仅包含唯一元素的 Java 阻塞队列

Question

有点像“阻塞集”。我如何实现阻塞队列，其中忽略添加已在集合中的成员？

Answer 1

我写这个类是为了解决类似的问题:

/**
 * Linked blocking queue with {@link #add(Object)} method, which adds only element, that is not already in the queue.
 */
public class SetBlockingQueue<T> extends LinkedBlockingQueue<T> {

    private Set<T> set = Collections.newSetFromMap(new ConcurrentHashMap<>());

    /**
     * Add only element, that is not already enqueued.
     * The method is synchronized, so that the duplicate elements can't get in during race condition.
     * @param t object to put in
     * @return true, if the queue was changed, false otherwise
     */
    @Override
    public synchronized boolean add(T t) {
        if (set.contains(t)) {
            return false;
        } else {
            set.add(t);
            return super.add(t);
        }
    }

    /**
     * Takes the element from the queue.
     * Note that no synchronization with {@link #add(Object)} is here, as we don't care about the element staying in the set longer needed.
     * @return taken element
     * @throws InterruptedException
     */
    @Override
    public T take() throws InterruptedException {
        T t = super.take();
        set.remove(t);
        return t;
    }
}