When a listener is removed, is it okay that the event be called on that listener one more time?

Question

Most people use a ConcurrentLinkedQueue or CopyOnWriteArrayList to collect listeners and notify them when something happens. The drawback of that is that it is possible that a listener can get a notification while it already unregistered himself. This happens if the manager (collection, ..., give it a name) is iterating over the collection of listeners and some listener unregisters himself during that iteration.

So, the 'remove listener' contract is broken. Would you consider that as a problem?

Answer 1

Even if you checked the listener was still registered, in a multithreaded environment you still couldn't be sure it was still registered by the time you call it. Even for non-multithreaded situations, the standard implementation is to work on a copy.