Object-Oriented Programming in Java - Java in a Nutshell - page 130

}
class Circle extends Shape {
public static final double PI = 3.14159265358979323846 ;
protected double r ; // Instance data
public Circle ( double r ) { this . r = r ; } // Constructor
public double getRadius () { return r ; } // Accessor
public double area () { return PI * r * r ; } // Implementations of
public double circumference () { return 2 * PI * r ; } // abstract methods.
}
class Rectangle extends Shape {
protected double w , h ; // Instance data
public Rectangle ( double w , double h ) { // Constructor
this . w = w ; this . h = h ;
}
public double getWidth () { return w ; } // Accessor method
public double getHeight () { return h ; } // Another accessor
public double area () { return w * h ; } // Implementation of
public double circumference () { return 2 *( w + h ); } // abstract methods
}
Each abstract method in Shape has a semicolon right after its parentheses. They
have no curly braces, and no method body is defined. Using the classes defined in
Example 3-5 , we can now write code such as:
Shape [] shapes = new Shape [ 3 ]; // Create an array to hold shapes
shapes [ 0 ] = new Circle ( 2.0 ); // Fill in the array
shapes [ 1 ] = new Rectangle ( 1.0 , 3.0 );
shapes [ 2 ] = new Rectangle ( 4.0 , 2.0 );
double totalArea = 0 ;
for ( int i = 0 ; i < shapes . length ; i ++)
totalArea += shapes [ i ]. area (); // Compute the area of the shapes
Notice two important points here:
• Subclasses of Shape can be assigned to elements of an array of Shape . No cast is
necessary. This is another example of a widening reference type conversion
(discussed in Chapter 2 ).
• You can invoke the area() and circumference() methods for any Shape
object, even though the Shape class does not define a body for these methods.
When you do this, the method to be invoked is found using virtual method
lookup, which means that the area of a circle is computed using the method
defined by Circle , and the area of a rectangle is computed using the method
defined by Rectangle .