Abstract Classes and Interfaces in Java: Purpose, Declaration, and Implementation, Study notes of Object Oriented Programming

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Object-Oriented

Programming:

Polymorphism

Chapter 10

What You Will Learn

 What is polymorphism?

 How to declare and use virtual

functions for abstract classes

Introduction

Polymorphism

• Enables “programming in the general”
• The same invocation can produce “many forms” of results

Interfaces

• Implemented by classes to assign common functionality to possibly unrelated classes

Polymorphism

When a program invokes a method

through a superclass variable,

• the correct subclass version of the method is called,
• based on the type of the reference stored in the superclass variable

The same method name and signature

can cause different actions to occur,

• depending on the type of object on which the method is invoked

Polymorphism Promotes Extensibility

 Software that invokes polymorphic

behavior

• independent of the object types to which messages are sent.

 New object types that can respond to

existing method calls can be

• incorporated into a system without requiring modification of the base system.
• Only client code that instantiates new objects must be modified to accommodate new types.

Demonstrating Polymorphic Behavior

A superclass reference can be aimed at

a subclass object

• a subclass object “ is-a” superclass object
• the type of the actual referenced object, not the type of the reference, determines which method is called

A subclass reference can be aimed at a

superclass object only if the object is

downcasted

View example, Figure 10.

Abstract Classes and Methods

 Abstract classes

• Are superclasses (called abstract superclasses)
• Cannot be instantiated
• Incomplete subclasses fill in "missing pieces"

 Concrete classes

• Can be instantiated
• Implement every method they declare
• Provide specifics

Abstract Classes and Methods

 Purpose of an abstract class

• Declare common attributes …
• Declare common behaviors of classes in a class hierarchy

 Contains one or more abstract methods

• Subclasses must override

 Instance variables, concrete methods

of abstract class

• subject to normal rules of inheritance

Keyword abstract

Use to declare a class abstract

Also use to declare a method

abstract

Abstract classes normally contain

one or more abstract methods

All concrete subclasses must

override all inherited abstract

methods

Abstract Classes and Methods

 Iterator class

• Traverses all the objects in a collection, such as an array
• Often used in polymorphic programming to traverse a collection that contains references to objects from various levels of a hierarchy

Beware! Compile Time Errors

 Attempting to instantiate an object of

an abstract class

 Failure to implement a superclass’s

abstract methods in a subclass

• unless the subclass is also declared abstract.

Creating Abstract Superclass Employee

abstract superclass Employee,

Figure 10.

• earnings is declared abstract No implementation can be given for earnings in the Employee abstract class
• An array of Employee variables will store references to subclass objects earnings method calls from these variables will call the appropriate version of the earnings method

Polymorphic interface for the

Employee hierarchy classes.

Note in Example Hierarchy

 Dynamic binding

• Also known as late binding
• Calls to overridden methods are resolved at execution time, based on the type of object referenced

 instanceof operator

• Determines whether an object is an instance of a certain type