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Chapter 9a

Abstract Classes & Dynamic Binding

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Abstract Classes

• All classes so far have been concrete classes– Classes that can be used to create object

• We can also create abstract classes– Classes that can not be used to create objects– Abstract classes can be inherited as the basis for

other classes

• The purpose of an abstract class is to be inherited by another class

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Abstract Classes

• An abstract class is a class that contains at least one abstract method

• An abstract method– Defined using the keyword abstract– A method that has no method statements

• The class that inherits the abstract class must override the abstract methods

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Abstract Class Definitionpublic abstract class Animal

{

private String name;

public String getName( )

{

return name;

}

public abstract void speak( );

}

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Inheriting Abstract Classes

public class Dog extends Animal

{

*

*

public void speak( )

{

screen.println(“Woof”);

}

}

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Declaring Objects

• Declaring an object of an abstract class is illegal

Animal someAnimal = new Animal( );

• Declaring an object of a class that inherits an abstract class

Dog myLab = new Dog(“Belle”);

myLab.speak( );

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Abstract Classes

• Why?– Abstract classes allow for the definition of class

methods without having to define the method’s code

• Each class that inherits the abstract class defines its version of the code for each abstract method

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Binding

• Binding– refers to the point in time when all the information

needed to call a function is known

• Two types of binding– Early (Compile time)– Late (Run time)

• Dynamic binding is another name for late binding

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Dynamic Binding

• A subclass object is also considered to be an object of the superclass– Every Dog object is also an Animal object

• While we can not create an object of an abstract class we can create a reference of the abstract class

• As a result a reference of an abstract class type can “point” to any object of a class that inherited the abstract class

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Abstract Class References

public class Cow extends Animal

public class Dog extends Animal

Animal ref; // reference

Cow aCow = new Cow(“Bossie”);

Dog aDog = new Dog(“Sophie”);

ref = aCow;

ref.speak( );

ref = aDog;

ref.speak( );

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Dynamic Binding

• Two different methods named speak( ) are executed

• Which version is executed is determined at run-time based on the reference address

ref = aCow;

ref.speak( );

ref = aDog;

ref.speak( );

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Significance

• A abstract class reference allows you to act upon a diverse group of similar class objects

• No objects of type Animal but a reference of type Animal allows you to access object of type Cow and Dog

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Arrays of Subclass Objects

public class Cow inherits Animal

public class Dog inherits Animal

Animal[ ] ref = new Animal[2]; // reference array

Cow aCow = new Cow(“Bossie”);

Dog aDog = new Dog(“Sophie”);

ref [0] = aCow;

ref [1] = aDog;

for ( i = 0; i < 2; i++)

ref [i].speak( );