Highlights of Polymorphism

• In Java, when you have a parent/child relationship between 2 classes, you can have a parent-class variable refer to an object of the child class.

• If you use the parent reference to call a method, Java will first look in the child class for the version of the method to call.

• This is used when you have 1 parent with 2 or more child classes.

• The parent class reference can be a variable, parameter, or an array.

• For example:

  • You write a class called MyShape. It has a child class called MySquare and another one called MyCircle.

  • All 3 classes have a method called draw(). (So each of the child classes overrides the draw() method in MyShape).

  • You then have an graphical program (either an application or an Applet) using these classes, and want to draw a circle and a square, but you always want to print your name to the left of the object. (You should be thinking "function" or "method" right now!)

  • You can then write your main-class code like this:

            public void paint(Graphics g)
            {
                MySquare    aSquare= new MySquare(Color.red);
                MyCircle    aCircle = new MyCircle(Color.blue);
                
                draw(aSquare, g, 25);
                draw(aCircle, g, 200);
            }
    
            public void draw(MyShape shape, Graphics g, int row)
            {
                g.drawString("Cathy Bishop:", row, 25);
                shape.draw(row, 100);
            }
    
    
    

• How would we do this with an array rather than as an argument to a method?