Creating Custom Listeners In Java

Question

I was wondering how it would be possible to create my own listener for something, and to be able to use it just like any other listener, ex.

public interface Listener {

    public void onListen(Event e);

}

And then this in my class

public class test implements Listener {

    Object.add(new Listener() { 

        @Override
        public void onListen(Event e) {

        }
    });
}

I guess what i'm really asking also is where do I define how to check if something happens pertaining to the listener that I create?

If any of this doesn't make sense, please tell me and I will clarify it.

If at best this makes no sense to you, i'll try this. How would I make a listener to check if the selected option in a combobox has changed to a different option. I don't actually need a combobox listener though, that was just an example.

Answer 1

Here is a short example:

    private static class Position {
    }

    private static class Ball {
        private ArrayList<FootballObserver> observers = new ArrayList<FootballObserver>();
        private Position ballPosition = new Position();

        public void registerObserver(FootballObserver observer) {
            observers.add(observer);
        }

        public void notifyListeners() {
            for(FootballObserver observer : observers) {
                observer.notify(ballPosition);
            }
        }

        public void doSomethingWithFootballPosition() {
            //bounce etc
            notifyListeners();
        }
    }

    private static interface FootballObserver {
        void notify(Position ball);
    }

    private static class Player implements FootballObserver {
        public void notify(Position ball) {
            System.out.println("received new ball position");
        }
    }

    public static void main(String... args) {
        FootballObserver player1 = new Player();
        Ball football = new Ball();
        football.registerObserver(player1);
        football.doSomethingWithFootballPosition();
    }

Answer 2

There is no trouble creating the listeners that you need, but you only can apply it to classes that expect it (so you cannot do (new Object()).addMyListener )

You will need to add to the classes where you want to use the listener an addMyListener(MyListener myListener) method. It just stores the listeners that you pass to it in a list for later user. It would be a good idea creating an interface with the addMyListener method (and a fireMyListeners() ) method too.

Of course, you must also provide the code to call the fireMyListeners() method, too. Then the fireMyListeners() will just loop through the listeners and call theirs notification method.