kofa <>
Wed, 27 Jun 2007 15:09:48 -0000

I have a problem that I have been unable to code cleanly with
generics. I suspect it is not possible because of no run-time generics
info. The problematic code (a cast, highlighted in capitals in
comments) is in dispatchEvent(Event e) below.

Suppose I have a hierarchy of events. XEvent extends Event, YEvent
extends Event etc.

The purpose is to provide a type-safe event listener and an event

I could create the interface:
public interface <T extends Event> EventListener {
  void eventRaised(T event);

And also:
public interface EventManager {
  <T extends Event> void addListener(EventManager<T> listener,
Class<T> type);
  <T extends Event> void removeListener(EventManager<T> listener,
Class<T> type);
  void raiseEvent(Event event);

I think the fact that I need to specify T in addListener twice (in
listener type and class type) already shows something's wrong...

Now implementing this seems impossible without casts and unchecked
public class EventManagerImpl implements EventManager {
  // no way to express the binding between event subclass as Map key
and event type of listener
  private final Map<Class<? extends Event>, Set<EventListener<?
extends Event>>> myListenersByType = new HashMap<Class<? extends
Event>, Set<EventListener<? extends Event>>>();

  // addListener, removeListener omitted for brevity

  public void dispatchEvent(Event event) {
    for (Map.Entry<Class<? extends Event>, Set<EventListener<? extends
Event>>> entry : myListenersByType.entrySet()) {
      if (entry.getKey().isInstance(event)) {
        for (EventListener<? extends Event> listener:
entry.getValue()) {
          // UGLY CAST HERE - could this be avoided?
          ((EventListener<Event>) listener).eventRaised(event);


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