1. 1 COSC2007 Data Structures II Chapter 9 Class Relationships

2. Topics Inheritance & Polymorphism Abstract Class Class Relationship

3. Object Oriented Programming Main components of OOP Inheritance (Base/Parent Class – Sub/Child Class) Encapsulation Polymorphism Inheritance and polymorphism are the subject of chapter 9

4. Object Oriented Programming Main components of OOP Inheritance (Base/Parent Class – Sub/Child Class) Encapsulation Polymorphism Inheritance and polymorphism are the subject of chapter 9

5. Inheritance Inheritance example Example: Class Ball is derived from class Sphere Ball inherits Sphere's attributes and methods Ball theName: String equals(): boolean displayStatistics (): void Sphere theRadius: double radius (): double area(): double equals(): boolean displayStatistics (): void

6. Inheritance Class Ball extends Sphere Note that the Ball class has two data fields: 1) theRadius //inherited from Sphere Since theRadius is private in Sphere, Ball can only reference it through methods 2) theName //new to the Ball class What methods does Ball have? Ball Sphere

7. Inheritance Class Ball extends Sphere A closer look at Ball's constructors The default constructor in class Ball here automatically calls the default superclass constructor, then sets theName public Ball () { setName (“unknown”); } How about non-default constructor? Ball Sphere

8. Inheritance Method equals is overridden The method equals in class Sphere public boolean equals(Object rhs) { if (rhs instanceof Sphere) && (theRadius = = ((Sphere)rhs).theRadius)) return true; else return false; } // end equals

9. Inheritance Method equals is overridden In the class Ball, three conditions must be met public boolean equals(Object rhs) { return ( (rhs instanceof Ball) && (radius() == ((Ball)rhs).radius()) && (theName.compareTo(((Ball)rhs).theName)==0) ); }

10. Inheritance displayStatistics is also overridden Sphere's displayStatistics public void displayStatistics() { System.out.println("\nRadius = " + radius() + "\nDiameter = " + diameter() +"\nCircumference = " + circumference() + "\nArea = " + area() + "\nVolume = " + volume()); } // end Sphere's displayStatistics In the class Ball public void displayStatistics() { System.out.print("\nStatistics for a "+ name()); super.displayStatistics( ); } // end Ball's displayStatistics;

11. Inheritance Overridden methods Which method will the compiler use for these calls to displayStaticis()? Sphere mySphere=new Sphere(); Ball myBall=new Ball (); myBall.displayStaticis(); //what will print? mySphere.displayStatics(); //what will print?

12. Inheritance How overriding works When a method is called by an object, it is compared against the object’s class’s own method signatures The signature is the method name plus the argument list If a match is found, that method is invoked If a match is not found, the signature is compared against the superclass’s method signature. This process is repeated until a match is found. A match is guaranteed because otherwise the Java compiler would give an error.

13. Inheritance Dynamic binding Sometimes the compiler cannot decide which version of an overridden method to use The decision is then delayed until run time; until the program is executed Example: Ball myBall = new Ball(1.25, “golfball”); Sphere mySphere = myBall; mySphere.displayStatistics( ); Which displayStatistics method is used? polymorphism

14. Abstract Classes Suppose we have classes for shapes: Rectangle, Square, Triangle, etc. All these shape classes likely have methods for computing area() and circumference(). Would be nice to have a generic Shape class to help organize these classes - make it simpler to define an array of different shapes, for example. Make shape an abstract class. All of Shapes subclasses must implement the area() method, or be abstract. Each of these methods are implemented differently.

15. The abstract Shape class public abstract class Shape { public abstract double area(); public abstract double circumference(); } Shape CircleRect Square The Shape Hierarchy

16. Can now write code such as... Shape [] shapes = new Shape[3]; shapes[0] = new Circle(2.0); shapes[1] = new Rectangle(10.,20.); shapes[2] = new Circle(5.6); double totalArea=0.0; for (int I=0; I<shapes.length; I++) totalArea=totalArea + shapes[I].area();

17. Question public abstract class IsAbstractClass{ public void test () { S.O.P (“ I am a method”); } Is this class an abstract class? What are the rules?

18. Keywords final and static If a method is defined as final, the compiler can determine which form of a method to use at compile time This is static binding; sometimes called early binding Methods declared as static also use early binding, since only one version of the method is available for all classes Override a static method?

19. Relationship between Classes is-a Implies inheritance In an is-a relationship the subclass is-a type of the superclasss For example myBall is-a Sphere Whatever is true of Ball is true of Sphere You can use Sphere as a type for Ball because they are type compatible Is the other way around true?

20. Relationship between Classes is-part-of or has-a If class A is-part-of class B then there is no inheritance Some negotiation between A and B for responsibilities may be needed Example: Assume A contains a list that B uses Who sorts the list? A or B? Country /Province?

21. Relationship between Classes Object type compatibility and type casting The system will automatically type cast a subclass into a superclass since they are type compatible It does this since it is guaranteed that whatever is true of the superclass or interface is true of the subclass The opposite is not true It is up to you to make sure that an object of type Comparable or Object is type String before you explicitly type cast it

22. Casting What would happen? Why? Shape c = new Circle(); double r = c.getRadius(); Shape CircleRect Square

23. How do we use this Shape as a Circle? Is this ok? How about this one? Shape c = new Circle(); double r = ((Circle)c).getRadius(); forces c to act as a circle Shape square = new Shape (); double r = ((Circle)square).getRadius();

24. Widening Conversions This is a widening conversion. obj -- an Object variable -- now references something more specific, namely a String. String s = new String(“hello”); Object obj; obj = s;

25. Narrowing Conversions and Casting String s = new String(“bye”); String t; Object obj; obj = s; t = obj; // NO NO NO !!! // you must CAST t = (String) obj;

26. 26 Review ______ is the ability of a class to derive properties from a previously defined class. Encapsulation Simulation Inheritance Polymorphism

27. 27 Review The class from which another class is derived is known as the ______. base class subclass child class final class

28. 28 Review A subclass inherits all of the following members of its superclass EXCEPT ______. public methods data fields constructors protected methods

29. 29 Review ______ enables the reuse of existing classes. Encapsulation Inheritance Polymorphism Simulation

30. 30 Review A method in a subclass is said to ______ an inherited method if it has the same method declarations as the inherited method. copy override overload cancel

31. 31 Review The keyword ______ is used in the class declaration of a subclass to indicate its superclass. inherits extends implements super

32. 32 Review A superclass method can be accessed by a subclass, even though it has been overridden by the subclass, by using the ______ reference. super final Static new

33. 33 Review The constructor of a subclass can call the constructor of the superclass by using the ______ reference. extends new super import

34. 34 Review Inheritance should only be used when a(n) ______ relationship exists between the superclass and the subclass. is-a has-a has-many similar-to

35. 35 Review Dynamic binding is also known as ______. early binding late binding package binding inheritance binding

36. 36 Review ______ is the ability of a variable name to represent, during program execution, instances of different but related classes that descend from a common superclass. Inheritance Containment Polymorphism Encapsulation

37. 37 Review If the field modifier ______ is specified in a method definition, the method cannot be overridden by a subclass. public protected final abstract

38. 38 Review If a method definition in a superclass has the field modifier ______, a subclass is required to override the method. static protected Final abstract

39. 39 Review Methods declared as ______ use static binding. protected final public abstract

40. 40 Review A method that has the same name but a different set of parameters as an existing method is said to ______ the original method. bind cancel Override overload

41. 41 Review Which of the following is true about an abstract class? it can be instantiated it can contain zero or more abstract methods it cannot be inherited by other classes it cannot contain data fields