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    <p>This is my attempt at creating a weak-reference copy-on-write array set. <strong>I rewrote it based on the list implementation that you provided in your question.</strong> <code>iterator</code> method is the only major addition and a minor change in the <code>add</code> method to make the behavior compatible with a set. Then I removed some methods that are not needed by the set implementation.</p> <p><strong>Note</strong>: I did basic testing of this code and it passed. But this might not be the most efficient implementation nor is it fully tested. Use it at your own risk. </p> <p><strong>Update</strong>: Added <code>removeReleased()</code> call to <code>remove()</code> method so that <code>iterator()</code> will not return the removed elements while iterating.</p> <pre><code> public class CopyOnWriteWeakReferenceSet&lt;E> extends AbstractSet&lt;E> { private static final long serialVersionUID = -3127064371410565215L; private CopyOnWriteArraySet&lt;WeakReference&lt;E>> items; public CopyOnWriteWeakReferenceSet() { items = new CopyOnWriteArraySet&lt;WeakReference&lt;E>>(); } public CopyOnWriteWeakReferenceSet(Collection&lt;E> c) { items = new CopyOnWriteArraySet&lt;WeakReference&lt;E>>(); addAll(c); } public boolean add(E element) { if (contains(element)) return false; return items.add(new WeakReference&lt;E>(element)); } @Override public boolean remove(Object o) { boolean removed = false; E element = null; for (WeakReference&lt;E> ref : items) { element = ref.get(); if (element != null && element.equals(o)) { ref.clear(); removed = true; break; } } if (removed) removeReleased(); return removed; } @Override public boolean contains(Object o) { List&lt;E> list = new ArrayList&lt;E>(); for (WeakReference&lt;E> ref : items) { if (ref.get() != null) { list.add(ref.get()); } } boolean contains = list.contains(o); list.clear(); list = null; return contains; } public int size() { removeReleased(); return items.size(); } private void removeReleased() { for (WeakReference&lt;E> ref : items) { if (ref.get() == null) { items.remove(ref); } } } @Override public Iterator&lt;E> iterator() { final Iterator&lt;WeakReference&lt;E>> iter = items.iterator(); return new Iterator&lt;E>() { @Override public boolean hasNext() { return iter.hasNext(); } @Override public E next() { return iter.next().get(); } @Override public void remove() { iter.remove(); } }; } public static void main(String[] args) { CopyOnWriteWeakReferenceSet&lt;String> set = new CopyOnWriteWeakReferenceSet&lt;String>(); set.add("3"); set.add("1"); set.add("2"); set.add("1"); //testing iterator multiple times System.out.println("----------------------------"); System.out.println("Iterator test start"); System.out.println("----------------------------"); for (String str : set) { System.out.println(str); } System.out.println("----------------------------"); for (String str : set) { System.out.println(str); } System.out.println("----------------------------"); for (String str : set) { System.out.println(str); } System.out.println("----------------------------"); //Trying iteration using iterator object Iterator&lt;String> strIter = set.iterator(); while(strIter.hasNext()) { System.out.println(strIter.next()); //This remove call will throw //java.lang.UnsupportedOperationException //even on the original CopyOnWriteArraySet. //strIter.remove(); } System.out.println("----------------------------"); System.out.println("Iterator test end"); System.out.println("----------------------------"); System.out.println("----------------------------"); System.out.println("Remove test start"); System.out.println("----------------------------"); System.out.println("Set size now " + set.size()); set.remove("3"); System.out.println(set.contains("3")); //false for (String str : set) { System.out.println(str); //1 and 2 } System.out.println("Set size now " + set.size()); //2 System.out.println("----------------------------"); System.out.println("Remove test end"); System.out.println("----------------------------"); } } </code></pre>
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    1. POIs there an open-source implementation of a weak-reference copy-on-write set for Java?
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    1. USNarendra Yadala
    1. USNarendra Yadala
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    1. POIs there an open-source implementation of a weak-reference copy-on-write set for Java?
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      1. This table or related slice is empty.
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    1. COA `contains` method for a set really shouldn't iterate over the entire set.
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      1. USAlexey Romanov
    2. CO@AlexeyRomanov I am not sure if it is possible for a weak reference set to avoid iterating over the whole set to find if the element exists in it. Even a normal (non-weak reference) `CopyOnWriteArraySet` is backed by a `CopyOnWriteArrayList` and even that iterates over the entire set to implement `contains`.
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