Workbook 1 Welcome Visual Aids Notice
Contents
1. 4 Moodle NZ Net Page 15 of 19 One thing this change does do is break our existing program now that we have explicitly added a constructor we can no longer use the simple constructor Create two Person instances Person myPerson new Person Person anotherPerson new Person 7 Set their names myPerson setName Richard anotherPerson setName Huang However we can now combine these lines into the simpler J Create two Person instances Person myPerson new Person Richard Person anotherPerson new Person Huang And or we can add a simple constructor which is often used to initialize instance variables to some neutral value public Person name The complete MorePeople class now looks like this public class MorePeople public static void main String args 4 Create two Person instances Person myPerson new Person Richard Person anotherPerson new Person Huang String namel myPerson getName String name2 anotherPerson getName output a message System out println namel says hello to name2 A class is loosely called an object and an instance is loosely called an object too Naturally this is confusing to beginners Class The class is the template the blueprint the definition of the object Instance The instance is created when the constructor is called it occupies space in the computer s memory To use a building analogy t2. myPerson setName Harold Having set the name we get it back into a variable String name myPerson getName calling a method The method getName The following line prints a greeting to the screen in the class Person is being called here System out printin Hello T name Whatever it returns is assigned to the String The sign in the middle of the brackets is an operator that joins two Strings together concatenates name in the SayHello class If you compile SayHello this happens 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOME RIGHTS RESERVED 4 Moodle NZ Net Page 11 of 19 The two classes link up SayHello is using class Person If you now right click SayHello you can run the main method directly Click the void main String args part then new SayHello void main String args TEE OK and the console ought to output the message Compile Inspect Hello Harrola Remove 1 4 Blue Terminal Window Options Or whatever you chose to pass as a name Hello Richard This is your first Java program Why make things so complicated Clearly what we see here is overkill in terms of getting a message printed on the screen In Java as in most other programming languages this can be done much more s
3. sl System o t printli s2 trim between the instance sl sl System Out printin indexOf s2 name and the System out printin indexOf s2 trim method name instance method Output If the method returns aa a value you will need Pg a variable to hold it j as seen on the next page A bit more on variables We mentioned already that variables are memory locations inside the computer If we add these statements to our program String name Richard name Jones It is important to remember that the first value is completely lost the computer memory location now looks like this Internal Computer Named instance reference Memory variable 01110101 01110110 01110111 01111000 01111001 01111010 01111011 Every instance variable needs to be given a type The type describes the kind of data the memory location holds might be text numbers special values Our instance variable holds String or text data 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License l SOME RIGHTS RESERVED 4 Moodle NZ Net Page 13 of 19 A bit more on instances Once we have a class defined more instances can easily be created in a program and their methods used with the dot form we saw in SayHello public class MorePeople public Static v
4. the first programmer this does not happen to Quite often you will see an ugly yellow highlight and an error message of some kind Usually these error messages are confusing to beginning programmers Some beginner problems 5 author Richard e version 20090511 jpublic class Person 10 instance variables replace the exi privat String name 1 Constructor for objects of class Pe 5 L initialise instance variables name Richard invalid method declaration return type required eee Person initialise instance variables name Richard return the name illegal start of expression public Person et initialise instance variables name Richard f An example of a method replace this comment the name return the name f public String getName An example of a method replace 1 return p the name f public String getName return name Means this The constructor name must be the same as the class name Person with a capital letter Check did you leave a closing bracket off the previous method or constructor Message was spelled with a lowercase letter before Cannot find symbol is usually a spelling mistake Also check that you haven t confused a 1 with an or other similar characters The semi colon is missing from the end of the statement You left out the closing brace of the class I m sure you
5. 4 Moodle NZ Net Workbook 1 Welcome Welcome to the workshop and to our online learning environment Moodle Please see the User Manual for details of the system This workbook will cover the following topics during the first week e Introducing the BlueJ IDE e Intro to Object Oriented Programming OOP e what is an object e constructors e data members and methods e interaction with an object e Putting objects to work e simple assignment e methods e parameters e return values By the end of the week you should be able to create a simple program Visual Aids We have visual guides and jargon boxes to help you get to grips with the applications and many technical terms you must use Where you see this logo we have Flash demos in the workshop course to help you visualize the workings of the examples The workshop also uses screencasts to walk you through some of the tasks Notice This workbook is intended for use with the Java Programming Workshop run at http ib computing net Every effort has been made in the preparation of this workbook to ensure the accuracy of the information presented However the information contained in this book is sold without warranty either express or implied The author publisher will not be held liable for any damages caused or alleged to be caused directly or indirectly by this workbook This work is released under a creative commons license There are legal restrictions on your use o
6. choices amp Bluel Create New Class 22 Select a new Class and enter the name Compie Person then click OK You see a yellow Class Name i o Person box appear with the name Person in it Class T 0 Abstract Class Interface Applet amp Buek OOPWorkchop nnn Double click the box labelled Person to see the code in the editor Project Edit Tools View Help New Class This is sca ry eee _ s i Don t panic Compile 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License 4 Moodle NZ Net Page 5 of 19 Introduction to Object Oriented Programming Machine code digis Often thought of as Class Edit Tools Options The Code Editor i g f Te ee So aditar i 1 s and O s this is the Compile Und This editor is part of the i IDE A nice thing about language that Write a description of class Hello here it IS the Way it colour computers actually k author ur name codes the different types run through their e version a version number or a date eave 2 of text for us For silicon chips eae example the nice blues J 10 lace the exar and greys are 11 private int x Syntax error 12 comments i i a o y A spelling mistake 14 Constructor for objects of class Hello E v We will delete mos
7. cut and paste code from this document you may need to re type these in BlueJ Alternatively you can get the source code from the code folder in Week 1 of the course 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOME RIGHTS RESERVED 4 Moodle NZ Net Page 6 of 19 To help you out a bit we will put definitions of these terms in a word list every now andthen With that background let s examine the code class Java programs are A class made up of objects public class Person the class defines the object A class is a template or blueprint for object instances Classes are generally variable public and by convention start with a capital letter upper case followed by A variable is an area small letters lowercase Public means that other classes can create instances of computer memory from the template you can actually do something with an instance l given a convenient The entire class code must appear between two braces one opening and one name by the It is conventional to line them up vertically or sometimes horizontally You will notice that after the opening brace the source code is indented This makes it more readable for humans Instance variable private String name Richard An instance variable is a named location in compu
8. e that name is given a value This is called assignment Assignment means giving an instance variable a value private String name Richard Terminology introduced so far aclass class name 1Jpublic class Person ra a private String name Richard al instance variable data type assienment An introduction to classes So we have a class it is a definition of some real world object we want to model with our computer program The world is made up of these objects Every object has two features that we can try to include usually in a simplified way in our computer programs properties and functions Name is a property of a person Our class doesn t have any functions yet but we will consider that shortly Page 7 of 19 data type Data is stored differently to text if it is a number so that arithmetic can be done assignment Putting data in an instance variable instance variable used to store data in a class we choose the name there are some rules Can only hold one data value at a time When we create an instance variable with a statement like this we are said to declare the variable statement A Java statement is a command or instruction to the computer It ends with the semi colon When we want to create objects that we can use in our programs we are Said to be creating an object instance or just instance from a class The correct way to do this in Java i
9. eople in the abstract have a set of qualities or properties the most commonly used one might be their name The instance variables of a class store those properties A class can define people in the abstract To deal with a real person we need to create an instance object based on that class People and other objects also typically have functionality occasionally a person wants to change their name The methods of a class define that functionality Here is a compact version of a Person class public class Person class private String name instance variable public Person String newName 77 Constructor setName newName public String getName return name Method public void setName String newName Method name newName There really isn t that much to it after all Useful classes will have more instance variables and more methods Pictorially we might represent the class like this Person class name derinition String name instance variable property Person String constructor create instance String getName methods fonctions setName String 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOME RIGHTS RESERVED 4 Moodle NZ Net Page 19 of 19 Using BlueJ to interact with Object Instances One of the nice featur
10. es of BlueJ is that we can actually create and alter objects on the Workbench If you right click on the Person class you will get a menu one of the items in that menu is new Person If you click this you will call the constructor and be asked what name you want to give to the instance Once the object is on the Workbench you can inspect its properties for example you can call the method getName by clicking it new Person Right click over the class name Open Editor ile l Compil i Click new Person Inspect Remove sE Bluel Sreate Object 4 Constructor for objects of class Person Person You can accept the suggested name or change it Name of Instance Cea x An instance is placed on the Workbench person Person The instance is called person1 unless you person Person renamed it Blue Method Result iai If you right click the person1 instance you can an a run the method getName to see the value of returned oe the name instance variable ce 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net CME RIGHT RESERVED This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License
11. f the work See the screenshot on the next page or go directly to http creativecommons org licenses by nc sa 3 0 nz 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net SIME RIGHTS RESERVED This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License 4 Moodle NZ Net Page 2 of 19 Ee creative Y commons Attribution Noncommercial Share Alike 3 0 New Zealand aoe You are free ME a to Share to copy distribute and transmit the work to Remix to adapt the work Under the following conditions Attribution You must attribute the work in the manner specified by the author or licensor but notin any way that suggests that they endorse you or your use of the work Noncommercial You may not use this work for commercial purposes Share Alike If you alter transform or build upon this work you may distribute the resulting work only under the same or similar license to this one With the understanding that Waiver Any ofthe above conditions can be waived if you get permission from the copyright holder Other Rights In no way are any of the following rights affected by the license Your fair dealing or fair use rights The authors moral rights Rights other persons may have either in the work itself or in how the work is used such as publicity or privacy rights Notice F
12. he class is like the blueprint for a house you can t live in it yet or sell it or do anything with it Only when the constructor the builder is called and the instance created can you do 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOME RIGHTS RESERVED 4 Moodle NZ Net Page 16 of 19 something with the house To stretch the analogy a little further the same blueprint can be used to create many houses Anatomy of a class public class Person Class Name private String name Instance variable s public Person String newName Constructor setName newName public String getName Method with return value return name public void setName String newName Method with parameter name newName 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net SIME RIGHTS RESERVED This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License 4 Moodle NZ Net Page 17 of 19 rr Assignment 2 You can have more than one instance variable in a class pu ublic Class City instance variables first and last name private String name private String country To complete this class would require A constructor named C
13. he method setName is SLUICE declared as void public void setName String newName name newName The setName method also has a parameter inside the brackets following the method name the parameter is used to pass information into the method The keyword void indicates that it does not return any values Our class now has functions as well as a property parameter name is a property of a Person A method may have one or more getName can tell us what the name is parameters used to pass information to the method setName String newName can assign a value to name Our class now looks like this public class Person private String name public String getName return name public void setName String newName name newName Note We no longer assign the name directly to the instance variable we use the setName method This will allow us to have instances of the Person class with different names as we shall see First let s try compiling our class 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOMME EDIE RESERVED Moodle NZ Net Compiling Page 9 of 19 Click the compile button if a message appears near the bottom of the screen class compiled no syntax errors then that is excellent progress However you won t be
14. ick the download link From here you B ued TF can select the version you need for your operating system B o Step 1 Get Java home Check the system requirements right side of this page Make sure you have an appropriate version of Java installe about BlueJ Step 2 Download the installation package download lt a Latest official release documentation BlueJ Ai 2 5 0 extensions for Windows 5 1Mb bluejsetup 250 exe help amp Info nee AE a resources all other systems executable jar file 4 8 Mb blue 250 jar News lt If your browser tries to display the file instead of downloading it then use Save link as which you get when you right click or shift click the link Once you have loaded the file go_to the Installation Instructions Double click or drag to the applications folder the downloaded file to install BlueJ see the installation Instructions for further detailed information After it is installed you have an option to read the readme file and or launch BlueJ itself Starting with BlueJ Normally BlueJ will ask you to confirm which version of Java it is planning to use you may want to browse for a newer version if you have just installed the latest JDK Normally you can just accept the default 6 Bluel code When you get to the main screen you will see this window Project Edit looks View sHelp You can click the New Class button or right click control click in the window to get a popup menu of
15. imply The use of classes as definitions or blueprints is central to Object Oriented Programming it combines attributes or properties of abstract objects together with the actions or methods that define the object s behaviour As programs get more complex the OOP approach encapsulates properties and actions together in a single unit that can be built upon extended as necessary Example of a useful class We have actually already been using a more useful class without making it obvious String is a class that handles character strings A character string is a sequence of Unicode characters and there are various methods that can operate on these strings Some examples are Examples of String data HeLlo Woridt the cat sat nervously on the mat 221b Baker Strect Notice that the start and end of the St ring is delimited by quotation marks this allows us to pad the Strings with spaces if we want to s the cat sat nervously on the mat 7 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOME RIGHTS RESERVED 4 Moodle NZ Net Page 12 of 19 Examples of String methods dot notation String sl Hello World To call a method of String s2 ell an instance of a class place a dot System out printin equals s2
16. ity logically this would take two parameters Four methods that set and get both instance variables for example public String getName return name Create the class and a main program that outputs Paris 1S in France New York is in the USA for example Help with getting started on this assignment is included in the course Javadoc Comments When you create a new Class in BlueJ you see comments in a certain style As we said before the comments are for humans the compiler simply ignores them they don t get turned into bytecodes at all A Class that keep details of a person author Richard version 20090511 Pe It is an excellent habit to write comments for yourself and for people that want to help you with your programming The symbol is useful as it can help document your classes automatically 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOME RIGHTS RESERVED 4 Moodle NZ Net Page 18 of 19 Review of the concepts covered so far We have covered a lot of ground in a very short time so you are probably quite confused This is perfectly normal Computer programming attempts to model the real world in some way To do this it needs to represent objects Objects have properties and functions Think of people p
17. near the NetBeans nor the EE i fth l version op O e page Windows Vista users must use JDK 6 read this for details Sun Java Solari Communities My SDN Account Y Join SDH Details about system S m Sun Developer Network SDN regquirements are here v SUM p l APIs Downloads Products Support Training Participate ri a On MacOS X a recent Java SDK version is installed by default You do not need to install Java on this system SDN Home gt Java Technology gt Java SE gt Java SE Downloads It s time Download the complete platform and rugtime environment Get the JDK download Other downloads Previous Blue versions Source code Latest Release Next Release Early Access En eed eo ed cas iced ok oe a Ee hee a ot aces Sc JRE 6 Update 13 This release includes the highly anticipated 64 bit Java Plug In for 64 bit Select the Operating System you are using from the drop down list continue agree and download the file Read the installation instructions for your operating system and install the software 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOME RIGHTS RESERVED 4 Moodle NZ Net Page 4 of 19 To download BlueJ Go to the website http www bluej org and cl
18. oid main String args Create two Person instances Person myPerson new Person Person anotherPerson new Person if Sen their nanes myPerson setName Richard anotherPerson setName Huang I Get their names back String namel myPerson getName String name2 anotherPerson getName output a message System out printlin namel says hello to name2 Here we use the class blueprint to create two new object instances nyPerson and anotherPerson The lines in blue like Create two Person instances are comments these are for humans to be able to understand what is being done the compiler ignores all comments Notice that each instance has its own copy of the instance variable name myPerson setName Richard anotherPerson setName Huang Compile and run the program if when you get syntax errors refer back to the boxes on page 8 for help Assignment 1 Write a new class that prints out the message Vidyut Hiremath and Moubin Aboualzahab are friends or something similar 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOME RIGHTS RESERVED 4 Moodle NZ Net Page 14 of 19 The constructor The constructor is very important in Java so important that every class has one whether we put it there o
19. or any reuse or distribution you must make clear to others the license terms of this work The best way to do this is with a link to this web page 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net CME RIGHTS RESERVED This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License 4 Moodle NZ Net Page 3 of 19 Introducing Blue Integrated Development We will use the BlueJ Integrated Development Environment in this course You do not absolutely have to use this but things will be easier Environment of you do You will certainly need the JDK Java Development Kit as Or IDE this is a package a Ree of several tools to help If you don t have the JDK programmers It usually Or if you have an older version that does not work with the most consists of an editor some recent BlueJ release templates a debugger and l l a help system Notice that according to the System Requirements information on the BlueJ We will use many of these You need to have Java downloads page MacOS X features in BlueJ installed on your system users will not need to install the You need one of the following versions JDK JDK 6 or JDK 5 Note thst only the version The JDK can be installed from the location labeled JDK will work Not the one labeled JRE nor the one that comes with http java sun com javase downloads index jsp click the link
20. r not For our Person class Java quietly added a constructor method like this to the class public Person Like methods the constructor has braces to mark its start and end The constructor always has the same name as the class and never has any return value which distinguishes from a method which must always have one or void This constructor does nothing obvious however it does work behind the scenes such as allocating space inside the computer s RAM Random Access Memory to store any data belonging to an instance In the case of our Person class the String name needs to be stored somewhere the compiler can tag it for later use One useful use for a constructor is to initialize an instance variable Here is our class all nicely commented and laid out easily for humans to read pee A class to store a Person s name a public class Person private String name The name of the person x Constructor for objects of class name xf public Person String newName setName newName fee Returns the value of the name eJ public String getName return name xx Sets the value of the name a public void setName String newName name newName 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net SOME BIGHTE RESERVED This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License
21. s to use a constructor Methods A method is a block of statements sometimes only one that are put together under a name so that it can be used called when needed In a typical class there are methods for passing information to a class and for getting information out of the class SOME RIGHTS RESERVED 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License 4 Moodle NZ Net Page 8 of 19 public String getName return name public void setName String newName name newName method We choose the names for the methods and by convention they start with a a convenient way to lowercase letter Any internal words have capitals sometimes called CamelCase or BumpyText This makes longer names more readable for humans group statements together under a single name As for the class public means that other classes can use the method if it was private they could not The String keyword means that the getName return type method returns a String value to the place it was called from more later If This indicates that a the method has a return value then a return statement is also needed method returns a value we will ublic Strin etName eturn name a P 7 1ng g CEE j Shortly see this in Some methods do not have return types For example t
22. t of like writing publik 16 public Hello mf this code for now instead of public or 18 tie e ance e Oa even Public instead of 7 public BlueJ is useful for teaching as it does allow us to create our own templates similar to this one but we won t do that just yet Let s first change the comments and then enter the Java code as follows public class Person String name Richard This is a class the basis of pretty much everything in Java It isn t very useful yet You can cut and paste the code from the workbook if you like making sure to replace all of the existing code Or you can replace the bits that are different We will break everything down and define some jargon Getting the jargon right is important as otherwise we can t communicate when we need help from an expert Different experts have slightly different jargon unfortunately but we will try to be consistent Compilers and interpreters These are computer programs that turn ordinary text into machine code the stuff that computers actually run on the hardware The compiler in Java translates the source code our Person file into bytecodes a compact form which is in turn sent to an interpreter to be converted to machine code run on the computer If the compiler cannot turn the source code into bytecodes an error is given you will see a lot of these This is known as a syntax error i Many word processors use smart quotes if you
23. ter memory which we can use to store different types of data By convention an instance variable always starts with a lowercase letter In this case we are storing String data a string of characters alohabetic numeric or other symbols The name of the instance variable is name The private keyword means that it can only be accessed inside the Person class this is good practice It is quite common to think of an instance variable as a box with the difference that this type of box can only store one item of data Internal Computer Named instance reference Memory variable 01110110 name 01110111 PY 01111000 sd 01111001 balance 01111010 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License SOME RIGHTS RESERVED Moodle NZ Net As humans we find much easier to keep track of things that have names rather than numbers The computer tracks memory locations and everything else using binary machine code we think of it as 1s and Os Thus the ability to name memory locations is very useful Data Types You will notice that the instance variable name has the word String before it This denotes its data type the kind of data that can be stored in the variable Simple types are text and numbers Text values are called Strings Assignment Finally you se
24. will find ones not mentioned here as well SOME RIGHTS RESERVED 2010 Richard Jones 347 Ariki Street Karapiro Village Cambridge 3495 New Zealand richardnz moodlenz net This work is licensed under a Creative Commons Attribution Non commercial Share Alike 3 0 New Zealand License 4 Moodle NZ Net Page 10 of 19 Main programs There are a few ways to run computer programs written in Java We can output text at the console or we can build graphical user interfaces GUI s To start with we are just going to output text at the console when we have a few basic principles established we will do more interesting things with graphical objects like the boxes and buttons you are used to using To create a program in Java we will use another class one that has only one method called main public class SayHello public static void main String args Person myPerson new Person myFerso on setName Harolda String name myPerson getName SY Sctem OUL Dprintin Heillo rane We have seen something like the first two lines before We define a new class called SayHello We have no data members Then we have a special method called main Main always has this format for historical reasons which we won t go into here you can always Google it The next line constructs a new instance of class Person Person myPerson new Person The next line calls the method setName String newName inside class Person
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