Principles of Object Oriented Programming: Part 1: Abstraction and
Contents
1. Note that variables declared to be static exist even if no objects of that class exist Hence if no Employee objects exist then INUumEmployees 0 Because of this a means of accessing static data when there are no instances of the class is provided This is done by prefixing the static data item by the class name e g Employee iNumEmployees assuming the static data has been declared to be public and hence accessible outside of the class A method can also be declared as static Primarily this is of use to provide a means of accessing static data that has been declared private class data should rarely be declared public public class Employee System out println Num Employees Employee numPeople private static int 1NumEmployees public static int numPeople return 1NumEmployees Blocks of data and classes may also be declared to be static however this is of limited use Aside static is not a very good name It originated from C and referred to data that was allocated statically at compile time Whenever you see the keyword static in Java it is best to think once only per class Final data methods and classes The final keyword when applied to data makes it read only i e it cannot be changed Java 1 1 introduced the notion of a blank final variable i e a variable which is blank undefined at initialisation and can be assigned a value once before then becoming read only pri2. The finalize method should be used to free up any ae vocina Zay special resources which have been allocated 1 e file E handles db locks etc Thankfully Java s built in garage collection is very good and the finalize method is rarely needed Methods and Overloading Apart from the constructor and finalize methods a class consists of methods that operate upon class data Methods can be overloaded which is to say several different methods can have the same name provided they differ in terms of the parameters that they accept e g float getProduct float a float b ee float getProduct float a int b eee int getProduct int a int b teeny Note that an overloaded method cannot have several methods that share the same argument list but differ only in their return value as there is no means of determining which method should be called e g int CalculateWage String name float CalculateWage String name PAGE 25 HN A Computer Programming II MECIURE Data modifiers Below two useful data modifiers are briefly explored data modifiers alter how items are accessed and used Static data methods and classes By declaring a class data item to be static it signifies that all instances of that class share the static data item in common This is useful in a number of situations t For example consider an Employee class where it is important to know how many employees there are e g as sho
3. differing in terms of the parameters they accept Constructor Constructors never return any values public lt classname gt ee Constructor public lt classname gt lt arglist gt An example showing two constructors public class Counter one the default no argument l Pn a constructor and the other accepting two private int iCounter iMaximum integers follows l public Counter iCounter 0 iMaximum 100 public Counter int initCount int initMax iCounter initCount setMaximum initMax public void setMaximum int iMax iMaximum iMax PAGE 24 N Computer Programming II ATURE E The procedure when an object is created follows e g SomeClass myObj new SomeClass 1 Sufficient memory to store the class data is allocated This memory is then cleared te all values set to null 0 etc this guarantees starting values for class data 2 The class constructor is called Note the constructors of parent classes are always called first either explicitly with Super or implicitly The rest of the constructor s code is then called Destructors the finalize method The finalize method should one exist is guaranteed to be called before the object is deleted The finalize method The finalize method accepts no parameters nor does it public class lt classname gt return any value There can be only one finalize method for a class Destructor
4. TH EET I Computer Programming II PERE Contents of lecture 2 1 Principles of Object Oriented Programming e Object Oriented OO principles e Basics of OOP using Java 2 OOP Java Style e Classes and methods e Static and Final keywords e Objects and References within Java Principles of Object Oriented Programming Part 1 Abstraction and Encapsulation Object Oriented Programming Simple Idea Object oriented programming OOP is based on a few simple ideas all of which make good common sense From this point of view OOP is nothing more than a straightforward concept which is 100 true However two factors one of which 1s artificial contrive to make OOP more difficult to understand They are e Surfeit of Zen problem In order to fully appreciate any one part you need to understand most of the other parts e Use of complex technical sounding terminology which does not really encapsulate the simplicity of the principles behind OO programming See example below Example Object Oriented Programming is characterised by inheritance and dynamic binding C supports inheritance through class derivation Dynamic binding is provided by virtual class functions Virtual functions provide a method of encapsulating the implementation details of an inheritance hierarchy Note that the example shown above drawn from an actual textbook is technically accurate however it is also utterly useless
5. an existing language provided data type which might not be appropriate Obviously the expense of this additional flexibility and modeling power is the need for the programmer to accurately model the problem within the language a process that can be time consuming Indeed one of the key challenges of object oriented programming is the creation of a one to one mapping between the objects and types found within the problem and those created by the programmer within the developed software The process of object oriented software development can be considered as initially modeling the problem and then finally so ving the problem both this lecture and the next lecture aim to show you how you can model problems using Java s object oriented capabilities Object oriented languages can be summarised by the following five key points 1 Everything is an object an object being an entity that can store data and can perform certain types of operation i e object black box that can store stuff and do stuff 2 An object oriented program involves a bunch of objects telling each other what to do by sending messages to one another i e the program consists of objects working in harmony to accomplish some task Each object can be formed as an agglomeration of other objects whilst still being regarded as a single entity from the outside e g complexity can be built up in a program while hiding it behind the simplicity of objects 3 Every obj
6. and finally how instances of the classes should interact to solve the problem i e you firstly model the problem and then secondly solve the problem You may have noticed that there has been no coverage on how to identify which objects need to be modeled given a problem description Unfortunately as with determining relevant data and functionality the ability to select appropriate objects is something that improves through the accumulation of experience and not something that can be profitably taught Step 3 will be explored within the next lecture Step 4 can only really be built up from accumulated practical experience 1 e by actively solving problems PAGE 23 EET N Computer Programming II padi Object oriented Programming Java Style Class declarations within Java Within the Java programming language the class syntax definition is as follows Class syntax public class lt classname gt where lt classname gt is the name given to the class Java requires that each class be stored in the corresponding Java file lt classname gt java Constructors Each class has a constructor that is called whenever an instance of that class is created If the class does not explicitly define a constructor then default methods are invoked which might range from doing nothing to calling the relevant constructor of a superclass public class lt classname gt Several constructors can be defined for a class
7. are accessible from classes in the same package 1 e directory only A class can be given either package access or public access int iSum compute Members are accessible in the package and in subclasses of this class Note that protected is less protected that the default package access protected int iSum protected compute Members are accessible anywhere the class is accessible A class can be given either package or public access public int iSum public compute co In Java package access broadly equates to public access between all Java files in the same directory e g Directory C SamsProject 3 files Control java Process java Printout java Directory C JillsProject 1 file Project java Assume Printout java contains a method PrintResults which has package access If so PrintResults can freely be called from within either the Process or Control classes However it cannot be called from Project as Project java is found within a different directory Note in general data should not be declared as public instead helper read write methods should be provided PAGE 28 PUTER C Computer Programming Il aaa Objects within Java Java uses indirect addressing when creating or manipulating objects instantiations of a class Otherwise stated you can only manipulate an object through a reference variable What does this mean Basically that you never deal directly with an object
8. e programmer s cognitive resources to tackle larger and more difficult problems which would have been prohibitively complex to write using only assembly language Solving different types of problem Non imperative languages Imperative languages such as assembly or C require the programmer to think about how to solve the problem in terms of the structure and capabilities of the computer Hence it is not sufficient for the programmer to know how to solve the problem they must also be capable of expressing a solution in terms of basic actions that a computer can perform e g adding looping manipulating data etc Obviously for certain problems this may not always be an easy translation As a solution to this difficulty a number of languages were developed that permit the programmer to more directly model the problem i e at a conceptually higher level with the compiler performing the translation into code that can be run on a CPU Examples of such languages include LISP APL and PROLOG which each present different views of the world i e particular approaches as to how problem solutions should be expressed For example LISP assumes all problems ultimately boil down to lists and list manipulation APL assumes all problems are mathematically algorithmic in nature and PROLOG assumes all problems can be expressed in terms of rules and decisions chains Each of the above non imperative languages enables the programmer to elegantly expres
9. ect has a type 1 e belongs to a particular class 4 All objects of a particular type can receive the same messages and operate in a similar manner Aside Aristotle was probably the first person to carefully study the concept of type he spoke of the class of fishes and the class of birds The idea that all objects whilst being unique are also part of a class of objects that have characteristics and behaviors in common was used directly in the first object oriented language Simula 67 which introduced the keyword class Aside Simula as the name suggests was created for developing simulations such as the classic bank teller program where you have a bunch of tellers customers accounts transactions units of money etc However whilst Simula was the first object oriented language it is Smalltalk that holds the honor of being the first successful and widely known OO language in part Java is based on Smalltalk Pace 19 T Computer Programming Il PECIURE Differences between classes and objects Objects and classes can be defined and differentiated as follows Class A template or type describing the fields data and methods that are grouped together to represent something A class is also known as a non primitive type more on this later Object A variable constructed according to a class template able to hold values in its datafields and able to have its methods called There are typically ma
10. ed or alternatively an important piece of functionality may be omitted The programmer goes about determining which functionality is important in much the same way as they determine which data items are important OOP Principle 2 Encapsulation Encapsulation is one step beyond abstraction Whilst abstraction involves reducing a real world entity to its essential defining characteristics encapsulation extends this idea by also modeling and linking the functionality of that entity Consider the following data items and operations Multiplication Truthhood Multiplication Shifting Addition Multiplication Addition Boolean Encapsulation links the data to the operations that can be performed upon the data For example it makes sense to determine the truth hood of a Boolean variable however it does not make sense to multiply two Boolean variables Float together hence encapsulation ensures data is only used in an appropriate manner PAGE 22 N Computer Programming II ATURE Encapsulation gives classes OOP makes use of encapsulation to enforce the integrity of a type i e to make sure data is used in an appropriate manner by preventing programmers from accessing data in a non intended manner e g asking if an Integer is true or false etc Through encapsulation only a predetermined group of functions can access the data The collective term for datatypes and operations methods bundled togethe
11. em out println iTest The above code will return a value of 4 This value is changed to 5 by the method However once the addOne method finishes the value is discarded PAGE 31 Method et Computer Programming Il SERTORE E After this lecture you should explore the second practical pack which should enable you to investigate the material in this lecture Learning Outcomes Once you have explored and reflected upon the material presented within this lecture and the practical pack you should e Understanding the general methodology encapsulated by object oriented programming e Understand the principles behind both Data Abstraction and Encapsulation and be able to apply these principles to extract relevant object data and object functionality given a straightforward problem description e Have knowledge of the syntax and functionality on offer within Java when constructing classes and be able to successfully employ Java to create appropriate classes given a straightforward class design e Understand the differences between classes objects and references within Java including primitive and non primitive data items and the consequences thereof when comparing or passing objects references More comprehensive details can be found in the CSC735 Learning Outcomes document PAGE 32
12. for teaching the fundamentals of OOP Hopefully the coverage of OOP and Java within this course will try to avoid unnecessary technical baggage Pace 16 T Computer Programming Il MECIURE The Development of Object Oriented Languages A fundamental and necessary part of programming is abstraction 1 e taking an idea concept or object and expressing it in terms of the constructs provided by the programming language using abstraction a person might be represented as a bundle of variables and methods e g fHeight fWeight HairColour eat Food item sleep run etc As programming languages have evolved they have offered more powerful and sophisticated forms of abstraction In this section we will briefly explore how the forms of abstraction have evolved leading to the introduction of object oriented approaches Machine Language and Assembly Language We start our brief coverage with machine language which can be viewed as 10101111 directly talking to the processor in the language it understands i e in binary 01010011 opcodes instructions 11011001 Understandably programmers found machine language incredibly cumbersome 11110110 to use the strings of 1 s and 0 s that go together to form the op codes of machine language are not particularly easy to remember or understand e g the sequences 10111010 and 10101011 look somewhat similar to one another and their action when processed by the CPU is
13. g due care that the names are always correctly accessed Later imperative languages such as C enabled the programmer to declare variables with the compiler dealing with how the data is allocated and represented in memory and further ensuring that the data item is correctly accessed Pace 17 4 Computer Programming II MEGTURE Adding in assembly Adding in C In assembly we have mov loc al Move specified memory address contents into CPU register add loc2 al Add specified memory address contents to CPU register mov al res Move CPU register contents into the specified memory address In C we have result vall val2 Add value of variable vall to value of variable val2 storing result in specified variable Notice that the C code is considerably more compact and readily comprehended This is in part due to it syntactical similarity to normal mathematical notation and the use of objects instead of memory offsets In the above examples we are not really adding anything new in terms of programming functionality instead we are simply providing higher level abstractions that mean the programmer can think about the problem at a higher level without worrying about how objects loops etc will be expressed in terms of machine code i e the compiler does some of the work for the programmer Apart from quickening development times and reducing the number of program bugs such languages free up th
14. instead you always deal with an intermediary to the object Consider the following Java statements assuming we have a Car class already defined 1 Car tomCar 2 tomCar new Car Ford 3 Car paulCar 4 paulCar tomCar 5 tomChar new Car Mazda The code fragment represents a fictional case where Tom originally has a Ford car which he then shares with Paul Finally Tom gets a new car a Mazda When executed the above code will result in the following data items being created tomCar Create a new reference tomCar initially it refers to no objet Name ford Model Milage tomCar is assigned to a new Car object tomCar new Car Ford tomCar Car1 object ae Name Ford Model Milage paulCar Create a new reference paulCar eee tomCar Car1 object ae Name Ford Model Milage paulCar Assign paulCar to tomCar Car2 object Name Mazda Model Milage Car1 object Name Ford Model Milage PAGE 29 a Computer Programming Il MECTURE When a variable of any class type is declared it is simply a reference variable that can hold a pointer to an instance of the relevant class This is a process known as indirect addressing i e tomCar is an intermediary that holds a pointer to a Car object As such this has important consequences when it comes to comparing objects or passing objects into methods as will be shown Aside Classes are an example of non prim
15. itive data Java also contains primitive data types such as int float boolean char etc there are a total of 8 primitive types A primitive type is defined as not being composed of any other types whereas non primitive classes contain other data items and methods etc As primitive types are not objects they do not use indirect addressing instead they use direct addressing Compare and contrast Non primitive data Primitive data Integer iValue int iValue 2 iValue new Integer 2 iValue Integer1 object Comparing objects Consider the following code fragment String first new String Hello String second new String Hello Which is realised as follows Str1 object iai Str2 object iii Consider the statement if first second Y will this be evaluated as true or false In this case it will be evaluated as false the reason being that the statement is interpreted as meaning Does the first reference refer to the same object as the second reference Evidently they do not refer to the same object hence the statement evaluates as false This is a common source of program errors It is important to be aware if a comparison is to be made between references i e pointers to objects or between objects i e the data internal to an object For this example in order to compare the object s contents we should use the String method equals e g if first equals Ssecond which compare
16. l LECTURE 2 Abstraction and the programmer The task of the programmer given a problem is to determine what data needs to be extracted 1 e abstracted in order to adequately design and ultimately code a solution Normally with a good problem description and or good customer communication this process is relatively painless It is difficult to precisely describe how a programmer can determine which data items are important In part it involves having a clear understanding of the problem Equally it involves being able to see ahead to how the problem might be modeled solved Given such understandings it is normally possible to determine which data items will be needed to model the problem However please note the ability to abstract relevant data is a skill that is largely not taught but rather one that is refined through experience Failure at this stage can have two possible forms e Something unnecessary was abstracted 1 e some data is redundant This may or may not be a problem but it is bad design e Something needed was not abstracted This is a more serious problem usually discovered later in the design or coding stages and entails that the design needs to be changed to incorporate the missing data item code changed etc Functional Abstraction The process of modeling functionality suffers from the same pitfalls and dangers as found when abstracting the defining characteristics 1 e unnecessary functionality may be extract
17. membered as A PIE In what follows each of these four concepts will be introduced and further explored Aside Whilst A PIE may be easy to remember the principles build upon one another in the following order Abstraction Encapsulation Inheritance and Polymorphism Aside As mentioned OOP is not a new idea in fact it s very old by computing standards The first object oriented programming language was Simula 67 produced over 30 years ago OOP Principle 1 Abstraction Data Abstraction In order to process something from the real world we have to extract the essential characteristics of that object Consider the following example Registration Vehicle Identification Number License plate Current Owner Tax due date Car description Service history Petrol mileage history License plate Work description Billing info Owner Data abstraction can be viewed as the process of refining away the unimportant details of an object so that only the useful characteristics that define it remain Evidently this is task specific For example depending on how a car is viewed e g in terms of something to be registered or alternatively something to be repaired etc different sets of characteristics will emerge as being important Formulated as such abstraction is a very simple concept and one which 1s integral to all types of computer programming object oriented or not PAGE 21 Sel TER H Computer Programming I
18. not readily apparent Assembly language was in part introduced to overcome the difficulties associated with using machine code It was formed on the idea of assigning more meaningful labels to each machine language instruction hence the sequences 10111010 and 10101011 might be labeled as ADD and SUB Assembly language can be viewed as an abstracted form of machine language as it adds an extra layer of meaning by assigning textual labels to each op code However it 1s important to recognise that assembly language did not introduce any new functionality it was solely designed to make the task of programming more straightforward for the programmer Later Imperative Languages Languages which can be considered as superceding assembly language 1 e imperative languages such as Fortran BASIC C etc all extend assembly language not by offering any significantly new functionality but by introducing higher level concepts i e abstractions that significantly improved the ability of the programmer to express his or her ideas in code Some examples follow Data types in assembly vs Data types in C In assembly language no distinction is made between program code and data it s all just bytes in memory Hence if we wish to write an assembly language program that can store several names e g Bob Bill and Sue it will be necessary to set aside a block of memory space in which to store the ASCII representation of each name takin
19. ny objects for any given class We say an object is an instance of a class or belongs to a class Diagrammatically the relationship between objects and classes can be viewed as follows 4 Interface Class Ws lt gt User manual CD Player i aie blueprints 4 Objects CD Players In order to use an object all that 1s needed is knowledge of its interface which provides details of methods and data available to the programmer 1 e those declared as public etc Java is a hybrid language Languages such as Java and C are considered as hybrid languages in that they permit it would be more accurate to say require multiple programming styles For example C extended C an imperative language by introducing object oriented ideas Hence when writing a C program the programmer must think in terms of the classes and objects needed to solve the problem but ultimately the classes must be expressed written in terms of basic CPU operations i e in terms of loops data manipulation etc The same is true of Java which drew it inspiration from a number of languages including C and Smalltalk Pace 20 nN i Computer Programming Il CECIERE The first half of the pie Abstraction and Encapsulation Principle ingredients of OOP There are four principle concepts upon which object oriented design and programming rest They are Abstraction Polymorphism Inheritance and Encapsulation 1 e easily re
20. r with access restrictions public private etc is a class Classes are useful for the following reasons e Classes can be used to model many real world entities thereby permitting object oriented languages to model a wide range of problems e Bundling data and functionality together produces self contained units which are more maintainable reusable and deployable e By enforcing the idea of encapsulation upon the programmer object oriented languages make it more difficult for the programmer to produce bad code Object Oriented Design An Overview Broadly speaking OOD Object Oriented Design can be broken down into the following processes Step 1 Identify all objects arising from the problem description Step 2 Initially model the problem by defining classes for each object in terms of the data needed to represent the object and the operations that can be performed on the object abstraction and encapsulation Step 3 Further model the problem by defining relationships between classes inheritance and polymorphism Step 4 Devise an algorithm that instantiates a number of objects from the defined classes and shows how these objects can interact with one another to solve the problem The above process is somewhat abstract however it simply amounts to working out what classes are needed to model the problem what data and methods these classes should contain how the classes should relate to one another
21. s solutions when tackling the particular class of problem that they aim to model the programmer does not need to worry about how the underlying implementation will be expressed in machine language Whilst these languages often provide higher level abstractions beyond standard imperative languages they are limited in that design becomes cumbersome and awkward when tackling problems that fall outside of the modelled domain Pace 18 17 Computer Programming II MEGIURE Having the entire pie and eating it So far we have seen that assembly language provides a simple direct abstraction of machine code designed to be more readily usable by the programmer Later imperative languages such as C introduced higher level notions such as user defined variables various forms of looping etc Languages such as PROLOG deviated from standard W imperative languages by offering an abstraction particularly suited to a certain class of problem Object oriented languages extend this idea one step further by providing higher level abstractions without being restricted to a certain class of problem This is accomplished by providing capabilities within the language e g classes etc whereby the programmer can define a higher level abstraction tailored to the problem at hand The basic idea is that the program can be adapted to the problem by defining new types of object and relationship specific to that problem rather then being forced to use
22. s the String s contents The above principle also impinges upon multiple references to the same object e g Pace 30 EN 4 Computer Programming Il _ Obj A new Obj ae coarse A Obj1 object JD A P l Title B setTitle B A getTitle This returns B Evidently changing an object s data through one particular reference will affect the data that is retrieved by another reference that points to the same object Passing references into methods Consider the following code fragment Emplyee tom new Employee Tom processEmployee tom Whenever Java calls the processEmployee method it passes a copy of the reference to the object 1 e Emp1 object Name Tom Hence any changes made to the Employee object s data inside the processEmployee method remain after the method has returned As such this provides an elegant means of changing an object s data within a method However the programmer must be mindful not to inadvertently change an object s data from within the method There is one cravat to the above outlined procedure it only applies to non primitive data i e that which is accessed through a reference When primitive data e g int float etc is passed into a method a copy of the actual data is made and any changes within the method are not reflected when the method returns e g int iTest 4 public addOne int iValue addOne iTest iValuet Syst
23. vate final iUserlnputValue iUserlnputValue getUserInput Valid iUserlnputValue Invalid now Blank final variables are useful when the data value cannot be obtained before run time or is too complex to pre calculate A method or class that is defined to be final cannot be extended or modified by any sub classes that inherit from the method or class This is useful to ensure that the functionality of a method cannot be modified or to ensure that a class cannot be extended via inheritance Aside final also has a number of more subtle uses If a method or class is defined to be final then the Java Virtual Machine can execute method calls faster this has to do with polymorphism where Java has to check the type of each object If declared final Java does not need to perform the check For example the Java Math class is primarily declared to be final for this reason methods within the Math class are also static hence no instances of the class need to exist to call the method e g Math pow etc PAGE 27 Computer Programming Il LECTURE 2 Access Modifiers A summary of the different access modifiers follows none often called package access protected Members are not accessible outside the class Making a constructor private prevents the class from being instantiated Making a method private means that it can only be called from within the class private int iSum private compute Members
24. wn opposite public class Employee private static int iNumEmployees public Person iNumEmployees Based on the code every time a new instance of the Employee class is instantiated the constructor public static int numPeople increments iNumEmpolyees a static dataitem return iNumEmployees shared across all instances of Employee The following is a bit dangerous Anytime an Employee object is deleted by Java s public finalize garbage collection the finalize method ensures the iNumEmployees iNumEmployees variable is decremented However depending upon the CPU load it may take a long time before the Java VM gets around to deleting the object and hence invoking the destructor Static data items can be visualized as follows public class Employee public static int iNumEmployees private String name private String dept private int iPay lt Methods gt iNumEmployees 3 po name Dave name Sam name Mary dept Support dept Design dept Boss iPay 14500 iPay 21400 iPay 32500 PAGE 26 nN Computer Programming II MECTURE Each instance of Employee holds a certain amount of information in particular each Employee object contains a unique name dept and iPay variable However notice that the static iNumEmployees is not stored by any one Employee object rather it is global across all instances of that class
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