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copied!<p>If you want better concurrency than full synchronization, there is one way I know of to do it, using a ConcurrentHashMap as the backing map. The following is a sketch only.</p> <pre><code>public final class ConcurrentHashSet<E> extends ForwardingSet<E> implements Set<E>, Queue<E> { private enum Dummy { VALUE } private final ConcurrentMap<E, Dummy> map; ConcurrentHashSet(ConcurrentMap<E, Dummy> map) { super(map.keySet()); this.map = Preconditions.checkNotNull(map); } @Override public boolean add(E element) { return map.put(element, Dummy.VALUE) == null; } @Override public boolean addAll(Collection<? extends E> newElements) { // just the standard implementation boolean modified = false; for (E element : newElements) { modified |= add(element); } return modified; } @Override public boolean offer(E element) { return add(element); } @Override public E remove() { E polled = poll(); if (polled == null) { throw new NoSuchElementException(); } return polled; } @Override public E poll() { for (E element : this) { // Not convinced that removing via iterator is viable (check this?) if (map.remove(element) != null) { return element; } } return null; } @Override public E element() { return iterator().next(); } @Override public E peek() { Iterator<E> iterator = iterator(); return iterator.hasNext() ? iterator.next() : null; } } </code></pre> <p>All is not sunshine with this approach. We have no decent way to select a head element other than using the backing map's <code>entrySet().iterator().next()</code>, the result being that the map gets more and more unbalanced as time goes on. This unbalancing is a problem both due to greater bucket collisions and greater segment contention.</p> <p>Note: this code uses <a href="http://guava-libraries.googlecode.com" rel="noreferrer">Guava</a> in a few places.</p>

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