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 look in the child class of the actual object (in other words, it will use the class that was instantiated) for the version of the method to call.

• If the method was overridden, it will call the child version.

• If the method was not overridden, it will call the parent version because that is what the child class inherits.

• Note that the exact code for the method being called is resolved at run-time. (This is called late-binding or dynamic-binding.)

• The method signature must be defined in the parent class - this allows the syntax of the call to be verified at compile-time. (This is called early-binding).

• The parent-class reference can be a variable, parameter, array, or other Collection such as an ArrayList.

• 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 paint(). (So each of the child classes overrides the paint() method in MyShape).

  • You then have a graphical program (either an application or an Applet) using these classes, and want to randomly fill an array (or ArrayList) with shapes.

  • You can then write code like these snippets:

    
    ArrayList<MyShape> shapes = new ArrayList<MyShape>();
    ...
    MyShape shape;
    for (int i = 0; i < 10; i++)
    {
        shape = generateRandomShape(); // randomly returns a MyCircle or a MySquare
        shapes.add(shape);
    }
    ...
    public void paint(Graphics g)
    {
        for (MyShape shape:shapes)
        {
            shape.paint(g);
        }
    }
    
    

• How would we do this with an argument to a method rather than using an ArrayList?