Abstraction (Cont.)

---

An Abstraction Example
Consider a classic “shape” example, perhaps used in a computer-aided design system or game simulation. The base type is shape and each shape has a color, size, and so on. From this, specific types of shapes are derived (inherited)—circle, square, triangle, and so on—each of which may have additional characteristics and behaviors.

For example, some behaviors may be different, such as the area of a shape. The type hierarchy embodies both the similarities and differences between the shapes. Note that the type of area has been changed from double to int.

AbstractDemo.java (the abstract keyword)

view source print? 01 // Java program to illustrate the concept of abstraction 02 public class AbstractDemo { 03   public static void main( String[ ] args ) { 04     Shape s1 = new Circle   ( args[0], Double.parseDouble( args[1] ) ); 05     Shape s2 = new Rectangle( args[2], Double.parseDouble( args[3] ), 06                                        Double.parseDouble( args[4] ) ); 07     System.out.print( s1.toString( ) + " " ); 08     System.out.print( s2.toString( ) ); 09   } 10 }

Shape.java (the abstract keyword)

view source print? 01 abstract class Shape { 02   String color; 03   04   // These are abstract methods. 05   abstract int area( ); 06   public abstract String toString( ); 07   08   // Abstract classes can have constructors. 09   public Shape( String color ) { this.color = color; } 10   11   // This is a concrete method. 12   public String getColor( ) { return color; } 13 }

Circle.java (the abstract keyword)

view source print? 01 class Circle extends Shape { 02   double radius; 03   public Circle( String color, double radius ) { 04     // Calling Shape constructor 05     super( color ); 06     this.radius = radius; 07   } 08   @Override 09   int area( ) { return (int) ( Math.PI * Math.pow( radius, 2 ) ); } 10   @Override 11   public String toString( ) { return super.color + " " + area( ); } 12 }

Rectangle.java (the abstract keyword)

view source print? 01 class Rectangle extends Shape { 02   double length; 03   double width; 04   05   public Rectangle( String color, double length, double width ) { 06     // Calling Shape constructor 07     super( color ); 08     this.length = length; 09     this.width  = width; 10   } 11   @Override 12   int area( ) { return (int) ( length*width ); } 13   @Override 14   public String toString( ) { return super.getColor( ) + " " + area( ); } 15 }

shell> java AbstractDemo           Output:           Result: