When registering for Java events, duplicate listeners are usually allowed?

Question

Should the addListener method check for duplicate registrations when called? If so, what should happen when a duplicate is found?

final public class exampleCanFire {
    public void addFooListener(FooListener listener) {
      // Before adding listener to private list of listeners, should I check for duplicates?
    }
}

Answer 1

I prefer to store them in Listand not check for duplicates. Some advantages of this approach:

• Listeners are notified in a deterministic manner and can potentially mark events as “consumed,” because of which they do not apply to subsequent listeners.
• CopyOnWriteArrayList, ConcurrentModificationException ( ) ).

Answer 2

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Answer 3

, Set:

Set<FooListener> listeners = new HashSet<FooListener>();

public boolean addFooListener(FooListener listener) {
  return listeners.add(listener);
}

Answer 4

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