Classes And Methods
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The document discusses key concepts in object-oriented programming including classes, methods, interfaces, properties, and nested classes. It provides examples of class definitions in various languages like Java, C++, and C# to illustrate concepts like encapsulation, visibility modifiers, constructors, and accessor/mutator methods. It also covers topics like separation of definition and implementation, interfaces, properties, and class data fields.
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Classes And Methods
- 1. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested CLASSES AND METHODS Muhammad Adil Raja Roaming Researchers, Inc. cbnd April 14, 2015
- 2. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested OUTLINE I 1 INTRODUCTION 2 ENCAPSULATION AND INSTANTIATION 3 CLASS DEFINITIONS 4 METHODS 5 INTERFACES IN JAVA 6 PROPERTIES 7 INNER OR NESTED CLASSES 8 CLASS DATA FIELDS 9 SUMMARY 10 REFERENCES
- 3. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested INTRODUCTION I Chapters 4 and 5 discuss two sides of OOP. Chapter 4 discusses the static, compile time representation of object-oriented programs. Chapter 5 discusses the dynamic, run time behavior Both are important, and both chapters should be understood before you begin further investigation of object-oriented programming.
- 4. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested SAME IDEAS DIFFERENT TERMS I All OOP languages have the following concepts, although the terms they use may differ: CLASSES: object type, factory object. INSTANCES: objects MESSAGE PASSING: method lookup, member function invocation, method binding. METHODS: member function, method function INHERITANCE: subclassing
- 5. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested ENCAPSULATION AND INSTANTIATION I ENCAPSULATION The purposeful hiding of information, thereby reducing the amount of details that need to be remembered/communicated among programmers. A SERVICE VIEW The ability to characterise an object by the service it provides, without knowing how it performs its task. INSTANTIATION The ability to create multiple instances of an abstraction.
- 6. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested INTERNAL AND EXTERNAL VIEWS I TWO VIEWS OF SOFTWARE Encapsulation means there are two views of the same system. The outside, or service view, describes what an object does. The inside, or implementation view, describes how it does it.
- 7. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested BEHAVIOR AND STATE I A class can also be viewed as a combination of behavior and state. BEHAVIOR: The actions that an instance can perform in response to a request. Implemented by methods. STATE: The data that an object must maintain in order to successfully complete its behavior. Stored in instance variables (also known as data members, or data fields).
- 8. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested CLASS DEFINITIONS I We will use as a running example the class definition for a playing card abstraction, and show how this appears in several languages. Languages include Java, C++, C#, Delphi Pascal, Apple Pascal, Ruby, Python, Eiffel, Objective-C and Smalltalk.
- 9. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested CLASS DEFINITIONS II A TYPICAL EXAMPLE, CLASS DEFINITION IN C++ class PlayingCard { public : enum Suits {Spade , Diamond , Club , Heart } ; Suits s u i t ( ) { return suitValue ; } int rank ( ) { return rankValue ; } private : Suits suitValue ; int rankValue ; } ;
- 10. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested CLASS DEFINITIONS III Note syntax for methods, data fields, and visibility modifiers. (Will see more on syntax later).
- 11. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested VISIBILITY MODIFIERS I The terms public and private are used to differentiate the internal and external aspects of a class. Public features can be seen and manipulated by anybody – they are the external (interface or service) view. Private features can be manipuated only within a class. They are the internal (implementation) view. Typically methods are public and data fields are private, but either can be placed in either category.
- 12. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested VISIBILITY MODIFIERS II A C-SHARP CLASS DEFINITION enum Suits {Spade , Diamond , Club , Heart } ; class PlayingCard { public Suits s u i t ( ) { return suitValue ; } public int rank ( ) { return rankValue ; } private Suits suitValue ; private int rankValue ; } C# class definitions have minor differences, no semicolon at end, enum cannot be nested inside class, and visibility modifiers are applied to methods and data fields individually.
- 13. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested VISIBILITY MODIFIERS III JAVA CLASS DEFINITION class PlayingCard { public int s u i t ( ) { return suitValue ; } public int rank ( ) { return rankValue ; } private int suitValue ; private int rankValue ; public static f i n a l int Spade = 1; public static f i n a l int Diamond = 2; public static f i n a l int Club = 3; public static f i n a l int Heart = 4; }
- 14. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested VISIBILITY MODIFIERS IV Java also applies visibility modifiers to each item indivually. Does not have enumerated data types, uses symbolic constants instead.
- 15. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested STATIC AND FINAL I Notice how symbolic constants are defined in Java: static means that all instance share the same value. One per class. Similar meaning in many languages. final is Java specific, and means it will not be reassigned. (C++ has const keyword that is similar, although not exactly the same). STATIC AND FINAL public static f i n a l int Spade = 1; public static f i n a l int Diamond = 2; public static f i n a l int Club = 3; public static f i n a l int Heart = 4;
- 16. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested METHODS I Although syntax will differ depending upon language, all methods have the following: A name that will be matched to a message to determine when the method should be executed. A signature, which is the combination of return type and argument types. Methods with the same name can be distinguished by different signatures. A body, which is the code that will be executed when the method is invoked in response to a message.
- 17. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested METHODS II AN EXAMPLE FROM C-SHARP class PlayingCard { / / constructor , i n i t i a l i z e new playing card public PlayingCard ( Suits is , int i r ) { s u i t = i s ; rank = i r ; faceUp = true ; } / / operations on a playing card public boolean isFaceUp ( ) { return faceUp ; } public int rank ( ) { return rankValue ; } public Suits s u i t ( ) { return suitValue ; } public void setFaceUp ( boolean up ) { faceUp = up ; } public void f l i p ( ) { setFaceUp ( ! faceUp ) ; } public Color color ( ) { i f ( ( s u i t ( ) == Suits . Diamond ) | | ( s u i t ( ) == Suits . Heart ) ) return Color . Red ; return Color . Black ; } / / private data values private Suits suitValue ; private int rankValue ; private boolean faceUp ; }
- 18. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested METHODS III CONSTRUCTOR class PlayingCard { / / constructor , i n i t i a l i z e new playing card public PlayingCard ( Suits is , int i r ) { s u i t = i s ; rank = i r ; faceUp = true ; } . . . } A constructor is a method that is used to initialize a newly constructed object. In C++, Java, C# and many other languages. It has the same name as the class.
- 19. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested METHODS IV ACCESSOR (OR GETTER) METHODS class PlayingCard { . . . / / operations on a playing card public int rank ( ) { return rankValue ; } public Suits s u i t ( ) { return suitValue ; } . . . private int rankValue ; } An accessor (or getter) is a method that simply returns an internal data value: Why Use an Accessor? There are many reasons why an accessor is preferable to providing direct access to a data field. You can make the data field read-only.
- 20. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested METHODS V It provides better documentation that the data field is accessible It makes it easier to later change the access behavior (count number of accesses, whatever). Some conventions encourage the use of a name that begins with get, (as in getRank()), but this is not universally followed. SETTERS (OR MUTATORS) class PlayingCard { / / operations on a playing card public void setFaceUp ( boolean up ) { faceUp = up ; } . . . / / private data values private boolean faceUp ; }
- 21. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested METHODS VI A setter (sometimes called a mutator method) is a method that is used to change the state of an object. Mutators are less common than accessors, but reasons for using are similar. Constant data fields. Some languages allow data fields to be declared as constant (const modifier in C++, final in Java, other languages have other conventions). Constant data fields can be declared as public, since they cannot be changed.
- 22. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested ORDER OF METHODS I For the most part, languages don’t care about the order that methods are declared. Here are some guidelines: List important topics first. Constructors are generally very important, list them first. Put public features before private ones. Break long lists into groups List items in alphabetical order to make it easier to search. Remember that class definitions will often be read by people other than the original programmer. Remember the reader, and make it easy for them.
- 23. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested SEPARATION OF DEFINITION AND IMPLEMENTATION I In some languages (such as C++ or Object Pascal) the definition of a method can be separated from its implementation. They may even be in a different file: SEPARATION OF DEFINITION AND IMPLEMENTATION class PlayingCard { public : . . . Colors color ( ) ; . . . } ; PlayingCard : : Colors PlayingCard : : color ( ) { / / return the face color of a playing card i f ( ( s u i t == Diamond ) | | ( s u i t == Heart ) ) return Red ; return Black ; }
- 24. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested SEPARATION OF DEFINITION AND IMPLEMENTATION II Notice the need for fully-qualified names.
- 25. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested CONSIDERATIONS IN METHOD DEFINITIONS I In C++ you have a choice to define a method in the class interface, or separately in an implementation file. How do you decide? Readability. Only put very small methods in the class definition, so that it is easier to read. Semantics. Methods defined in class interface may (at the descretion of the compiler) be expanded in-line. Another reason for only defining very small methods this way.
- 26. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested PREPARING FOR CHANGE I An interface is like a class, but it provides no implementation. Later, another class can declare that it supports the interface, and it must then give an implementation. AN INTERFACE IN JAVA public interface Storing { void writeOut ( Stream s ) ; void readFrom ( Stream s ) ; } ; public class BitImage implements Storing { void writeOut ( Stream s ) { / / . . . } void readFrom ( Stream s ) { / / . . . } } ;
- 27. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested PREPARING FOR CHANGE II We will have much more to say about interfaces later after we discuss inheritance.
- 28. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested PROPERTIES I Properties are a way to define getters and setters, but allow them to be used as if they were simple assignments and expressions: PROPERTIES w r i t e l n ( ’ rank i s ’ , aCard . rank ) ; (∗ rank i s property of card ∗) aCard . rank = 5; (∗ changing the rank property ∗)
- 29. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested PROPERTIES II PROPERTIES IN C-SHARP public class PlayingCard { public int rank { get { return rankValue ; } set { rankValue = value ; } } . . . private int rankValue ; } Omitting a set makes it read-only, omitting a get makes it write-only.
- 30. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested SOFTWARE COMPONENTS I Some languages (C++ or Java) allow a class definition to be given inside another class definition. Whether the inner class can access features of the outer class is different in different languages. INNER CLASSES IN JAVA class LinkedList { . . . private class Link { / / inner class public int value ; public Link next ; } }
- 31. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested CLASSE DATA FIELDS I Idea is that all instances of a class can share a common data field. Simple idea, but how to resolve the following paradox. All instances have the same behavior: Either they all initialize the common area, which seems bad, or Nobody initializes the common area, which is also bad. Different languages use a variety of mechanisms to get around this. See text for details.
- 32. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested SUMMARY I In this chapter we have examined the static, or compile time features of classes: The syntax used for class definition. The meaning of visibility modifiers (public and private). The syntax used for method definition. Accessor or getter methods, and mutator or setter methods. Variations on class themes. Interfaces. Properties. Nested classes. Class data fields.
- 33. Introduction Encapsulation and Instantiation Class Definitions Methods Interfaces in Java Properties Inner or Nested REFERENCES I Images and content for developing these slides have been taken from the follwoing book. An Introduction to Object Oriented Programming, Timothy Budd. This presentation is developed using Beamer: Frankfurt, spruce.