reference to throw The object is in effect returned from the method even though that object type isn t normally what the method is designed to return A simplistic way to think about exception handling is as a different kind of return mechanism although you get into trouble if you take that analogy too far You can also exit from ordinary scopes by throwing an exception In either case an exception object is returned and the method or scope exits Listing 1 4 Throwing an exception results in an unexpected return from the method throw new Exception Anything after the throw statement would not be executed unless the thrown exception is HAN DLED
174. ng Do not override the equals and hashCode methods in the Apple and Orange classes e Create appleEquals method in the Apple class and create orangeEquals method in the Orange class e Override the hashCode method in the Fruit class and return the same value so the equals is called by the Set implementation e Override the equals method in the Fruit class for something like this public boolean equals Object obj 3 HTTP EN WIKIBOOKS ORG WIKI JAVA 20PROGRAMMINGS2FAPI 2FJAVA LANG OBJECTS 23EQ0UALS 28 29 20METHOD 4 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS 2FAPIS2FJAVA LANG OBJECTS23 5 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FAPIS2FJAVA LANG OBJECTS 23HASHCODES28 529 520METHOD 6 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FAPIS2FJAVA LANG OBJECTS23 7 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMING 2FAPIS2FJAVA LANG OBJECTS23 153 Collection Classes boolean ret false if this instanceof Apple amp amp obj instanceof Apple ret this appleEquals obj else if this instanceof Orange amp amp obj instanceof Orange ret this orangeEquals obj else Can not compare Orange to Apple ret false return ret Note only the objects that have the same hashCode will be compared you are responsible to override the equal and hashCode methods The default imple mentations in Object won t work
175. ng a problem it is not enough to clone the Customer object we also need to clone the referenced objects The solution e Make the Activity class also cloneable e Make sure that if the Activity class has other changeable object references those has to be cloned as well as seen below e Change the Customer class clone method as follows public class Customer implements Cloneable Activity _activity public Customer clone throws CloneNotSupportedException Customer clonedCustomer Customer super clone Clone the object referenced objects if _activity null 131 Creating Objects clonedCustomer setActivity Activity _activity clone return clonedCustomer Note that only mutable objects needs to be cloned References to unchangeable objects such as String be used in the cloned object without worry 23 4 Creating object receiving from a remote source When an object is sent through a network the object needs to be recreated at the receiving host Object Serialization The term Object Serialization refers to the act of converting the object to a byte stream The byte stream can be stored on the file system or can be sent through a network At the later time the object can be re created from that stream of bytes The only requirement is that the same class has to be available at both times when the object is serialized and also when the object is re created
176. o easy to use Many potential problems are caught by the compiler One area where Java was criticized was regarding the container objects Container objects are objects that contain other objects Before Generics were introduced there was no way to ensure that a container object contains only one type of objects When an object was added to a container it was automatically cast to Java Object When it was taken out an explicit cast was needed Normally an explicit cast is checked by the compiler String st This is a String Integer integer Integer st Compilation Error But in the case of container classes the compiler was not able to catch an invalid type casting 1 Collection collString new ArrayList 2 collString add This is a String 3 Integer integer Integer collString get 0 No Compilation Error RunTime CastException Just looking at line 3 we do not know what type of objects collString contains If that contains Integers then the code is fine The below code using Generic Collection lt String gt collString mew ArrayList lt String gt collString add This is a String Js Integer integer Integer collString get 0 Compilation Error collStringis a container object that can contain only String objects nothing else so when we get out an element it can be casted only to class that normally a String can be casted 118 Note for C
177. ollection is created the actual Element type will replace the E Objects put into a collection are upcasted to Object class It means that you need to cast the object reference back when you get an element out from the collection It also means that you need to know the type of the object when you taking it out For this reason we usually add objects 148 Collection or Map of the same type to a collection If a collection contains different types of objects we will have difficulty finding out the type of the objects obtained from a collection at run time With the use of the parameterized type lt E gt the Element type that can be put into the collection can be specified when the collection object is created Collection lt Integer gt ageList new ArrayList lt Integer gt ageList add new Integer 46 Integer can be added ageList add 50 Compilation error ageList can have only Integers inside ageList isacollection that can contain only Integer objects as elements No casting is required when we take out an element Integer age ageList get 0 For more information about Generics please go to JAVA PROGRAMMING GENERICS 29 3 Collection or Map The Java JDK contains two high level Interfaces e java util Collection e java util Map Implementations for those interfaces are notinterchangeable Collections are used for collecting Java objects Maps are used for
178. omething other than a container class For example the type of String class is Class lt String gt and the type of Serializable class is Class lt Serializable gt This can be used to improve the type safety of your reflection code In particular since the newInstance method in Class now returns a T you can get more pre cise types when creating objects reflectively Now we can use the newInst ance method to return a new object with exact type without casting Customer cust Utility createAnyObject Customer class No casting public static lt T gt T createAnyObject Class lt T gt cls T ret null try ret cls newInstance catch Exception e Exception Handling return ret And the above code without Generics Customer cust Customer Utility createAnyObject Customer class Casting is needed public static Object createAnyObject Class cls Object ret null try ret cls newInstance catch Exception e Exception Handling 120 Variable Argument return ret Get exact type when getting JavaBean property using reflection See the following code where the method will return the exact type of the Java Bean property based on how it will be called Using reflection get a Java Bean property by its name public static lt T gt T getProperty Object bean String propertyName if bean n
179. onsist of a front end and a back end part both running on a separate computer The idea is that the business logic would be on the back end part of the program which would be reused by all the clients Here the challenge is to achieve a separation between front end user interface code and the back end business logic code The communication between the front end and the back end can be achieved by two ways One way is to define a data communication PROTOCOL between the two tiers The back end part would listen for an incoming request Based on the PROTOCOL it interprets the request and sends back the result in data form The other way is to use JAVA REMOTE INVOCATION RMI With the use of RMI a remote object can be created and used by the client In this case Java objects are transmitted across the network More information can be found about client server programming with sample code at the CLIENT SERVER CHAPTER in this book Web Applications For applications needed by lots of client installations the client server model did not work Maintaining and upgrading the hundreds or thousands of clients caused a problem It was not practical The solution to this problem was to create a unified standard client for all applica tions and that is the BROWSER 3 HTTP EN W PEDIA ORG W INTERNETS20EXPLORER 4 HTTP JAVA COM 5 HTTP EN W BOOKS ORG W JAVAS20PROGRAMMINGS2F
180. ors a project was initiated in December of 19907 whose aim was to create a programming tool that could render obsolete the C and C programming languages Engineer Patrick Naughton had become extremely frustrated with the state of Sun s C and C APIs appli cation programming interfaces and tools While he was considering to move towards NEXT he was offered a chance to work on new technology and the Stealth Project was started a secret nobody but he knew This Stealth Project was later named the Green Project when James Gosling and Mike Sheridan joined Patrick Over the period of time that the Green Project teethed the prospects of the project started becoming clearer to the engineers working on it No longer was its aim to create a new language far superior to the present ones but it aimed to target the language to devices other than the computer Staffed at 13 people they began work in a small office on Sand Hill Road in Menlo Park Califor nia This team would be called Green Team henceforth in time The project they underwent was chartered by Sun Microsystems to anticipate and plan for the next wave in computing For the team this meant at least one significant trend that of the convergence of digitally controlled consumer devices and computers 3 2 Reshaping thought The team started thinking of replacing C with a better version a faster version a responsive version But the one thing they hadn t thought of as of y
181. owever get you acquainted with the basics of the language in order for you to create that masterpiece of a software you dream of creating one day People have written operating systems in Java they have programmed robots in the language and managed power stations and conveyor belts in factories thus you can do anything and everything with Java Your applications and code need not fear any bounds because with Java possibilities are endless all you need is creativity HTTP EN WIKIPEDIA ORG WIKI OPENOFFICE ORG HTTP EN WIKIPEDIA ORG WIKI MINECRAFT HTTP EN WIKIPEDIA ORG WIKI RUNESCAPE HTTP EN WIKIPEDIA ORG WIKI TIVO HTTP EN WIKIPEDIA ORG WIKI SKY 2B HTTP EN WIKIPEDIA ORG WIKI ANDROID 20 280PERATINGS20SYSTEMS29 CANDO A 2 About This Book The Java Programming Wikibook is a shared effort in amassing a comprehensive guide of the complete Java platform from programming advice and tutorials for the desktop computer to programming on mobile phones The information presented in this book has been concep tualised with the combined efforts of various AUTHORS AND CONTRIBUTORS and anonymous editors The primary purpose of this book is to teach the Java programming language to an audience of beginners but its progressive layout of tutorials increasing in complexity it can be just as helpful for intermediate and experienced programmers Thus this book is meant to be used as acollection of tutorials building upon on
182. owser The HTML output represents both the presention logic and the results of the business compu tations This represents a huge problem and there is no real application relying only on servlets to handle the presention part of the responsibility There are two main solutions to this Use a template tool Store the presentation part in an HTML file marking the areas that need to be replaced after business logic computations e Use JSP See next section Wikipedia also has an article about SERVLETS Client Computer Server Computer Browser EJ Application Server lt html gt HttpServlet JSP doPost request response lt html gt lt a href http gt E ap Process form request lt Embedded Java gt lt form action http gt ie doGet request response lt a Bas lt form gt lt form gt lt html gt Process GET request lt html gt Data base Figure 12 Figure 4 Jsp Execution Java Server Pages JSP code Figure 4 JSP isan HTML file with embedded Java code inside The first time the JSP is accessed the JSP is converted to a Java Servlet This servlet outputs HTML which has inside the result of the business logic computation There are special JSP tags that helps to add data dynamically to the HTML Also JSP technology allows to create custom tags Using the JSP technology correctly business logic computations should not be in the embedded Java part of the JSP JSP s
183. ped by Tek Tools Itis basically a Java editor which does not include wizards and GUI tools Itis best suited to experienced Java programmers in small and midsized develop ment teams The latest version is 4 0 you can DOWNLOAD 11 1 For more info see KAWA FROM TEK TOOLS See a JAVAWORLD ARTICLE It looks that there is no new development for Kawa 11 HTTP WWW ECLIPSEPLUGINCENTRAL COM 12 HTTP WWW BLUEJ ORG 13 HTTP WWW OLABS COM KAWA 14 HTTP WWW TEK TOOLS COM KAWA 15 HTTP WWW JAVAWORLD COM JAVAWORLD JW 06 2000 JW 0602 IW KAWA HTML 39 Compilation 6 13 Ant For comprehensive information about all aspects of Ant please see the ANT WIKIBOOK Antis a build management tool designed to replace make as the tool for automated builds of large Java applications Like Java and unlike make Ant is designed to be platform independent Building a Java application requires certain tasks to be performed Those tasks may include not only compiling the code but also copying files packaging the program into a jar file running tests and so on Some of these tasks may depend upon others having been done previously not creating a jar unless the program has been complied for instance It might also be a good idea to not execute all tasks every time the program is complied e g to only compile changed source files Ant makes all of these things easy The tasks and their dependencies are defined in a build xml file
184. pile gt lt java classname main class gt lt classpath gt lt path refid classpath gt lt pathelement location classes dir gt lt classpath gt lt java gt lt target gt 41 Compilation lt target name clean build depends clean build gt lt target name main depends clean run gt lt project gt 6 14 The JIT compiler The standard JIT compiler runs on demand When a method is called repeatedly the JIT compiler analyzes the bytecode and produces highly efficient machine code which runs very fast The JIT compiler is smart enough to recognize when the code has already been compiled so as the application runs compilation happens only as needed As Java applications run they tend to become faster and faster because the JIT can perform runtime profiling and optimization to the code to meet the execution environment Methods or code blocks which do not run often receive less optimization those which run often so called hotspots receive more profiling and optimization 42 7 Execution There are various ways in which Java code can be executed A complex Java application usually uses third party APIs or services In this section we list the most popular ways a piece of Java code may be packed together and or executed 7 1 JSE code execution Java language first edition came out in the client server era Thick clients were developed with rich GUI interfaces
185. ple do if OK equals state and do not do lt strike gt if state equals OK lt strike gt If the state variable is null you get a NullPointerException in the second example but not in the first one 174 See also CATEGORY JAVA PROGRAMMING 30 14 See also W JAVA PLATFORM W JAVA API W JAVA VIRTUAL MACHINE 0 e ccc includes a Java to machine language compiler COMPARISON OF JAVA TO C w JINIP ECLIPSE IDE http eclipse org w NETBEANS Another open source IDE w OPTIMIZATION OF JAVA 7 HTTP EN W BOOKS ORG W CATEGORY 3AJAVA 20PROGRAMMING 8 HTTP EN W PEDIA ORG W JAVAS20PLATFORM 9 HTTP EN W PEDIA ORG W Javas20API 10 HTTP EN Ww PEDIA ORG W JAVAS2ZOVIRTUALS20MACHINE 11 HTTP EN W PEDIA ORG W GNUS20COMPILERS20COLLECTION 12 HTTP EN W PEDIA ORG W COMPARISONS200F 20JAVAS20T0 20CPLUSPLUS 13 HTTP EN w PEDIA ORG W JINI 14 HTTP EN Ww PEDIA ORG W ECLIPSES2Z0328COMPUTINGS29 15 HTTP EN W PEDIA ORG W OPENS20SOURCES20 INTEGRATED S20DEVELOPMENTS 20ENVIRONMENT 16 HTTP EN Ww PEDIA ORG W NETBEANS 17 HTTP EN W PEDIA ORG W OPTIMIZATIONS200F 20JAVA 175 Throwing and Catching Exceptions 176 31 Links 31 1 External References JAVA CERTIFICATION PREPARATION GUIDES JAVA CERTIFICATION MOCK EXAMS 500 questions with exam simulator this is the older
186. pletviewer from the JDK can be used to run an applet 4 2 3 Annotation processing For more about annotation processing READ THIS In Java 1 5 alias Java 5 0 Sun added a mechanism called annotations Annotations allow the addition of meta data to Java source code and even provide mechanisms to carry that meta data forth into a compiled class files apt An annotation processing tool which digs through source code finds annotation state ments in the source code and executes actions if it finds known annotations The most com mon task is to generate some particular source code The actions apt performs when finding annotations in the source code are not hard coded into apt Instead one has to code particu lar annotation handlers in Java These handlers are called annotation processors The most difficult thing with apt is that Sun decided to use a whole set of new terminology apt can sim ply be seen as a source code preprocessor framework and annotation processors are typically just code generators 4 2 4 Integration of non Java and Java code javah A Java class can call native or non Java code that has been prepared to be called from Java The details and procedures are specified in the JNI Java Native Interface Commonly native code is written in C or C The JDK tool javah helps to write the necessary C code by generating C header files and C stub code 4 2 5 Class library conflicts extcheck This tool
187. ppear to compile correctly but the program behaves as if you were using an old copy of the source code or otherwise reports errors during runtime When this occurrs you may need to clean your compilation folder by either deleting the class files or using the Clean command from the IDE The next most popular way to build applications are using an IDE IDE stands for Integrated Development Environment examples of which are listed below 6 3 Packages Subdirectories and Resources Each Java top level class belongs to a package covered in the chapter about PACKAGES This may be declared in a package statement at the beginning of the file if that is missing the class belongs to the unnamed package For compilation the file must be in the right directory structure A file containing a class in the unnamed package must be in the current root directory if the class belongs to a package it must be in a directory with the same name as the package 5 Chapter 6 13 on page 40 6 Chapter 6 13 on page 40 7 Chapter 12 on page 75 36 Filename Case The convention is that package names and directory names corresponding to the package con sist of only lower case letters Example Top level package A class with this package declaration package example has to be in a direc tory named example Example Subpackages A class with this package declaration package org wikibooks en has to be in the org wikibooks en directory Java pro
188. properly Any similarity to an ordinary return from a method ends here because where you return is some place completely different from where you return for a normal method call i e you end up in an appropriate exception handler that might be far away many levels on the call stack from where the exception was thrown In addition you can throw any type of Throwable which is the exception root class Typically you ll throw a different class of exception for each different type of error The information about the error is represented both inside the exception object and implicitly in the name of the ex ception class so someone in the bigger context can figure out what to do with your exception Often the only information is the type of exception and nothing meaningful is stored within the exception object 1 HTTP EN WIKIBOOKS ORG WIK1 23EXCEPTIONS20HANDLERS 164 Catching an exception 30 2 Catching an exception To see how an exception is caught you must first understand the concept of a guarded region This is a section of code that might produce exceptions and is followed by the code to handle those exceptions 30 2 1 The try block Ifyou are inside a method and you throw an exception or another method that you call within this method throws an exception that method will exit in the process of throwing If you don t want a throw to exit the method you can set up a special block within that method to capture the
189. reference points at that will be permanent For example int varl int var2 10 20 myMethod varl var2 YDOBWNHE System out println varl varl var2 var2 The variable values did not change 8 O idas 10 void myMethod int vari int var2 11 12 ae 13 varl 14 var2 15 16 l I On line 7 the value of var1 is 10 and the value of var2 is 20 When the variables were passed in to the methods their values were copied This is called passing the parameter by value In java we do not represent an object directly we represent an object throught an object reference You can think of an object reference as a variable having the address of the object So the object reference passed in by value but the object itself is not For example 88 Functions MyObjOne obj new MyObjOne obj setName Christin String name obj getName The name attribute was changed to Susan inside the method void myMethod MyObjOne obj 9 10 obj setName Susan 11 S 12 obj new MyObjOne 13 ob setName Sonya 14 aps Lb 3 1 2 3 4 vise 5 myMethod obj 6 7 8 On line 2 we created an object on line 3 we set its name property to Christin On line 5 we called the myMethod obj Inside the method we changed the name to Susan through the passed in object reference So that change will stay Note however that after we reassign
190. ring values are a sequence of 0 or more Unicode char acters The null reference is another valid value for a String expression The operations on a String reference variable are those available for all reference types such as comparison operations and assignment The allowed operations on String object however are the set of methods in the java lang String class which includes length toString toLowerCase toUpperCase compareTo String anotherString and more In addition String objects also inherit the set of operations from the base class that String extends from which is java lang Object These operations include methods such as equals hashCode wait notifyAll and getClass private String name Marry Brown In the above example the name object reference s attributes are Type is String The referenced object is also String Both the java lang String class methods and java lang Object class methods are avail able for the object reference name private Object name Marry Brown In the above exam ple the name object references attributes are Type is Object The referenced object is String Only the java lang Object class methods are available for the object reference name 17 4 Array Types Arrays in Java are represented as a built in Array object As with object types they behave as a reference but have a few differences allowing them to allow easy access to sub elements
191. roblem since the compiler would not be able to distinguish between the two public void logit String param String err public void logit String err String param The compiler would give an error for the following method definitions as well public void logit String param public String logit String param String retType return retValue Note the return type is not part of the unique signature Why not The reason is that a method can be called without assigning its return value to a variable This feature came from C and C 86 Method Overriding So for the call logIt msg the compiler would not know which method to call 15 3 Method Overriding Obviously a method signature has to be unique inside a class The same method signature can be defined in different classes If we define a method that exist in the super class then we override the super class method The terminology for this is method overriding This is different from method overloading Method overloading happens with methods with the same name differ ent signature Method overriding happens with same name same signature between inherited classes The return type can cause the same problem we saw above When we override a super class method the return type also must be the same In fact if that is not the same the compiler will give you an error Method overriding is related dynamic
192. rsion 1 2 features a more robust JVM Instead of interpret ing byte code it down right converts the code straight into equivalent native code for the local system This process of conversion is called JUST IN TIME COMPILATION or JIT compilation 2 HTTP EN WIKIPEDIA ORG WIKI JUST IN TIMESZOCOMPILATION 23 The Java Platform This process only occurs when the byte code is executed for the first time Unless the byte code itself is changed the JVM uses the compiled version of the byte code on every successive exe cution Doing so saves a lot of time and processor effort allowing applications to execute much faster at the cost of a small delay on first execution Figure 3 A just in time compiler only compiles the byte code to equivalent native code at first execution Upon every successive execution the JVM merely uses the already compiled native code to optimize performance Java Runtime Environment Compiled Native JIT Compiler Application PEE wk Local Successive use Machine Java Byte code Bypass Compilation Figure 6 Native optimization The JVM is an intelligent virtual processor It has the ability to identify areas within the Java code itself that can be optimized for faster and better performance Based on every successive run of your Java applications the JVM would optimize it to run even better Note There are portions of Java code that do not require to be JIT compiled at runt
193. s Kk MyClass Empty Constructor ai public MyClass 91 Methods The Constructor is called automatically when an object is created with the new keyword A constructor may also be called from an other constructor see below public class MyClass private String memberField MyClass Constructor there is no input parameter public MyClass yClass Default Value MyClass Constructor there is one input parameter public MyClass String paraml memberField paraml In the above example the constructor with no input parameter calls the other constructor with the default initial value This gives an option to the user to create the object with the default value or create the object with a specified value 15 8 Static Methods We defined methods above as operations on an object Static methods are defined inside a class but they are not an operation on an object No object needs to be created to execute a static method they are simply global functions with input parameters and a return value public class MyObject static public String myStaticMethod return I am a static method Static methods can be referenced anywhere prefixed by the class name See below 92 Externallinks Call myStaticMethod System out println Output from the myStaticMethod MyObject myStatic
194. s a boolean primitive value yet it was given a Boolean wrapper object No problem Java 1 5 will automatically unbox this Boolean canMove new Boolean true if canMove System out println This code is legal in Java 1 5 CATEGORY JAVA PROGRAMMING DE JAVA_STANDARD PRIMITIVE DATENTYPEN FR PROGRAMMATION JAVA TYPES DE BASE IT JAVA TIPI DI DATI PT JAVA TIPOS DE DADOS PRIM RIOS 13 HTTP EN WIKIBOOKS ORG WIKI CATEGORYS3AJAVAS20PROGRAMMING 14 HTTP DE WIKIBOOKS ORG WIKI JAVA_STANDARD 33A 20PRIMITIVES20DATENTYPEN 15 HTTP FR WIKIBOOKS ORG WIKI PROGRAMMATION 20JAVAS2FTYPESS20DES20BASE 16 HTTP IT WIKIBOOKS ORG WIKI JAVAS2FTIPIS20DI 20DATI 17 HTTP PT WIKIBOOKS ORG WIKI JAVAS2FTIPOS 20DES20DADOSS 20PRIMSElLRIOS 102 18 java lang String UNKNOWN TEMPLATE Java Programming UNKNOWN TEMPLATE ifeq Types String ifeq Classes Objects and none TYPES Types none CLASSES OBJECTS AND TYPES 18 1 java lang String String is a special class built into the Java language defined in the java lang package The String class represents character strings String literals in Java programs such as abc are implemented as instances of this class For example String ste This ls string literal On the right hand side a String object is created represented by the string literal Its object refer ence is assigned to the st r variable Strings are immutable that is they cannot be modified once create
195. s do not have any run time mechanisms The compiler creates normal code to substitute the template similar to any hand written code In contrast Java Generics are built into the language The same Class object handles all the Generic type variations No additional code is generated no matter how many Generic objects are created with different type parameters For example Collection lt String gt collString new ArrayList lt String gt Collection lt Integer gt collInteger mew ArrayList lt Integer gt There is only one Class object created In fact at runtime both these objects appear as the same type both ArrayList s The generics type information is erased during compilation type era sure This means for example that if you had function that takes Collection lt T gt as an argu ment and that collection happened to be empty your function would have no way of instanti ating another T object because it doesn t know what T was The Class class itself is generic since Java 1 5 119 Generics public final class Class lt T gt extends Object implements Serializable GenericDeclaration Type AnnotatedElement The T type here represents the type that is handed to the Class object The T type will be substi tuted with the class being loaded 21 4 Class lt T gt Since Java 1 5 the class java lang Class is generic It is an interesting example of using generic ness for s
196. s his name static fields Static fields are common to any object of the same class For example a static field within the Employee class could keep track of the last ID number issued Only one static field exists for one class static methods Static methods are methods that do not affect a specific object other classes Sometimes it is useful to contain a class within another one if it is useless out side of the class or should not be accessed outside the class Constructors A special method that generates a new object Parameterized types Since 1 5 parameterized types can be assigned to a class during def inition The parameterized types will be substituted with the types specified at the class s instantiation It is done by the compiler It is similar to the C language macro define state ment where a preprocessor evaluates the macros public class Employee This defines the Employee class The public modifier indicates that it can be accessed by any other class private static int nextID Define a static field Only one copy of this will exist no matter how many Employees are created 125 Defining Classes private int myID private String myName indicates that access it public Employee String name name to the constructor Hi Define fields that will be stored for each Employee The private modifier only code inside the Employee class can
197. s owned by Sun Microsystems but later it was released to open source the term Java was a trademark of Sun Microsystems Sun released the source code for its HotSpot Virtual Machine and compiler in November 2006 and most of the source code of the class library in May 2007 Some parts were missing because they were owned by third parties not by Sun Mi crosystems The released parts were published under the terms of the GNU GENERAL PUBLIC LICENSE a free software license 3 5 Versions Unlike C and C Java s growth is pretty recent Here we d quickly go through the development paths that Java took with age 16 HTTP EN WIKIPEDIA ORG WIKI NATIONALS20CENTERS20FOR 20SUPERCOMPUTINGS 20APPLICATIONS 17 HTTP EN WIKIPEDIA ORG WIKI APPLET 18 HTTP EN WIKIPEDIA ORG WIKI DUKE 20 28MASC0T 29 19 HISTORY OF Java 29 Lindsey Clark S Retrieved 7 May HTTP EN WIKIPEDIA ORG WIK1 7 20MAY 2008 uTTP EN WIKIPEDIA ORG WIKI 2008 21 HuTtTp EN WIKIPEDIA ORG WIKI GNUS2Z0GENERALS20PUBLICS20LICENSE 17 History 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2006 2007 2008 2005 Earlier revisions 1 0 1 1 Java 2 version 1 2 1 4 Tiger version 1 5 Future versions Java was largely being developed and refined Java version 1 2 was by far the most stable With Java like languages being built by Involving more customers and for specific programmability on a computer and readily robust version of Java and was competitors J
198. s we go on So did you notice why we used a double in listing 1 3 and not a float The answer is pretty simple If we d used a float we would have to append the number with a f as a suffix so 10 5 should be 10 5f asin Listing 1 4 The correct way to define floating point numbers of type float float age 10 5f 10 6 Assignment statements Up until now we ve assumed the creation of variables as a single statement In essence we assign a value to those variables and that s just what it is called When you assign a value to a variable in a statement that statement is called an assignment statement Did you notice one more thing The semicolon It s at the end of each statement A clear indicator that a line of code is a statement is its termination with an ending semicolon If one was to write multiple statements itis usually done on each separate line ending with a semicolon Consider the example below Listing 1 5 Multiple assignment statements int a 10 int b 20 int c 30 You do not necessarily have to use a new line to write each statement Just like English you can begin writing the next statement where you ended the first one as depicted below Listing 1 6 Multiple assignment statement on the same line int a 10 int b 20 int c 30 69 Statements However the only problem with writing such code is it s very difficult to read it back It doesn t look that intimidating at first but once you v
199. sed as key java util WeakHashMap lt E gt entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use Non synchronized java util LinkedHashMap lt E gt This linked list defines the iteration ordering which is normally the order in which keys were inserted into the map insertion order Note that insertion order is not affected if a key is re inserted into the map java util IdentityHashMap This class implements the Map interface with a hash table us ing reference equality in place of object equality when comparing keys and values In other words in an IdentityHashMap two keys k1 and k2 are considered equal if and only if k1 k2 In normal Map implementations like HashMap two keys k1 and k2 are considered equal if and only if k1 null k2 null k1 equals k2 Not synchronized 158 Classes Diagram UML java util EnumMap All of the keys in an enum map must come from a single enum type that is specified explicitly or implicitly when the map is created Enum maps are represented inter nally as arrays This representation is extremely compact and efficient Not synchronized 29 6 Thread Safe Collections It is also called Concurrent Collections Most of the popular collection classes have implemen tations for both single thread and multiple thread environments The non synchronized imple mentations are always faster You can use the non synchronized implementations in multip
200. sibility of a class For our example public class Distance is the class declaration It consists in this case of two keywords pub11C and crass l fol lowed by the identifier Distance This means that we are defining a class named Distance Other classes or in our case the com mand line can refer to the class by this name The public keyword is an ACCESS MODIFIER which declares that this class and its members may be accessed from other classes The class keyword obviously identifies this declaration as a class Java also allows declarations of INTER FACES and as of Java 5 ANNOTATIONS Chapter 9 on page 61 3 4 HTTP EN W BOOKS ORG W JAVAS20PROGRA GS2FKEYWORDS 5 Chapter 9 2 on page 63 6 HTTP EN W BOOKS ORG W JAVAS20PROGRA GS2FSTRINGS20LITERALS 7 HTTP EN W BOOKS ORG W JAVAS20PROGRA GS2FOPERATORS 8 Chapter 9 3 on page 64 9 HTTP EN W BOOKS ORG W JAVAS20PROGRA GS2FWHITESPACE 10 HTTP EN W BOOKS ORG W JAVAS20PROGRA GS2FKEYWORDSS2FPUBLIC 11 HTTP EN W BOOKS ORG W JAVAS20PROGRA GS2FKEYWORDS 2FCLASS 12 Chapter 14 on page 83 13 Chapter 24 on page 135 14 HTTP EN W BOOKS ORG W JAVAS20PROGRA GS2FANNOTATIONS 53 Understanding a Java Program The class declaration is then followed by a block surrounded by curly braces which provides the class s definition The definition is the implementation of the class the declarat
201. source dir destdir classes dir classpathref classpath gt lt Copy all resources to the build directory all non java files see the section Packages Subdirectories and Resources above for more information gt lt copy todir classes dir gt lt fileset dir src dir excludes java gt lt copy gt lt target gt lt target name build depends compile gt lt mkdir dir dist dir gt lt copy todir dist dir lib flatten true gt lt path refid classpath gt lt copy gt lt path id dist classpath gt lt fileset dir dist dir lib includes jar gt lt path gt lt manifestclasspath property dist manifest classpath jarfile dist dir ant project name jar gt lt classpath refid dist classpath gt lt manifestclasspath gt lt jar destfile S dist dir S ant project name jar gt lt zipfileset dir S classes dir gt lt manifest gt lt attribute name Class Path value dist manifest classpath gt lt attribute name Main Class value main class gt lt manifest gt lt jar gt lt target gt lt target name run build depends build gt lt java jar dist dir S ant project name jar fork true gt lt classpath gt lt path refid classpath gt lt path location dist dir ant project name jar gt lt classpath gt lt java gt lt target gt lt target name run depends com
202. stination cloned object This is possible by implementing the Cloneable interface which allows the method Object clone to perform a field by field copy 11 3 Type When an object is created a reference to the object is also created The object can not be ac cessed directly in Java only through this object reference This object reference has a type as signed to it We need this type when passing the object reference to a method as a parameter Java does strong type checking Type is basically a list of features operations that can be performed through that object refer ence The object reference type basically is a contract that guarantees that those operations will be there at run time When a car is created it comes with a list of features operations listed in the user manual that guarantees that those will be there when the car is used When you create an object from a class by default its type is the same as its class It means that all the features operations the class defined are there and available and can be used See below new ClassName operations You can assign this to a variable having the same type as the class ClassName objRefVariable new ClassName objRefVariable operations 72 Multiple classes in a Java file You can assign the created object reference to the class super class or to an interface the class implements SuperClass objectRef new ClassName
203. t E gt Non synchronized update operation is faster than other lists easy to use for stacks queues double ended queues javax management AtributeList lt E gt Represents a list of values for attributes of an MBean The methods used for the insertion of Attribute objects in the AttributeList overrides the corre sponding methods in the superclass ArrayList This is needed in order to insure that the objects contained in the AttributeList are only Attribute objects javax management relation RoleList lt E gt A RoleList represents a list of roles Role objects It is used as parameter when creating a relation and when trying to set several roles in a relation via setRoles method It is returned as part of a RoleResult to provide roles successfully retrieved javax management relation RoleUnresolvedList lt E gt A RoleUnresolvedList represents a list of RoleUnresolved objects representing roles not retrieved from a relation due to a problem en countered when trying to access read or write to roles The basic implementation of the List interface is the ArrayList The ArrayList is not synchro nized not thread safe Vector is synchronized and thread safe Vector is slower because of the extra overhead to make it thread safe When only one thread is accessing the list use the ArrayList Whenever you insert or remove an element from the list there are extra overhead to reindex the list When you have a large list and you have
204. t String gt list2 makeAList One Two Three makeAList One Two Three Four In the above example calls the arguments must be of type String If we write lt extends Ob ject gt instead of T then we can pass any kind of objects regardles of their type List lt extends Object gt list3 makeAList One 10 new StringBuffer new LinkedList Note the number 10 in the above code will be converted autoboxed to Integer See also java util Arrays asList T a 21 6 Wildcard Types As we have seen above generics give the impression that a new container type is created with each different type parameter We have also seen that in addition to the normal type checking the type parameter has to match as well when we assign generics variables In some cases this is too restrictive What if we would like to relax this additional checking What if we would like to define a collection variable that can hold any generic collection regardless of the parameter type it holds Wildcard The wildcard type is represented by the character lt gt and pronounced Unknown or Any Type This Unknown type matches anything if it is used only by itself Any Type can be express also by lt extends Object gt Any Type includes Interfaces not only Classes So now we can define a collection whose element type matches anything See below Collection lt gt collUnknown
205. t0 new java awt Point x0 y0 pointl new java awt Point x1 yl 17 Chapter 15 7 on page 91 18 Chapter 14 on page 83 19 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FPARAMETERS 20 Chapter 9 3 on page 64 55 Understanding a Java Program While it is true that this class could be implemented in other ways such as simply storing the coordinates of the two points and computing the distance as y x1 xo y yo this class instead uses the existing java awt Point class This choice matches the abstract definition of this class to print the distance between two points on the plane We take advantage of existing behavior already implemented in the Java platform rather than implementing it again We will see later how to make the program more flexible without adding much complexity because we choose to use object abstractions here However the key point is that this class uses information hiding That is how the class stores its state or how it computes the distance is hidden We can change this implementation without altering how clients use and invoke the class Example Methods Methods are the third and most important type of class member This class contains three methods in which the behavior of the Distance class is defined printDistance main and intValue The printDistance method The printDistance method prints the distance between the two points to the standard output normally
206. te class bytecode files when compiled just as if they were in separate source files However including multiple classes in one file may increase the difficulty in examining the structure of a given application 1 Chapter 17 on page 97 73 Classes Objects and Types 11 5 Externallinks e CONSTRUCTORS INTERACTIVE JAVA LESSON CATEGORY JAVA PROGRAMMING DE JAVA STANDARD CLASS ES PROGRAMACI N EN JAVA CLASES FR PROGRAMMATION JAVA LES CLASSES EN JAVA IT JAVA CLASSI E OGGETTI NL PROGRAMMEREN IN JAVA KLASSEN PT JAVA INTRODU O S CLASSES CANDO WwW 74 HAH d HHH HA EP Y yuy W W W W W W W KIBOOKS KIBOOKS KIBOOKS ORG W KIBOOKS ORG W KIBOOKS KIBOOKS KIBOOKS TP JAVALESSONS COM CG R amp SUB FUN TP EN DE ES FR SJIT NL PT ORG W ORG W ORG W ORG W ORG W BIN FUN JAVA TUTORIALS MAIN CGI SES A07898 CODE KI CATEGORY 3AJAVAS20PROGRAMMING KI JAVAS20STANDARDS3A 20CLASS KI PROGRAMACI F3N S20ENS20JAVAS2FCLASES KI PROGRAMMATIONS 20JAVAS2FLES 20CLASSESG20ENS20JAVA KI JAVAS2FCLASSIS20E 200GGETTI KI PROGRAMMERENS2Z0INS20JAVA S2FKLASSEN KI JAVAS2FINTRODUSE7 E30 20 E0SS20CLASSES 12 Packages 12 1 Java Package Name Space Usually a Java application is built by many developers and it is common that third party mod ules classes are integrated The end product can easily contain hundreds of classes Class na
207. te compo nents that can be reused throughout the Enterprise The Enterprise Java Beans EJBs were supposed to address this An EJB is an application component that runs in an EJB container The client accesses the EJB modules through the container never directly The container manages the life cycle of the EJB modules and handles all the issues that arise from network enterpise computing Some of those are SECURITY ACCESS CONTROL OBJECT POOLING TRANSACTION MANAGEMENT EJBs have the same problems as any reusable code they need to be generic enough to be able to be reused and the changes or maintenance of EJBs can affect existing clients Many times EJBs are used unnecessarily when they are not really needed An EJB should be designed as a separate application in the enterprise fulfilling one function 28 HTTP EN WIKIPEDIA ORG WIKI MODEL VIEW CONTROLLER 29 HTTP EN WIKIPEDIA ORG WIKI ACCESSS20CONTROL 30 HTTP EN WIKIPEDIA ORG WIKI OBJECTS20PO0OL 31 HTTP EN WIKIPEDIA ORG WIKI TRANSACTIONS 20PROCESSING 48 Jini MVC Paradigm Diagram HTML static Browser Figure 14 Figure 6 MVC Execution Servlet EJB Controller Model JavaBean Data _ Combine J2EE components to create an MVC architecture This leads us to the three layers tiers as shown in Figure 6 In modern web applications with lots of static data and nice graphics how the data is present
208. ten A 67 Statements tiny or big if youre lucky box with your name on it Now think of variables as something similar They are tiny little boxes in the computer s memory that save something within themselves This something within them is called a value Note A variable is an identifier to a value in the system s memory 10 4 Data types Take a look at the code in Listing 1 1 Here the variable we have just created is age The word int tells us what is inside the age variable int actually stands for integer a number On the right to this variable is the value of the variable which is the number 24 Just like int we can use byte short long double float boolean char or String All these tell us what type of data is within a variable These are hence called data types We explored the statement in Listing 1 1 where the variable held an integer within in Let s put another type of data within it Let s say the number 10 5 The code would look something like this Listing 1 2 Putting a number with decimal point inside an integer variable int age 10 5 This is actually wrong By definition if you have been awake throughout your mathematics lectures you d know that integers are whole numbers 0 1 2 all the way up to infinity Anything with a decimal point is not an integer hence the statement by virtue of it is wrong What would make it right is when you assign a certain type to the variable that would ac
209. th commas To construct a try catch block use the following syntax try Guarded region that will catch any exception that occurs within this code catch Exception ex Caught exceptions are handled here finally This class is optional Code here is executed 169 Throwing and Catching Exceptions regardless of exceptions being thrown The original code can be modified to work correctly in multiple ways For example the following public void methodC throws CustomException SomeException try methodB catch AnotherException e Handle caught exceptions methodA The AnotherException from methodB will be handled locally while CustomException and SomeException will be thrown to the caller to handle it 30 7 Application Exceptions Application Exception classes should extend the java lang Exception class Some of the JDK classes also throw exception objects inherited from java lang Exception If any of those Exception object is thrown it must be caught by the application some point by a catch block The compiler will enforce that there is a catch block associated with an exception thrown if the thrown exception object is inherited from java lang Exception and it is not the java lang RuntimeException or its inherited objects However java lang RuntimeException or its inherited objects can be caught by the application but that is not enforced by the compiler
210. ts that are made up of more than one word 18 6 Modifying String cases The String Class also allows for the modification of cases The two methods that make this possi ble are toLowerCase and toUpperCase These methods are useful for example in the typi cal programming classroom assignment of evaluating whether or not a string is a palindrome String a String b WIKIBOOKS wikipedia In this example a call to a toLowerCase would return a result of wikibooks and b toUpperCase would return WIKIPEDIA 107 java lang String 18 7 See also JAVA API JAVA LANG STRING 0 JAVA API JAVA LANG STRINGBUFFER JAVA API JAVA LANG STRINGBUILDER JAVA PROGRAMMING API JAVA LANG STRINGBUFFER JAVA PROGRAMMING API JAVA LANG STRINGBUILDER 10 uTtTe JAVA SUN COM J2SE 1 5 0 DocS API JAVA LANG STRING HTML 11 utTte JAVA SUN COM JAVASE 6 DOCS API JAVA LANG STRINGBUFFER HTML 12 HTTP JAVA SUN COM JAVASE 6 DOCS API JAVA LANG STRINGBUILDER HTML 13 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FAPIS2FJAVA LANG STRINGBUFFER 14 HTTP EN WIKIBOOKS ORG WIKI JAVAS20PROGRAMMINGS2FAPIS2FJAVA LANG STRINGBUILDER 108 19 Arrays 19 1 Intro to Arrays Anarray is similar to a table of data keyed by number In Java an array is an object like all other objects Look at the following program import java util x public class ArrayExample static Scanner input new Scanner System in public static vo
211. ull propertyName null propertyName length 0 return null Based on the property name build the getter method name String methodName get propertyName substring 0 1 toUpperCase propertyName substring 1 T property null try java lang Class c bean getClass java lang reflect Method m c getMethod methodName null property T m invoke bean null catch Exception e Handle exception return property 21 5 Variable Argument Using Generics it is very easy to define a method with a variable number of arguments Before generics this was not possible in Java if a likewise feature was needed this was mostly done by passing an array The only requirement for using a variable number of arguments using Generics is that the arguments in the list must have the same type The following code illustrates a method that can be called with a variable number arguments Method using variable length argument list param lt T gt param args public static lt T gt List lt T gt makeAList T args List lt T gt argList new ArrayList lt T gt for int i 0 i lt args length i argList add args i List lt T gt argList Arrays asList args 121 Generics return argList The above method can be called with a variable number of arguments for example List lt String gt listl List l
212. und This snippet of code is comparing the String variable myString to Hello World The compareTo method returns a negative zero or positive number if the parameter is less than equal to or greater than the object on which it is called If myString was to be different even in the slightest manner we will get a value above or below 0 depending on the exact difference The result is negative if this String object lexicographically precedes the argument string The result is a positive integer if this String object lexicographically follows the argument string Take a look at the Java API for more details 18 4 Splitting a String Sometimes it is useful to split a string into separate strings based on a regular expression For more information on regular expressions see REGEX The String class has a split method since Java 1 4 that will return a String array See the following example String person Brown John 100 Yonge Street Toronto 416 777 9999 String personData person split String name personData 0 String address personData 1 String phone personData 2 8 HTTP JAVA SUN COM J2SE 1 4 2 D0CS API JAVA LANG STRING HTML COMPARETO JAVA LANG STRING 9 HTTP EN WIKIBOOKS ORG WIKI JAVAS2FREGEX 106 Creating substrings An other useful application could be to split the String text based on the new line character so you could process the text line by lin
213. ur JDK 4 2 1 The Java compiler The Java compiler tool named javac in the JDK is the most important utility found with the JDK In order to compile a Java source file say SomeApplication java to its respective Java byte code you would need to use the following command in Command Prompt on Windows and Terminal on Linux or Mac OS javac SomeApplication java This command would convert the SomeApplication java source file into its equiva lent Java byte code The resultant byte code would exist in a newly created file named SomeApplication class This process of converting Java source files into their equivalent byte codes is known as compilation Figure 4 The basic Java compilation process SomeApplication java SomeApplication class JAVA J Java J ave Compiler aa Source Byte code Figure 7 There are a huge array of tools available with the JDK that will all be explained in due time as you progress with the book For the readers convenience these tools are listed below in order of their usage 4 2 2 Applet development appletviewer Java applets require a particular environment to execute Typically this en vironment is provided by a browser with a Java plug in and a web server serving the applet 25 The Java Platform However during development and testing of an applet it might be more convenient to start an applet without the need to fiddle with a browser and a web server In such a case Suns ap
214. y if you know the data beforehand you can write DataType variable item 1 item 2 1tem n 143 Arrays All elements of the array will be automatically initialized with the default ualue for that datatype This is false for booleans 0 for all numeric primitive types and null for all ref erence types So for example the previous note created an array of DataType references all of which are initialized to null To access an item use the syntax variable i where is the index To set an item use the syntax variable i data To find the length of an array use the syntax variable length 28 3 Two Dimensional Arrays A two dimensional array is represented by an array of arrays Because an array is also an object like any other object having the Object as the super class it can be used to create an array where the element of the array is also an array In this way an array with any number of dimensions can be created Here are examples of two dimensional arrays with initializer blocks String twoDimArray a b c d e CEN gt wh nin Wa by ke 1 m n o int twoDimIntArray 0 1 2 3 4 10 11 12 13 14 20 21 22 23 24 Note that the above twoDimArray is equivalent to the following more verbose code String twoDimArray new String 3 for int i 0 i lt twoDimArray length i twoDimArray i new String 5 for int j 0 j lt twoDi

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