Object-Oriented Application Development (Part II)

1. Object-Oriented Application Development (Part II) Dr.Wanwipa Titthasiri Department of Computer Science

2. Objectives In this chapter you will: • Study the concepts of inheritance and polymorphism • Learn more about how Java’s predefined classes use inheritance and polymorphism • Explore how inheritance and polymorphism are used in programmer-defined classes • Learn the importance of overloading, overriding, and dynamic binding • Develop skills in using inheritance and polymorphism in object-oriented applications An Introduction to Java Programming and Object-Oriented Application Development

3. Inheritance • Inheritance is a relationship in which one class has access to public members of another • Java has predefined classes that inherit attributes and behaviors from other classes • Programmer-defined classes can inherit from predefined or programmer-defined classes An Introduction to Java Programming and Object-Oriented Application Development

4. Inheritance and Java’s Predefined Classes • Inheritance hierarchy: Classes farther down inherit from classes farther up • Superclass: A class that is at a higher level • Subclass: A class at a lower level • Direct superclass: A class immediately above a given class • Direct subclass: A class immediately below a given class An Introduction to Java Programming and Object-Oriented Application Development

5. Inheritance and Java’s Predefined Classes (continued) An Introduction to Java Programming and Object-Oriented Application Development

6. Inheritance and Java’s Predefined Classes (continued) • Subclasses have access to public and protected members of their superclasses • Is-a relationship: A subclass has an “is-a” relationship with its superclass • Is-a relationships are also called generalization relationships • A superclass is more general that its subclasses • Example: Number is more general than double An Introduction to Java Programming and Object-Oriented Application Development

7. Inheritance and Programmer-Defined Classes • Recall MusicWorldApp.java • CDOrder has a LineItem and LineItem has a CD object • Does not involve inheritance • Inheritance is needed when a general type can be broken down into specific types • Example: The information system in a university must model different types of people • A person is a general type of object • A Person class serves as a superclass An Introduction to Java Programming and Object-Oriented Application Development

8. Inheritance and Programmer-Defined Classes (continued) An Introduction to Java Programming and Object-Oriented Application Development

9. Inheritance and Programmer-Defined Classes (continued) • A student is a person, so Student is a subclass of Person • All attributes of Person are attributes of Student • A student is a more specialized instance of a person • Students have a college class (junior, senior etc.) • Faculty and staff do not have a college class • Only methods unique to Student are defined An Introduction to Java Programming and Object-Oriented Application Development

10. Inheritance and Programmer-Defined Classes (continued) An Introduction to Java Programming and Object-Oriented Application Development

11. Inheritance and Programmer-Defined Classes (continued) • The keyword extends means inherits from • The class Student inherits public and protected members of Person • The keyword super calls the superclass constructor • When a Student object is created, a Person object is also created • The keyword this refers to the object whose method or constructor is being called An Introduction to Java Programming and Object-Oriented Application Development

12. Inheritance and Programmer-Defined Classes (continued) • The object-defining classes Person and Student cannot be executed on their own • An application class is required to execute tasks required by the application • Application classes contain a main method • The JVM looks up the inheritance hierarchy for methods if it doesn’t find it in the calling class An Introduction to Java Programming and Object-Oriented Application Development

13. Inheritance and Programmer-Defined Classes (continued) An Introduction to Java Programming and Object-Oriented Application Development

14. Inheritance and Programmer-Defined Classes (continued) An Introduction to Java Programming and Object-Oriented Application Development

15. Apply the Concept • Develop a simple banking application • Checking accounts have a monthly service fee • Savings accounts pay interest monthly • Three tasks • Identify all object-defining classes required • Sketch a class diagram • Plan the application class with a flowchart • Nouns are modeled by objects • Bank account, checking account, savings account • Verbs are modeled with methods • Assess fee, pay interest An Introduction to Java Programming and Object-Oriented Application Development

16. Apply the Concept (continued) • Create a class diagram • Focus on customer, bank account, checking account, and savings account • Checking and savings accounts are specialized bank accounts • Class name in the top section, instance variables in the middle, methods in the bottom • Composition is represented by solid diamonds and inheritance is represented by open arrows • Application class is represented with a flowchart An Introduction to Java Programming and Object-Oriented Application Development

17. Apply the Concept (continued) An Introduction to Java Programming and Object-Oriented Application Development

18. Apply the Concept (continued) • Customer.java defines Customer objects • SSN, name, checking account, savings account • Constructor, accessor methods for accounts • BankAccount.java defines BankAccount objects, members common to all accounts • Account number, balance, accessor methods, toString • CheckingAccount.java and SavingsAccount.java nearly identical • Checking accounts incur a service charge, savings accrue interest An Introduction to Java Programming and Object-Oriented Application Development

19. Apply the Concept (continued) • Both CheckingAccount and SavingsAccount class definitions include extendsBankAccount • Constructors call the BankAccount constructor first using the keyword super • BankingApp.java is procedural with 5 parts • Preparation, declaration, input, processing, output • All individual classes must be in the same directory, and be compiled individually An Introduction to Java Programming and Object-Oriented Application Development

20. Polymorphism • Polymorphism refers to the ability of a single method name to be used in different ways • Different kinds of polymorphism • Overloading • Overriding • Static binding • Dynamic binding An Introduction to Java Programming and Object-Oriented Application Development

21. Overloading • Overloading occurs when a single method name is used more than once • In the same class • Different parameter lists (different signatures) • Example: The indexOf method in the String class • Parameter lists can differ in number, type or order of arguments An Introduction to Java Programming and Object-Oriented Application Development

22. Overloading (continued) An Introduction to Java Programming and Object-Oriented Application Development

23. Overriding • One method name is used in different classes within an inheritance hierarchy (same signature) • Each of the methods may do completely different things • A method in a subclass may override a method in its superclass • Example: The toString method in Object is overridden in the BankAccount class • If a method is not found in the named class, Java looks up the hierarchy An Introduction to Java Programming and Object-Oriented Application Development

24. Static (Early) and Dynamic (Late) Binding • When overriding is used, the Java compiler attempts to bind a method call to its method definition • Binding at compile time is known as static (early) binding • Dynamic binding means the method call is not bound to the definition until run time An Introduction to Java Programming and Object-Oriented Application Development

25. Static (Early) and Dynamic (Late) Binding (continued) • Step 1: ObjectA is created from ClassA • Step 2: ObjectB is created from ClassB • Assume ClassB is a subclass of ClassA • ClassA contains a method called methodA • ObjectA.methodA() the compiler checks ObjectA for methodA: static binding • Step 3: ObjectB=ObjectA • Java thinks ObjectA is an instance of ClassB • Step 4: ObjectA tries to call methodA • Java does not check ClassB for methodA An Introduction to Java Programming and Object-Oriented Application Development

26. Static (Early) and Dynamic (Late) Binding (continued) An Introduction to Java Programming and Object-Oriented Application Development

27. Apply the Concept • Recall that a bank offers checking (with a service charge) and savings (with interest) • For simplicity, three classes • PolySavingsAccount is a subclass of PolyBankAccount • PolyBankingApp is a driver • PolyBankAccount has a payInterest method but cannot pay interest An Introduction to Java Programming and Object-Oriented Application Development

28. Apply the Concept (continued) An Introduction to Java Programming and Object-Oriented Application Development

29. Apply the Concept (continued) An Introduction to Java Programming and Object-Oriented Application Development

30. Apply the Concept (continued) • PolyBankAccount has overloaded constructors • The method payInterest is called on an instance of PolySavingsAccount • Overrides payInterest in PolyBankAccount • Dynamic binding (Lines 24 – 29) • account3 created from PolyBankAccount • account3 now refers to PolySavingsAccount • account3 calls methods from PolySavingsAccount • The payInterest method is not bound to account3 until run time An Introduction to Java Programming and Object-Oriented Application Development

31. Apply the Concept (continued) An Introduction to Java Programming and Object-Oriented Application Development

32. Summary • A superclass is higher in the hierarchy and a subclass is lower • The inheritance relationship is an is-a relationship or generalization relationship • A subclass inherits public and protected members of its superclass • Polymorphism means a single method can have many implementations depending on its signature and location • Overloading and overriding (with static and early binding) are forms of polymorphism An Introduction to Java Programming and Object-Oriented Application Development