COSC2007 Data Structures II

COSC2007 Data Structures II Chapter 9Class Relationships

Topics • Inheritance & Polymorphism • Abstract Class • Class Relationship

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

Sphere theRadius: double radius (): double area(): double equals(): boolean displayStatistics (): void 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

Ball Inheritance Sphere • 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 Inheritance Sphere • 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?

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

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) ); }

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;

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?

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.

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

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.

The abstract Shape class public abstract class Shape { public abstract double area(); public abstract doublecircumference(); } The Shape Hierarchy Shape Circle Rect Square

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();

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?

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?

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?

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?

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

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

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

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

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;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

COSC2007 Data Structures II