Interfaces and Abstract Classes in Java

Objectives

• Distinguish between interfaces, abstract classes, and concrete classes
• Know and understand the purpose of an interface and an abstract class
• Know what is allowed in an interface, an abstract class, and a concrete class

Prerequisites

• Understand Java classes, objects, and methods
• Understand Java variable scope (local, instance, and class) and method scope (instance and class)

Definitions

abstract method

A method without a body (only a header followed by a semicolon)

concrete method

A normal method; a method with a body

interface

A class-like entity that has only class (static) data and abstract methods

abstract class

A class that cannot be used to create objects (cannot be instantiated) and may have abstract methods

concrete class

A normal class; a pattern or blueprint for creating objects

Purpose

• An interface is used as a parent in a realizes hierarchy and as the type of a reference variable.
• An abstract class is used as the parent in an inheritance hierarchy and as the type of a reference variable.

Rules

• It is not possible to create an object from an interface or abstract class.
• Interfaces may not contain instance data.
• Interfaces may not contain concrete instance methods.
• Concrete classes may not contain abstract instance methods.
• There is no such thing as an abstract class method. It doesn't make sense.
• Summary

  	(most abstract)		(least abstract)
  	interface	abstract class	concrete class
  Can be instantiated			√
  May contain class data	√	√	√
  May contain instance data		√	√
  May contain class methods	√	√	√
  May contain abstract instance methods	√	√
  May contain concrete instance methods		√	√

Example

import java.awt.Graphics2D;

interface Drawable {
	public static final double SPARKLE_COEFFICIENT = 0.26;  // class data

	public void draw(Graphics2D g);  // abstract method
}


class ChessBoard implements Drawable {
	public void draw(Graphics2D g) {  // concrete method
		// actual code here
	}
}


abstract class ChessPiece implements Drawable {
	protected String color;
	public abstract boolean move(int location);  // abstract method
}


class King extends ChessPiece {
	private boolean moved = false;  // instance data

	public boolean move(int location) {  // concrete method
		// actual code here
		moved = true;
		// actual return statement here
		return true;
	}

	public void draw(Graphics2D g) {  // concrete method
		// actual code here
	}
}


class Queen extends ChessPiece {
	public boolean move(int location) {  // concrete method
		// actual code here
		// actual return statement here
		return true;
	}

	public void draw(Graphics2D g) {  // concrete method
		// actual code here
	}
}


class Pawn extends ChessPiece {
	public boolean move(int location) {  // concrete method
		// actual code here
		// actual return statement here
		return true;
	}

	public void draw(Graphics2D g) {  // concrete method
		// actual code here
	}
}

Copyright © 2010, Maia L.L.C.  All rights reserved.