僅包含唯一元素的Java阻塞隊列

Question

有點像「阻塞集」。我如何實現一個阻塞隊列，其中添加已經在組中的成員將被忽略？

Answer 1

您可以覆蓋任何實施BlockingQueue<T>的add和put方法，以首先檢查元素是否已經在隊列中，例如，

@Override 
public boolean add(T elem) { 
    if (contains(elem)) 
     return true; 
    return super.add(elem); 
} 

Answer 2

您可以創建一個組成BlockingQueue，Set和鎖的新類。當你把（）你測試對照集合，同時拿着防止get（）運行的鎖。當你得到（）時，你從該集合中刪除該項目，以便它可以在將來再次放置（）。

Answer 3

class BlockingSet extends ArrayBlockingQueue<E> { 
    /*Retain all other methods except put*/ 
    public void put(E o) throws InterruptedException { 
     if (!this.contains(o)){ 
     super.put(o); 
     } 
    } 
} 

Answer 4

通過用於預知迭代順序和一定的時間增加，刪除和鏈接哈希set支持一種阻塞隊列實現包含操作：

There you go。

Answer 5

我寫了這個類來解決類似的問題：

/** 
* 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; 
    } 
}