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POSuitable collection class for event listeners in Java
 

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    <p>Related: <a href="https://stackoverflow.com/questions/1391918/does-java-have-a-linkedconcurrenthashmap-data-structure">Does java have a &quot;LinkedConcurrentHashMap&quot; data structure?</a></p> <hr> <p>I am looking for a collection class to hold references to event listeners.</p> <p>Ideally I would like the collection to have the following properties (in order of priority):</p> <ol> <li>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 <code>HashSet</code> whose iterator may return elements in the wrong order.</li> <li>Uses <code>WeakReference</code>s so that the listener list does not prevent the listeners from being garbage-collected.</li> <li>The collection is a <code>Set</code>, so duplicates are automatically removed.</li> <li>The <code>Iterator</code> 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.)</li> </ol> <p>I am aware of some classes that satisfy some but not all of these criteria. Examples:</p> <ul> <li><code>java.util.LinkedHashSet</code> (#1 and #3)</li> <li><code>java.util.WeakHashMap</code>, wrapped by <code>Collections.newSetFromMap</code> (#2 and #3)</li> <li><code>javax.swing.event.EventListenerList</code> (needs some extra synchronization) (#1 and #4)</li> <li><code>java.util.concurrent.CopyOnWriteArraySet</code> (#1, #3 and #4)</li> </ul> <p>But nothing with both #1 and #2. Does class like this exist in a library somewhere?</p>
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    1. COIs it really typical for listener objects to implement `equals`?
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    2. COAlso, whose job is it to retain a strong reference to each listener? If I wrote a listener to log various events, and registered it, I'd sure be surprised when it suddenly disappeared a few minutes later!
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    3. CONo the problem with equals is: `{Object o = new Object(); WeakReference r1 = new WeakReference(o), r2 = new WeakReference(o); return r1.equals(r2);}` returns `false`
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