CODEWITHSUNDEEP
java
Max
Max

Reputation: 3914

Override a function without extending the class

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.

Upvotes: 26

Views: 11469

Answers (3)

Alexandre Dupriez
Alexandre Dupriez

Reputation: 3036

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 a lambda, which should come along the next release 8 of Java SE.

Upvotes: 17

Michael Anderson
Michael Anderson

Reputation: 73520

I like to do this kind of thing with a delegate, or "strategy pattern".

public interface ADelegate {
   public void method1();
}

public class A {
    public A(){....}
    public ADelegate delegate;
    public final void method1() { delegate.method1(); }
};

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

Upvotes: 3

jensgram
jensgram

Reputation: 31498

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.

Upvotes: 50

Related Questions