1. ACM/JETT Workshop - August 4-5, 2005 1 02:Inheritance and Interfaces

2. ACM/JETT Workshop - August 4-5, 2005 2 Inheritance and Interfaces This is a relatively long lecture and we will cover a number of very important Object- Oriented concepts. Please note that all the features covered are used in the MarineBiologyCaseStudy.

3. ACM/JETT Workshop - August 4-5, 2005 3 Topics Inheritance Defining a class using inheritance in Java Overriding methods Abstract classes Substitutability. Interfaces Dynamic binding. Casting

4. ACM/JETT Workshop - August 4-5, 2005 4 Inheritance What is inheritance? A class (child class) –inherits the functionality of another class (parent class), and then –adds new functionality of its own. Java supports single inheritance. Inheritance is a requirement of all object-oriented systems.

5. ACM/JETT Workshop - August 4-5, 2005 5 Inheritance – An Example CheckingAccounts and SavingsAccounts are types of BankAccounts. They all have properties like acctId, balance, deposit and withdraw. But CheckingAccount and SavingsAccount also have some special properties of their own.

6. ACM/JETT Workshop - August 4-5, 2005 6 Why inheritance Inheritance is a technique for reuse. If a new class has a lot in common with a class that already exists, you can reuse parts of the existing class in the new class. The child class is defined by extending the parent class. The new ``child'' class has all characteristics of its ``parent'' plus any it adds itself.

7. ACM/JETT Workshop - August 4-5, 2005 7 Inheritance (an is-a relationship) Inheritance is a parent-child relationship between classes. The parent class is also called a base class or a superclass. The child class is also called a derived class or subclass.

8. ACM/JETT Workshop - August 4-5, 2005 8 Inheritance – Some Examples BankAccount is the base class (parent class, superclass) CheckingAccount and SavingsAccount are the derived classes (child classes, subclasses) SavingsAccount is-a BankAccount. CheckingAccount is-a BankAccount

9. ACM/JETT Workshop - August 4-5, 2005 9 Inheritance A child class inheriting from a parent class may –Define new instance data members and/or methods, and also may –Override methods inherited from parent class.

10. ACM/JETT Workshop - August 4-5, 2005 10 Extension and Overriding CheckingAccount and SavingsAccount inherit the deposit() and withdraw(). CheckingAccount adds a new method, overdraftProtection() and overrides withdraw() SavingsAccount adds a new method, annualInterest() Show class CheckingAccount Example1_22 Show class CheckingAccount Example1_22

11. ACM/JETT Workshop - August 4-5, 2005 11 Overriding a Method A subclass overrides a method contained in its super class. Both methods should have the same signature. Overriding a method allows the subclass – to extend the method in the super-class and –Modify the behavior in the super class as befits the context of the subclass.

12. ACM/JETT Workshop - August 4-5, 2005 12 Overriding a Method Example withdraw( ) in super class BankAccount successfully withdraws the amount if the amount <= balance. withdraw () in subclass CheckingAccount withdraws the amount if the amount <= balance or if the amount <= balance + overdraftProtection.

13. ACM/JETT Workshop - August 4-5, 2005 13 Inheritance private members in the super class are not accessible in the subclass. protected members in the super class –Are accessible in the subclass. –Are not accessible outside the package.

14. ACM/JETT Workshop - August 4-5, 2005 14 Inheritance – Some Examples Car and Bicycle override the method move()

15. ACM/JETT Workshop - August 4-5, 2005 15 What is an abstract class? Let us look at the classes, Shape, Circle and Rectangle, where Circle and Rectangle are both Shapes. We know that a shape has area, but we do not know how to compute area unless we know what kind of concrete shape it is. Then area() is an abstract method in class Shape. The class Shape itself is an abstract class.

16. ACM/JETT Workshop - August 4-5, 2005 16 Abstract classes An abstract class cannot be instantiated (no objects can be created from this class). In Java, an abstract class must be declared with the abstract keyword. An abstract class can contain one or more abstract methods.

17. ACM/JETT Workshop - August 4-5, 2005 17 Abstract classes An abstract method has no method body (definition), just a declaration with method signature. Why have an abstract method? An abstract method specifies a responsibility that the derived (subclasses) classes should fulfill. The concrete subclasses of an abstract class define the inherited abstract methods.

18. ACM/JETT Workshop - August 4-5, 2005 18 Abstract classes and abstract methods The area() in Shape class is declared, but not defined. This is an abstract method. Class Shape is an abstract class. Circle and Rectangle define the area() Show Shape classes Example2_22 Show Shape classes Example2_22

19. ACM/JETT Workshop - August 4-5, 2005 19 Mixing Object Types Assume we have a class Car and a class BankAccount. We have the following statements: // Create an instance of a Car Car myCar = new Car(); // Declare a variable of type BankAccount: BankAccount myAcct; //Assign myCar to myAcct. myAcct = myCar; // A compile-time error. Why?

20. ACM/JETT Workshop - August 4-5, 2005 20 Mixing Object Types myAcct = myCar; The object types do not match in the assignment statement.

21. ACM/JETT Workshop - August 4-5, 2005 21 Mixing Object Types Let us look at a slightly different situation: CheckingAccount cAcct = new CheckingAccount (); BankAccount myAcct; myAcct = cAcct; This assignment is OK, since an instance of a CheckingAccount is also of type BankAccount because (CheckingAccount is-a BankAccount). What is the rule?

22. ACM/JETT Workshop - August 4-5, 2005 22 Substitutability A variable defined to be of type base class can reference an object of any of the derived classes. Therefore, the following assignments are ok. BankAccount myAcct; myAcct = new CheckingAccount(); myAcct = new SavingsAccount();

23. ACM/JETT Workshop - August 4-5, 2005 23 Substitutability Is the following assignment ok? Here, we are assigning a variable of base class type to a variable of type derived class. CheckingAccount cAcct; cAcct = new BankAccount(); This will give a compile-time error.

24. ACM/JETT Workshop - August 4-5, 2005 24 Substitutability Given the Shape classes, are these following statements legal? Shape shape1 = new Shape(); shape1 = new Circle(); shape1 = new Rectangle(); Shape [] someShapes = new Shape[3]; someShapes[0] = new Circle(5); someShapes[1] = new Rectangle(5,10);

25. ACM/JETT Workshop - August 4-5, 2005 25 Object – the ancestor of all Java classes All classes in Java inherit from one common super class named Object. The Object class defines a set of basic methods that all Java classes may need. Some of these are: –clone() –equals() –toString() Classes typically override these methods to implement their specific functionality.

26. ACM/JETT Workshop - August 4-5, 2005 26 Java interfaces

27. ACM/JETT Workshop - August 4-5, 2005 27 Interfaces An interface in Java is a “strict abstract class” with only –Method declarations –No method definitions ie. no implementations. –Can also contain constants You cannot create an object from an interface.

28. ACM/JETT Workshop - August 4-5, 2005 28 Interfaces A class can implement an interface (s). When a class implements an interface, it should provide definitions for all methods declared in the interface. One interface can extend another, but –An interface cannot extend a class –A class cannot extend an interface –Classes implement an interface

29. ACM/JETT Workshop - August 4-5, 2005 29 Defining Interfaces Define an interface called Taxable public interface Taxable { // A constant public static final double TAX_RATE = 0.1; // A method declaration public double computeTax(); }

30. ACM/JETT Workshop - August 4-5, 2005 30 Implementing interfaces Class Stock implements the interface Taxable public class Stock extends Asset implements Taxable { public double computeTax(){ return ((stockPrice * numberOfStocks) * Taxable.TAX_RATE) } private double stockPrice; private int numberOfStocks; }

31. ACM/JETT Workshop - August 4-5, 2005 31 Benefits of Interfaces Separates a specification from an implementation. A Java interface is also a Java type. Is someStock of type Stock? Yes. Is someStock of type Taxable? Yes, since class Stock implements Taxable interface. Stock someStock = new Stock();

32. ACM/JETT Workshop - August 4-5, 2005 32 Some Commonly used Java interfaces Clonable Serializable Comparable, …

33. ACM/JETT Workshop - August 4-5, 2005 33 Dynamic Binding For this discussion we will again use the BankAccount classes.

34. ACM/JETT Workshop - August 4-5, 2005 34 Static vs Dynamic Binding Suppose we have the following code: CheckingAccount c1 = new CheckingAccount(); What is the type of the reference variable c1? It is CheckingAccount, as determined at compile time. CheckingAccount is called the variable’s static type.

35. ACM/JETT Workshop - August 4-5, 2005 35 Static vs Dynamic Binding Static binding is done by the compiler when it can determine the type of an object c1.withdraw(); This call to the withdraw() method will be bound to the withdraw() defined in class CheckingAccount.

36. ACM/JETT Workshop - August 4-5, 2005 36 Static vs Dynamic Binding Now, let us change the code to the following: BankAccount b; b = new CheckingAccount(); Is this assignment ok? Yes. What is the type of the reference variable b? It’s static type is BankAccount, as determined at compile time. Its dynamic type is CheckingAccount as determined at runtime.

37. ACM/JETT Workshop - August 4-5, 2005 37 Static vs Dynamic Binding BankAccount b; b = new CheckingAccount(); b.withdraw(); Is the call to the method, withdraw() result in execution of withdraw() in BankAccount or withdraw() in CheckingAccount? It will result in dynamically binding to CheckingAccount.withdraw().

38. ACM/JETT Workshop - August 4-5, 2005 38 Static vs Dynamic Binding The Rule : Whenever a reference refers to an interface or a base class, methods are dynamically bound Dynamic Binding is method implementation determined at runtime. Also known as Polymorphism

39. ACM/JETT Workshop - August 4-5, 2005 39 Checking an Object’s Type Its possible to check the actual type of an object using the instanceof operator. Example BankAccount myAcct; myAcct = new CheckingAccount(); If (myAcct instanceof CheckingAccount ) ….

40. ACM/JETT Workshop - August 4-5, 2005 40 Exercise Consider the classes below:

41. ACM/JETT Workshop - August 4-5, 2005 41 Exercise Given the interface and classes on the previous slide, are these following statements valid? Taxable [] assets = new Taxable[5]; assets[0] = new Stock(); assets[1] = new RealEstate();.. for (int i = 0;i < assets.length; ++i) double d = assets[i].computeTax();

42. ACM/JETT Workshop - August 4-5, 2005 42 Casting We know the following assignment is OK. Taxable oneAsset = new Stock(); oneAsset.computeTax(); // Ok oneAsset.getStockPrice(); // Not Ok // Cast it to Stock and then call // method if (oneAsset instanceof Stock) ((Stock)oneAsset).getStockPrice ();

43. ACM/JETT Workshop - August 4-5, 2005 43 Test your Understanding What is inheritance for? What is overriding a method? Can I assign an instance of a subclass to a variable of type super class? Or, vice versa? What is an abstract class? What is an abstract method? What is an interface? What is dynamic binding? What is casting? You will get some hands-on experience in using some of these features in Lab 3