public class MyEvent {}

public interface MyListener {
 void eventFired(MyEvent evt);
}

public class AnObservable {
 private List<MyListener> listeners;

 public void fire() {
   for (MyListener ml : listeners) ml.fire(new MyEvent());
 }

 public void addListener(MyListener ml) {
   listener.add(ml);
 }
}

public class AnObserver {
 private MyListener myListener = new MyListener() {
   public void eventFired(MyEvent evt) {
       // react to event being fired
   }
 };
}