Abstract Classes

AP Computer Science A

Chris Vollmer

Purpose of Abstract Classes

• An abstract class is a placeholder in a class hierarchy that represents a generic concept

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• An abstract class is a class that is declared abstract
• it may or may not include abstract methods.

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• Cannot be instantiated, but they can be subclassed.

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Java Documentation

Abstract Classes

• We use the modifier abstract on the class header to declare a class as abstract:

public abstract class Whatever

{

// contents

}

Abstract Classes

• Subclass usually provides implementations for all of the abstract methods in its parent class.
• if it does not, then the subclass must also be declared abstract.

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Abstract Class - Code

public abstract class MyAbstractClass

{� public abstract void abstractMethod( );�}

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public class MySubClass extends MyAbstractClass

{� public void abstractMethod( )

{� System.out.println("My method implementation");� }�}

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Abstract Classes

• An abstract class often contains abstract methods with no definitions (like an interface does)
• Unlike an interface, the abstract modifier must be applied to each abstract method
• Typically contains non-abstract methods (with bodies), further distinguishing abstract classes from interfaces
• A class declared as abstract does not need to contain abstract methods

• You want to share code among several closely related classes.
• You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
• You want to declare non-static or non-final fields.
• This enables you to define methods that can access and modify the state of the object to which they belong.

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When to Use?

Abstract Classes

• The child of an abstract class must override the abstract methods of the parent, or it too will be considered abstract
• An abstract method cannot be defined as static (because it has no definition yet)
• The use of abstract classes is a design decision – it helps us establish common elements in a class that is too general to instantiate

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abstract class GraphicObject

{� int x, y;� ...

void moveTo(int newX, int newY)

{� ...� }� abstract void draw();� abstract void resize();�}�

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class Rectangle extends GraphicObject

{� void draw() { definition }� void resize() { definition }

}

class Circle extends GraphicObject {� void draw() { definition }� void resize() { definition }�}��

Resources

• See Pets.java (page 411)
• See Pet.java (page 412)
• See Dog.java (page 413)
• See Snake.java (page 414)

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https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html