Re: Generics

"Oliver Wong" <>
Fri, 29 Jun 2007 15:20:36 -0400
"kofa" <> wrote in message

Copying manually, casting individual elements solves that:
public <T extends Event> Set<EventListener<? super T>> get(Class<?
extends T> key) {
Set<EventListener<? super T>> listeners = new HashSet<EventListener<?
super T>>();
for (Entry<Class<? extends Event>, Set<EventListener<?>>> entry :
myListenersByType.entrySet()) {
listeners.add((EventListener<? super T>) entry.getValue());
return listeners;

Is there a nicer way?

    Depends on your metric for "niceness". I haven't been able to fully
get rid of warnings. I can move the warning around to different locations
where they might make more sense. For example, you could have the
listeners provide a way to "cast themselves" to the proper type:

class EventImpl<T extends Event> implements EventListener<T> {
 Class<? extends T> eventType;

 public void eventRaised(T event) {
  // TODO Auto-generated method stub


 public <U extends Event> EventListener<? super U>
castSelfAsHandlerFor(Class<? extends U> clazz) {
     if (eventType.isAssignableFrom(clazz)) {
      return (EventListener<? super U>) this;
     } else {
      return null;

Or you could have the code in the event manager take advantage of the fact
that it just knows (but can't express this in the Java type system) that
Class<T> is mapped onto EventListener<T>:

 public <T extends Event> void raiseEvent(T event) {
  for (Class<? extends Event> key : myListenersByType.keySet()) {
   if (key.isInstance(event)) {
    Set<EventListener<? super T>> listeners = myListenersByType
      .get((Class<? extends T>)key);
    for (EventListener<? super T> listener : listeners) {

    - Oliver

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