Java Cookbook, Second Edition

Recipe 9.8 Polymorphism/Abstract Methods

Problem

You want each of a number of subclasses to provide its own version of one or more methods.

Solution

Make the method abstract in the parent class; this makes the compiler ensure that each subclass implements it.

Discussion

A hypothetical drawing program uses a Shape subclass for anything that is drawn. Shape has an abstract method called computeArea( ) that computes the exact area of the given shape:

public abstract class Shape { protected int x, y; public abstract double computeArea( ); }

A Rectangle subclass, for example, has a computeArea( ) that multiplies width times height and returns the result:

public class Rectangle extends Shape { double width, height; public double computeArea( ) { return width * height; } }

A Circle subclass returns r²

public class Circle extends Shape { double radius; public double computeArea( ) { return Math.PI * radius * radius; } }

This system has a very high degree of generality. In the main program, we can pass over a collection of Shape objects and here's the real beauty call computeArea( ) on any Shape subclass object without having to worry about what kind of shape it is. Java's polymorphic methods automatically call the correct computeArea( ) method in the class of which the object was originally constructed:

/** Part of a main program using Shape objects */ public class Main { Collection allShapes; // created in a Constructor, not shown /** Iterate over all the Shapes, getting their areas */ public double totalAreas( ) { Iterator it = allShapes.iterator( ); double total = 0.0; while (it.hasNext( )) { Shape s = (Shape)it.next( ); total += s.computeArea( ); } return total; } }

Polymorphism is a great boon for software maintenance: if a new subclass is added, the code in the main program does not change. Further, all the code that is specific to, say, polygon handling, is all in one place: in the source file for the Polygon class. This is a big improvement over older languages, where type fields in a structure or record were used with case or switch statements scattered all across the software. Java makes software more reliable and maintainable with the use of polymorphism.