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Related: Does java have a "LinkedConcurrentHashMap" data structure? I am looking for a collection class to hold references to event listeners. Ideally I would like the collection to have the following properties (in order of priority): Maintains insertion order. The earlier listeners may cancel the event, preventing it from being delivered to listeners added later. This will break if using a class such as HashSet whose iterator may return elements in the wrong order. Uses WeakReference s so that the listener list does not prevent the listeners from being garbage-collected. The collection is a Set , so duplicates are automatically removed. The Iterator is a thread-safe snapshot of the collection, unaffected by the addition of new listeners. Also allows events to be delivered on multiple threads. (This is not essential - I could iterate over a clone of the set instead.) I am aware of some classes that satisfy some but not all of these criteria. Examples: java.util.LinkedHashSet (#1 and #3) java.util.WeakHashMap , wrapped by Collections.newSetFromMap (#2 and #3) javax.swing.event.EventListenerList (needs some extra synchronization) (#1 and #4) java.util.concurrent.CopyOnWriteArraySet (#1, #3 and #4) But nothing with both #1 and #2. Does class like this exist in a library somewhere?

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