Discovering Classes and Objects - Beginning Java 7 - page 142

Listing2-36 ' s Shape classdeclaresanempty draw() methodthatonlyexiststobe
overridden and to demonstrate subtype polymorphism.
You can now refactor Listing 2-33 ' s Point class to extend Listing 2-36 's Shape
class,leave Circle asis,andintroducea Rectangle classthatextends Shape .You
canthenrefactor Listing2-34 ' s Graphics class's main() methodtotake Shape into
account. Check out the following main() method:
public static void main(String[] args)
{

Shape[] shapes = new Shape[] { new Point(10, 20), new
Circle(10, 20, 30),

new Rectangle(20, 30,

15, 25) };
for (int i = 0; i < shapes.length; i++)
shapes[i].draw();

}

Because Point and Rectangle directly extend Shape , and because Circle
indirectly extends Shape by extending Point , main() responds to
shapes[i].draw(); by calling the correct subclass's draw() method.
Although Shape makes the code more flexible, there is a problem. What is to
stop someone from instantiating Shape and adding this meaningless instance to the
shapes array, as follows?
Shape[] shapes = new Shape[] { new Point(10, 20), new
Circle(10, 20, 30),

new Rectangle(20, 30, 15,

25), new Shape() };
What does it mean to instantiate Shape ? Because this class describes an abstract
concept,whatdoesitmeantodrawagenericshape?Fortunately,Javaprovidesasolu-
tion to this problem, which is demonstrated in Listing 2-37 .

Listing 2-37. Abstracting the Shape class

abstract class Shape
{

abstract void draw(); // semicolon is required

}