Introduction to Polymorphism

• Imagine that you have a class hierarchy of shapes:

  Shape:

  • Polygon
    • Triangle
    • Quadrilateral
      • Parallelogram
        • Rectangle
          • Square
        • Rhombuse
      • Trapezoid
  • Circle
• Imagine that you have an applet that declares an array of Shape:

  Shape arrayOfShapes[] = new Shape[100];

• Imagine that you have a method that iterates through the array, 1 element at a time, and draws each shape encountered by invoking a draw method that exists in the Shape class, but is overridden in each of its subclasses.

public void paint(Graphics g) {

for (int i = 0; i < arrayOfShapes.length; i++)

arrayOfShapes[i].draw(g);

}

• Imagine that if you add a new subclass of Triangle, say Scalene (with its associated draw method), your draw method in Shape does not have to change at all. Your applet just works!

  You have imagined an example of polymorphism.