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The RabbitMQ Java client library allows Java applications to interface with RabbitMQ.

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// Copyright (c) 2007-Present Pivotal Software, Inc.  All rights reserved.
//
// This software, the RabbitMQ Java client library, is triple-licensed under the
// Mozilla Public License 1.1 ("MPL"), the GNU General Public License version 2
// ("GPL") and the Apache License version 2 ("ASL"). For the MPL, please see
// LICENSE-MPL-RabbitMQ. For the GPL, please see LICENSE-GPL2.  For the ASL,
// please see LICENSE-APACHE2.
//
// This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND,
// either express or implied. See the LICENSE file for specific language governing
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package com.rabbitmq.client.impl;

import java.util.HashSet;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Set;

/**
 * 

A generic queue-like implementation (supporting operations addIfNotPresent, * poll, contains, and isEmpty) * which restricts a queue element to appear at most once. * If the element is already present {@link #addIfNotPresent} returns false. *

* Elements must not be null. *

Concurrent Semantics

* This implementation is not thread-safe. * @param type of elements in the queue */ public class SetQueue { private final Set members = new HashSet(); private final Queue queue = new LinkedList(); /** Add an element to the back of the queue and return true, or else return false. * @param item to add * @return true if the element was added, false if it is already present. */ public boolean addIfNotPresent(T item) { if (this.members.contains(item)) { return false; } this.members.add(item); this.queue.offer(item); return true; } /** Remove the head of the queue and return it. * @return head element of the queue, or null if the queue is empty. */ public T poll() { T item = this.queue.poll(); if (item != null) { this.members.remove(item); } return item; } /** @param item to look for in queue * @return true if and only if item is in the queue.*/ public boolean contains(T item) { return this.members.contains(item); } /** @return true if and only if the queue is empty.*/ public boolean isEmpty() { return this.members.isEmpty(); } /** Remove item from queue, if present. * @param item to remove * @return true if and only if item was initially present and was removed. */ public boolean remove(T item) { this.queue.remove(item); // there can only be one such item in the queue return this.members.remove(item); } /** Remove all items from the queue. */ public void clear() { this.queue.clear(); this.members.clear(); } }




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