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 Javas 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

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

4
Inheritance and Javas 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

5
Inheritance and Javas Predefined Classes
(continued)
6
Inheritance and Javas 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

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

8
Inheritance and Programmer-Defined Classes
(continued)
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

10
Inheritance and Programmer-Defined Classes
(continued)
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

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 doesnt find it in the calling class

13
Inheritance and Programmer-Defined Classes
(continued)
14
Inheritance and Programmer-Defined Classes
(continued)
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

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

17
Apply the Concept (continued)
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

19
Apply the Concept (continued)

• Both CheckingAccount and SavingsAccount class
  definitions include extends BankAccount
• 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

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

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

22
Overloading (continued)
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

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

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

26
Static (Early) and Dynamic (Late) Binding
(continued)
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

28
Apply the Concept (continued)
29
Apply the Concept (continued)
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

31
Apply the Concept (continued)
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