并发设置队列

如果您想要比完全同步更好的并发性，那么我知道有一种方法可以使用ConcurrentHashMap作为支持映射。以下仅为草图。

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;  }}

用这种方法，一切都不是阳光。除了使用背景图之外

entrySet().iterator().next()

注意：此代码在一些地方使用了番石榴。