Polymorphic method declarations

[Thirugnanam Saravanan]

A reference variable can refer to any object of the same type as the declared reference, or it can refer to any subtype of the declared type!

I have seen this in K & B SCJP Book. What is the main advantage in this kind of method declarations ?

List list = new ArrayList();

What is the problem with this declaration ?

ArrayList al = new ArrayList();

When should we choose polymorphic method declarations?

[Gitesh Ramchandani]

In those cases where you want type specific methods to be executed. Like Animal class having an eat method. Now based on cat and dog subclasses object, you prefer to have the eat method of either dog/cat executed.

Example:

 
class Shape {
  void draw() {
  }
 
  void erase() {
  }
}
 
class Circle extends Shape {
  void draw() {
    System.out.println("Circle.draw()");
  }
 
  void erase() {
    System.out.println("Circle.erase()");
  }
}
 
class Square extends Shape {
  void draw() {
    System.out.println("Square.draw()");
  }
 
  void erase() {
    System.out.println("Square.erase()");
  }
}
 
class Triangle extends Shape {
  void draw() {
    System.out.println("Triangle.draw()");
  }
 
  void erase() {
    System.out.println("Triangle.erase()");
  }
}
 
// A "factory" that randomly creates shapes:
 
class RandomShapeGenerator {
  private Random rand = new Random();
 
  public Shape next() {
    switch (rand.nextInt(3)) {
    default:
    case 0:
      return new Circle();
    case 1:
      return new Square();
    case 2:
      return new Triangle();
    }
  }
}
 
public class ShapesDemo {
  private static RandomShapeGenerator gen = new RandomShapeGenerator();
 
  public static void main(String[] args) {
    Shape[] s = new Shape[9];
    // Fill up the array with shapes:
    for (int i = 0; i < s.length; i++)
      s[i] = gen.next();
    // Make polymorphic method calls:
    for (int i = 0; i < s.length; i++)
      s[i].draw();
 
  }
}
   

[ February 08, 2008: Message edited by: Gitesh Ramchandani ]