Override a function without extending the class

Question

Suppose I have a class A:

public class A {
    public A(){....}
    public void method1() {...}
};

And an instance of that class:

A anA = new A();

Is there any way to override the method1() only for anA? This question arises when I write a small painting program in which I have to extend the JPanel class several times just to make minor changes to the different panels that have slightly different characteristics.

jensgram · Accepted Answer · 2012-08-13 06:35:12Z

You can do the following:

A anA = new A() {
    public void method1() {
        ...
    }
};

This is the same as:

private static class myA extends A {
    public void method1() {
        ...
    }
}

A anA = new myA();

Only with the exception that in this case myA can be reused. That's not possible with anonymous classes.

True, however technically this is creating an anonymous class that extends the class A. — Stephen C, Aug 13 '12 at 6:37
@StephenC Indeed. I have updated the answer. Thanks for pointing out. — jensgram, Aug 13 '12 at 6:38

score 11 · Answer 2

You can create a an new anonymous class on the fly, as long as you are using the no-arg constructor of your class A:

A anA = new A() {

  @Override
  public void method1() {
    ...
  }
};

Note that what you want to do is very close to what is known as closure, which should come along with next release 8 of Java SE.

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Override a function without extending the class

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