9. Inheritance

1. 9. Inheritance 9.1 Subclasses 9.2 Polymorphism 9.3 Abstract Classes 9.4 Modifiers and Access 9.6 Object-Oriented Design with Use Cases and Scenarios

2. Objectives • Use inheritance to relate classes • Use polymorphism to improve design • Use abstract classes to achieve implementation independence • Understand the use of modifiers in specifying access • Develop an object-oriented design from use cases and scenarios

3. Subclasses • Inheritance allows deriving a new class from an existing one • The existing class is called the parent class, or superclass, or base class • The derived class is called the child class or subclass • The child class inherits characteristics: data and methods, of the parent • Java only support single inheritance

4. Vehicle Car Subclasses • The inheritance (child-parent) relationships are often shown graphically in a class diagram like • Inheritance exhibits an is-a relationship, meaning the child is a more specific version of the parent

5. Class Hierarchies • Inheritance relationships create hierarchy of classes • Two children of the same parent are called siblings • An inherited member is continually passed down the line • Class hierarchies often have to be extended and modified to keep up with changing needs

6. Deriving Subclasses • To create a subclass, we extend a class, inheriting all the attributes and behavior of that class public class Car extends Vehicle { // class contents } • The keyword extends tells us that Car is a subclass of the superclass Vehicle • A subclass usually have data and methods defined in addition to the ones inherited

7. Deriving Subclasses • Words.java (extra) Book.java (extra) Dictionary.java (extra) • BankAccount.java SavingAccount.java UseSavingAccount.java CheckingAccount.java UseCheckingAccount.java

8. Deriving Subclasses • A child class inherits the public methods of its parent, but it may choose to override some of them to implement its own specific behavior Messages.java (extra) Thought.java (extra) Advice.java (extra) • Overloading - multiple methods in the same class with the same name but different signatures • Overriding - two methods of the same name, one in a parent class and one in a child class, that have the same signature

9. Reference and Inheritance • An object reference can refer to an object of its class, or to an object of any class related to it by inheritance • For example, if the Christmas is a subclass of the Holiday class, then a Holiday reference can be used to point to a Christmas object Holiday day; day = new Christmas();

10. The Keyword super • Constructors are not inherited, even though they are publicly visible • Java uses the keyword super to reference the superclass, it is often used to invoked the parent’s constructor • Words2.java (extra) Book2.java (extra) Dictionary2.java (extra)

11. Polymorphism • Polymorphism - “many forms” • A polymorphic reference is one which can refer to different types of objects at different times • A reference to the superclass can actually refer to an object in any subclass • An interface can also be used to create polymorphic reference • An object reference variable has • Compile-time type - the variable’s declared type • Run-time type - the type of the object to which the variable currently refers

12. Polymorphism • Compile-time vs. run-time types Animal herman; // compile-time type Animal Animal zelda; // compile-time type Animal herman = new Lion(); // run-time type Lion zelda = new Giraffe(); // run-time type Giraffe Animal[] two = {herman, zelda}; feed(two); where public void feed(Animal[] theZoo) { for (int i = 0; i < theZoo.length; i++) theZoo[i].eat(); }

13. Polymorphism • Animal.java Lion.java Giraffe.java UseAnimals.java • Duck.java UseNewAnimals.java • Withdraw.java

14. Abstract Classes • An abstract class is a placeholder in a class hierarchy that represents a generic concept • An abstract class cannot be instantiate • We use the modifier abstract on the class header to declare a class as abstract • An abstract class often contains abstract methods (like an interface does), though it doesn’t have to • The child of an abstract class must override the abstract methods of the parent, or it too will be considered abstract

15. Abstract Classes • The keyword final - Specify that a method cannot be overridden • An Abstract method cannot be defined as final or static • The use of abstract classes is a design decision; it helps us establish common elements in a class that is too general to instantiate • Polymorphism often works with abstract classes • Operations of an abstract class will work for objects of any subclass

16. A Shape Classes • Shape is an abstract concept • Shape has subclass like Line, Circle, etc. that have instances • Shape.java • Line.java • Circle.java • UseShape.java

17. Package • A java package corresponds to a directory in the directory structure of the host machine • To put a class in a named package, use a package statement as the first statement in the file • For example: package personData; • The compiled byte code file (.class) must be placed under the directory with the same name of the package • An import statement tells Java to look in the directory for classes referenced in the program

18. Package • Using import statement, we can ommit the package prefix of the classes • In addition to the default directories that Java will automatically search, the • classpath enviroment variable specifies user-defined directories to search • For example: javac -classpath .;c:\booklr Test.java java -classpath .;c:\booklr Test

19. Modifiers and Access • public - anywhere the class name is accessible • protected - in the package that contains the class in which the member is declared, and in any subclass of that class • (no modifier) - in the package that contains the class in which the member is declared • private - only in the class in which the member is declared • The visibility package: A.java, B.java, C.java, D.java

20. OO Design • Understand the problem • Solve the problem by identifying objects • Each object provides certain services • Objects use each other's services • A use case defines one function that the system should provide • For each case, several scenarios to describe the interaction among the user and • other parts of the system to provide the function

21. OO Design Example • Problem - Simple automatic teller machine • Use cases • deposit • withdrawal • get balance • Several scenarios for each use case • at least one for success • several for failure • AtmScreen.java

22. A Success Scenario for Deposit • The user asks the teller to accept an ATM card • The teller asks the user to enter a PIN • The user asks the teller to accept a PIN • The teller asks the user to select a transaction type • The user asks the teller to accept a deposit • The teller asks the user to select an account type • The user asks the teller to accept a saving account type • The teller asks the bank to find the account of the chosen type for the user with the specified PIN • The bank gives the teller a reference to the account

23. A Success Scenario for Deposit • The teller asks the user to specify an amount • The user asks the teller to accept an amount • The teller asks the account to de[posit the specified amount • The teller asks the user to select another transaction

24. A Failure Scenario • The user asks the teller to accept an ATM card • The teller asks the user to enter a PIN • The user asks the teller to accept a PIN • The teller asks the user to select a transaction type • The user asks the teller to accept a get balance transaction type • The teller asks the user to select an account type • The user asks the teller to accept a checking account type • The teller asks the bank to find the account of the chosen type for the user with the specified PIN • The bank gives the teller a null account • The teller asks the user to select another transaction, ...