Chapter 11: Inheritance and Polymorphism

1. Chapter 11: Inheritance and Polymorphism JavaProgramming: From Problem Analysis to Program Design, Second Edition

2. Chapter Objectives • Learn about inheritance. • Learn about subclasses and superclasses. • Explore how to override the methods of a superclass. • Examine how constructors of superclasses and subclasses work. • Learn about polymorphism. • Examine abstract classes. • Become aware of interfaces. • Learn about composition. Java Programming: From Problem Analysis to Program Design, Second Edition

3. Inheritance • Inheritance lets you create new classes from existing classes. • Any new class that you create from an existing class is called a subclass or derived class. • existing classes are called superclasses or base classes. • The subclass inherits the properties of the superclass. Rather than create completely new classes from scratch. • you can take advantage of inheritance and reduce software complexity. Java Programming: From Problem Analysis to Program Design, Second Edition

4. Inheritance • Single inheritance: • Subclass is derived from one existing class (superclass). • Multiple inheritance: • Subclass is derived from more than one superclass. • Not supported by Java. • In Java, a class can only extend the definition of one class. Java Programming: From Problem Analysis to Program Design, Second Edition

5. Inheritance modifier(s)class ClassName extendsExistingClassName modifier(s) { memberList } Java Programming: From Problem Analysis to Program Design, Second Edition

6. Inheritance: classCircle Derived from classShape public class Circle extends Shape { . . . } Java Programming: From Problem Analysis to Program Design, Second Edition

7. Inheritance • The private members of the superclass are private to the superclass. • The subclass can directly access the public members of the superclass. • The subclass can include additional data and method members. • The subclass can override (redefine) the public methods of the superclass. However, this redefinition applies only to the objects of the subclass, not to the objects of the superclass. • All data members of the superclass are also data members of the subclass. Similarly, the methods of the superclass (unless overridden) are also the methods of the subclass. (Remember Rule 1 when accessing a member of the superclass in the subclass.) Java Programming: From Problem Analysis to Program Design, Second Edition

8. Using Methods of the Superclass in a Subclass • The subclass can give some of its methods the same name as given by the superclass. • To override a public method of the superclass in the subclass, the corresponding method in the subclass must have the same name, the same type, and the same formal parameter list. • If the corresponding method in the superclass and the subclass has the same name but different parameters, then this is method overloading in the subclass, which is also allowed. Java Programming: From Problem Analysis to Program Design, Second Edition

9. UML Class Diagram: classRectangle Java Programming: From Problem Analysis to Program Design, Second Edition

10. UML Class Diagram: classBox Java Programming: From Problem Analysis to Program Design, Second Edition

11. Inheritance • When writing a method’s definition of a subclass, to specify a call to the public method of the superclass: • If subclass overrides public method of superclass, specify call to public method of superclass: super.MethodName(parameter list) • If subclass does not override public method of superclass, specify call to public method of superclass: MethodName(parameter list) Java Programming: From Problem Analysis to Program Design, Second Edition

12. classBox public void print() { super.print(); System.out.print("; Height = " + height); } public void setDimension(double l, double w, double h) { super.setDimension(l, w); if (h >= 0) height = h; else height = 0; } public double area() { return 2 * (getLength() * getWidth() + getLength() * height + getWidth() * height); } Java Programming: From Problem Analysis to Program Design, Second Edition

13. Defining Constructors of the Subclass • A constructor typically serves to initialize instance variables. • When we instantiate a subclass object, this object inherits the instance variables of the superclass • but the subclass object cannot directly access the private instance variables of the superclass. • As a consequence, the constructors of the subclass can (directly) initialize only the instance variables of the subclass. • When a subclass object is instantiated, to initialize the (private) instance variables it must also automatically execute one of the constructors of the superclass. Java Programming: From Problem Analysis to Program Design, Second Edition

14. Defining Constructors of the Subclass • Call to constructor of superclass: • Must be first statement. • Specified by super parameter list. • If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error.  public Box(){ super(); height = 0;} public Box(double l, double w, double h){ super(l, w); height = h;} Java Programming: From Problem Analysis to Program Design, Second Edition

15. Objects myRectangle and myBox Rectangle myRectangle = new Rectangle(5, 3); Box myBox = new Box(6, 5, 4); Java Programming: From Problem Analysis to Program Design, Second Edition

16. Protected Members of a Class Members of a class are classified into three categories : Public Private Protected So, If a member of a superclass needs to be accessed in a subclass, and still prevent its direct access outside the class ,that member is declared using the modifier protected. Java Programming: From Problem Analysis to Program Design, Second Edition

17. Protected Members of a Class Java Programming: From Problem Analysis to Program Design, Second Edition

18. Protected Members of a Class public void setData(char ch, double v, int a){super.setData(v);bCh = ch; //initialize bCh using the //assignmentstatement dA = a;} Java Programming: From Problem Analysis to Program Design, Second Edition

19. Protected Members of a Class Dclass objects can not directly access bCh. DClass dObject = new DClass; dObject.bCh = ‘&’; Java Programming: From Problem Analysis to Program Design, Second Edition

20. Protected Members of a Class In an inheritance hierarchy, the public and protected members of a superclass are directly accessible, in a subclass, across any number of generations, that is, at any level and they are also accessible by methods of other classes in the same package. Java Programming: From Problem Analysis to Program Design, Second Edition

21. Protected Members of a Class Even though the public and protected data members of a super class are directly accessible in a subclass, in the inheritance hierarchy, it should be the responsibility of the superclass to properly initialize these data members. Java Programming: From Problem Analysis to Program Design, Second Edition

22. The classObject • In ch8 we have seen the method toString • The default definition of the method toString returns the class name followed by the hash code of the object. • Where is the method toString definied? Java Programming: From Problem Analysis to Program Design, Second Edition

23. The classObject • The method toString comes from the class Object, and it is a public member of this class. • In Java, if you define a class and do not use the reserved word extends to derive it from an existing class, then the class you define is automatically considered to be derived from the class Object. • The class Object directly or indirectly becomes the superclass of every class in Java. Java Programming: From Problem Analysis to Program Design, Second Edition

24. The classObject: Equivalent Definition of a Class public classClock { //Declare instance variables as given in Chapter 8 //Definition of instance methods as given in Chapter 8 //... } public classClock extendsObject { //Declare instance variables as given in Chapter 8 //Definition of instance methods as given in Chapter 8 } Java Programming: From Problem Analysis to Program Design, Second Edition

25. The classObject • Using the mechanism of inheritance, public members of classObject can be overridden and/or invoked by object of any class type. • Because every Java class is directly or indirectly derived from the class Object, it follows that the method toString becomes a public member of every Java class. Therefore, if a class does not override this method, whenever this method is invoked, the method’s default definition executes. Java Programming: From Problem Analysis to Program Design, Second Edition

26. Some Constructors and Methods of the classObject Java Programming: From Problem Analysis to Program Design, Second Edition

27. Hierarchy of Java Stream Classes Java Programming: From Problem Analysis to Program Design, Second Edition

28. Polymorphism • Java allows us to treat an object of a subclass as an object of its superclass. In other words, a reference variable of a superclass type can point to an object of its subclass. • There are situations when this feature of Java can be used to develop generic code for a variety of applications. Java Programming: From Problem Analysis to Program Design, Second Edition

29. Polymorphism Person name, nameRef; //Person is the superclass PartTimeEmployee employee,employeeRef;//PartTimeEmployee //is the subclass name = new Person("John", "Blair"); employee = new PartTimeEmployee("Susan", "Johnson", 12.50, 45); nameRef = employee; System.out.println("nameRef: " + nameRef); nameRef: Susan Johnson wages are: $562.5 Java Programming: From Problem Analysis to Program Design, Second Edition

30. Polymorphism • Subclass objects can be treated as superclass objects. This makes sense because the subclass has members corresponding to each of the superclass members. • Assignment in the other direction is not allowed because assigning a superclass object to a subclass reference would leave the additional subclass members undefined. Java Programming: From Problem Analysis to Program Design, Second Edition

31. Polymorphism • Referring to a superclass object with a subclass reference is a syntax error. • Assigning a subclass object to a superclass reference, and then attempting to reference subclass-only members with the superclass reference , is a syntax error. Java Programming: From Problem Analysis to Program Design, Second Edition

32. Polymorphism Even though nameRef is declared as a reference variable of the type Person when the statement System.out.println("nameRef: " + nameRef); Excutes to output nameRef,the method toString of the class PartTimeEmployee executes. This is called late binding, dynamic binding, or run-time binding; that is the method to be executed is determined at execution time, not compile time. Java Programming: From Problem Analysis to Program Design, Second Edition

33. Polymorphism In a class hierarchy, • several methods may have the same name and the same formal parameter list. • Moreover, a reference variable of a class can refer to either an object of its own class or an object of its subclass. • Therefore, a reference variable can invoke a method of its own class or of its subclass(es). Java Programming: From Problem Analysis to Program Design, Second Edition

34. Polymorphism Binding means associating a method definition with its invocation. In early binding, a method’s definition is associated with its invocation when the code is compiled. In late binding, a method’s definition is associated with the method’s invocation at execution time. Java uses late binding for all methods Java Programming: From Problem Analysis to Program Design, Second Edition

35. Polymorphism • The term polymorphism means to assign multiple meanings to the same method name. • In Java, polymorphism is implemented using late binding. • The reference variable name or nameRef can point to any object of the classPerson or the classPartTimeEmployee. • These reference variables have many forms, that is, they are polymorphic reference variables. They can refer to objects of their own class or to objects of the classes inherited from their class. Java Programming: From Problem Analysis to Program Design, Second Edition

36. Polymorphism Example Superclass: Quadrilateral (رباعي) Subclasses: Rectangle, Square, Parallelograms, Trapezoids. An operation (such as calculating the area) that can be performed on an object of class Quadrilateral can also be performed on an object of class Rectangle. Such operations can also be performed on other kinds of Quadrilateral . When a request is made through a superclass reference to use a method, Java chooses the correct overridden method polymorphically in the appropriate subclass associated with the object. Java Programming: From Problem Analysis to Program Design, Second Edition

37. final • You can declare a method of a class final using the keyword final. For example, the following method is final. public final void doSomeThing() { //... } • If a method of a class is declared final, it cannot be overridden with a new definition in a derived class. • In a similar manner, you can also declare a class final using the keyword final. • If a class is declared final, then no other class can be derived from this class. • Java does not use late binding for methods that are private, marked final, or static. Java Programming: From Problem Analysis to Program Design, Second Edition

38. Polymorphism • You cannot automatically make reference variable of subclass type point to object of its superclass. This is a syntax error • An explicit cast can be used to convert a superclass reference to a subclass reference. • This can only be done when the superclass reference is actually referencing (pointing to) a subclass object. • If asubclass object has been assigned to a reference of its superclass, it is acceptable to cast that object back to its own type. This must be done in order to send that object any of its messages that do not appear in the superclass. Java Programming: From Problem Analysis to Program Design, Second Edition

39. Polymorphism • Suppose that supRef is a reference variable of a superclass type. Moreover, suppose that supRef points to an object of its subclass. • You can use an appropriate cast operator on supRef and make a reference variable of the subclass point to the object. • On the other hand, if supRef does not point to a subclass object and you use a cast operator on supRef to make a reference variable of the subclass point to the object, then Java will throw a ClassCastException—indicating that the class cast is not allowed. Java Programming: From Problem Analysis to Program Design, Second Edition

40. Polymorphism Person name, nameRef; //Person is the superclass PartTimeEmployee employee,employeeRef;//PartTimeEmployee //is the subclass name = new Person("John", "Blair"); employee = new PartTimeEmployee("Susan", "Johnson", 12.50, 45); nameRef = employee; Java Programming: From Problem Analysis to Program Design, Second Edition

41. Polymorphism employeeRef = (PartTimeEmployee) name; will throw a ClassCastException because name points to an object of the class Person. However, the following statement is legal: employeeRef = (PartTimeEmployee) nameRef; Java Programming: From Problem Analysis to Program Design, Second Edition

42. Polymorphism • Operator instanceof: Determines whether a reference variable that points to an object is of a particular class type. • p instanceof BoxShape • This expression evaluates to true if p points to an object of the classBoxShape; otherwise it evaluates to false: Java Programming: From Problem Analysis to Program Design, Second Edition

43. Abstract Methods • A method that has only the heading with no body. • Must be declared abstract. public void abstract print(); public abstract object larger(object, object); void abstract insert(int insertItem); Java Programming: From Problem Analysis to Program Design, Second Edition

44. Abstract Classes • A class that is declared with the reserved word abstract in its heading. • An abstract class can contain instance variables, constructors, finalizers, and non-abstract methods. • An abstract class can contain abstract methods. • If a class contains an abstract method, the class must be declared abstract. • You cannot instantiate an object of an abstract class type. You can only declare a reference variable of an abstract class type. • You can instantiate an object of a subclass of an abstract class, but only if the subclass gives the definitions of all the abstract methods of the superclass. Java Programming: From Problem Analysis to Program Design, Second Edition

45. Abstract Class Example public abstract classAbstractClassExample { protected intx; public void abstractprint(); public voidsetX(inta) { x = a; } publicAbstractClassExample() { x = 0; } } Java Programming: From Problem Analysis to Program Design, Second Edition

46. Interfaces • A class that contains only abstract methods and/or named constants. • We have seen the Actionlistener in ch6. • The WindowListener to handel winow events • The MouseListener to handel mouse events Java Programming: From Problem Analysis to Program Design, Second Edition

47. Interfaces • Why does Java have these interfaces? • After al they are also classes! Java Programming: From Problem Analysis to Program Design, Second Edition

48. Interfaces • The answer: • Java doesn’t support multiple inheritance. • A java program might have a variety of GUI component that generate a variety of events. • These events are handeled by separate interfaces. • Therefore program might need to use more than one such interface. Java Programming: From Problem Analysis to Program Design, Second Edition

49. Interfaces • The mechanism that was used in ch6 to handle an event is inner class mechanism . • Rather than using inner class mechanism, the class containing the java program canitself be created on top of an interface. public class RectangleProgram extends JFrame implements ActionListener { //… } We still have to register the listener by using the reference this Java Programming: From Problem Analysis to Program Design, Second Edition

50. Interfaces • To be able to handle a variety of events, Java allows a class to implement more than one interface. • That is how Java implements multiple inheritance, which is not true multiple inheritance. Java Programming: From Problem Analysis to Program Design, Second Edition