scijava-listeners

public interface MyListener
{
 void somethingChanged();
}
public class Listenable
{
 // Create a variant of Listeners.List
 private final Listeners.List< MyListener > listeners = new Listeners.SynchronizedList<>();
 // Use Listeners.List.list to call registered listeners
 private void notifyListeners() {
 listeners.list.forEach( MyListener::somethingChanged );
 }
 // expose only Listeners (not Listeners.List) to allow un/registering listeners
 public Listeners< MyListener > myListeners() {
 return listeners;
 }
}
public class Listening
{
 public Listening( Listenable l ) {
 l.myListeners().add( this::notifyMe );
 }
 void notifyMe() {
 System.out.println( "something changed!");
 }
}