Module 8 “Polymorphism and Inheritance”

1. Module 8“Polymorphismand Inheritance”

2. Outline • Understanding Inheritance • Inheritance Diagrams • Constructors in Derived Classes • Type Compatibility • Polymorphism • Method Overriding • The Object Class • Abstract Classes and Methods • Revisiting Polymorphism

3. Object-Oriented Programming (OOP) • Three key guiding principles • Encapsulation • Inheritance • Polymorphism

4. Inheritance • The ability of a class to reuse the status and behavior of another class while adding its own functionality. • The status of a class is the collection of its attributes (or fields). • The behavior of a class is the collection of its methods.

5. Inheritance Example • Class Person • Attributes: Name • Methods: • Constructors for Person • setName( … ) • getName ( ) • writeOutput ( ) • hasSameName ( )

6. Inheritance Example • Class Student inherits from Person • Able to use existing Person functionality • It has all of its attributes and methods. • But incorporates additional functionality • Attributes: studentNumber • Methods: • Constructors for Student • reset ( …) • getStudentNumber( ), setStudentNumber( … ) • equals (… )

7. Inheritance Example • Person is the base (or parent) class • Student is the derived (or child) class • Base classes are more general. • Derived classes are more specific. • Inheritance greatly enhances the ability to reuse existing code. • This simplifies the development time. • And makes design much simpler and cleaner.

8. Inheritance Example • In Java, a derived class can only have a single base class. • Other languages such as C++ allows inheritance from multiple classes. • Syntax: class <derived> extends <base>

9. Inheritance Example • View program listing 8.4class Person (base class) • View program listing 8.5 class Student (derived class) • View demo program, listing 8.6 class InheritanceDemo Sample screen output

10. Private Class Members • Private class members (i.e. attributes and methods) are NOT inherited. • Thus, cannot be manipulated by the derived classes. • Because they are “private” to the base class. • Likewise, private methods in the base class cannot be used in the derived class. • Unless declared as “public” or “protected”

11. Derived Classes A class hierarchy

12. Inheritance Diagrams

13. Inheritance Diagrams

14. Constructors in Derived Classes • A derived class does NOT inherit the constructors from the base class • Constructor in a derived class must invoke constructor from base class • Use the reserved word super • Must be first action in the child constructor

15. The this keyword - Again • Also possible to use the this keyword • Use to call any constructor in the class • When used in a constructor, this calls constructor in the same class • Unlike super, which invokes the constructor of the base class

16. Programming Example • A derived class of a derived class • View program listing 8.7class Undergraduate • Has all public members of both • Person • Student • This reuses the code in superclasses

17. Programming Example More details of the class hierarchy

18. Type Compatibility • In the class hierarchy • Each Undergraduate is also a Student • Each Student is also a Person • An object of a derived class can serve as an object of the base class • Note this is NOT typecasting • An object of a child class can be referenced by a variable of an ancestor type

19. Type Compatibility • Be aware of the "is-a" relationship • A Studentis aPerson • This is the basis for polymorphism • Another relationship is the "has-a" • A class can contain (as an instance variable) an object of another type • If we specify a date of birth variable for Person – it "has-a" Date object

20. Polymorphism • From the Greek • “poly” = many • “morph” = form, figure, silhouette • The ability of a class method to do different things based on the object it is acting upon. • Example: Animal.makeNoise(); • Dog: barks • Cat: mews

21. Polymorphism Two types of polymorphism • Method Overloading: • Multiples implementations of the same method occur in the same class. • Each differs in the number and types of the method arguments. • Java invokes the closest one that matches the actual arguments being passed to the method. • System.out.println ( int ); • System.out.println( char ); • System.out.println ( String ) ; • System.out.println ( boolean );

22. Polymorphism Two types of polymorphism • Method Overriding: • Multiples implementations of the same method occur in different classes along the same hierarchy. • A child class “overrides” the implementation of a method provided by its base class. • Examples: • Cat.makeNoise( ) overrides Animal.makeNoise( ) • SiameseCat.makeNoise( ) overrides Cat.makeNoise( )

23. Method Overriding • Go back to listings 8.4, 8.5 and 8.6 • Method writeOutput in Student class • Person class also has writeOutput • Method in child class with same signature overrides method from the parent class • Student objects use the overridden method. • Overridden method must return same type.

24. Overriding vs. Overloading • Do not confuse overriding with overloading • Overriding takes place in the subclass – new method with same signature • Overloading takes place in the same class – new method with different signature

25. The final Modifier • It is possible to specify that a method cannot be overridden in a subclass • Add modifier final to the method headingpublic final void specialMethod() • An entire class may be declared final • Thus cannot be used as a base class to derive any other class

26. Calling an Overridden Method • Reserved word super can also be used to call a base class method in the derived implementation • Useful if the derived implementation is an extension of the base (overridden) one.

27. Programming Example • Go back to program listing 8.7class Undergraduate • Notice how the writeOutput method invokes the one from its parent class (Student).

28. The Object Class • Java has a class that is the ultimate ancestor of every class • The class Object • Thus, it is possible to write a method with formal parameter of type Object • Actual parameter in the call can be object of any type • Example: method println(Object theObject)

29. The Object Class • The Object class has some methods that every Java class inherits • Examples • The equals()method • The toString()method • The toString() method is called when println(theObject)is invoked • Better to define your own toString to handle this.

30. A Better equals Method • The programmer of a class should override the equals() method of Object • View code of sample override, listing 8.8 public boolean equals (Object theObject)

31. Abstract Classes and Methods • So far, our base classes provide an implementation for all of its methods. • This means that objects from base classes can actually be created. • e.g. Person p = new Person( ); • What if we cannot provide an implementation for a particular method? • e.g. Animal.makeNoise();

32. Abstract Classes and Methods • We declare the method as abstract. • No implementation is provided in the base class. • Forces any derived class to implement the method. • Notice the semicolon at the end, meaning that the method has no body. • Different from a method with an empty body.

33. Abstract Classes and Methods • A class with at least one abstract method becomes an abstract class. • No objects can be created out of an abstract class. • You will derive other classes from it. • And create objects of the derived classes. • e.g. no Animal objects. • but of type Cat, Dog, Bird, Moose, etc.

34. Abstract Classes and Methods

35. Extending Abstract Classes Implements the abstract method

36. Extending Abstract Classes

37. Revisiting Polymorphism • Method overriding is one of the major sources of polymorphism. • How does it work? • The base class declares a method. • The derived class redefines (overrides) the method. • Must have the same signature.

38. Revisiting Polymorphism • A variable of any base class can “hold” an object of any of its derived classes. • Functionality is restricted to what the base class can “see” in the derived class. • All members of the base class. • Overridden methods in the derived class. • The implementation in the derived class prevails when the overridden method is invoked. • Called “late binding”

39. An Example • a2 declared as Animal (base class) • But created as Cat (derived class) • Therefore, “late bound” to Cat at runtime.

40. Another Example

41. Another Example Cat.makeNoise( ) SiameseCat.makeNoise( )

42. An Even More Powerful Example • Suppose the following classes have been created: • Dog extends Animal • Overrides makeNoise( ) as “WofWofWof” • Cow extends Animal • Overrides makeNoise( ) as “Mooooooooo” • Birdie extends Animal • Overrides makeNoise( ) as “Tweet Tweet”

43. An Even More Powerful Example

44. An Even More Powerful Example This is the program output: