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

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// Copyright (c) 2007-2020 VMware, Inc. or its affiliates.  All rights reserved.
//
// This software, the RabbitMQ Java client library, is triple-licensed under the
// Mozilla Public License 2.0 ("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
// rights and limitations of this software.
//
// If you have any questions regarding licensing, please contact us at
// [email protected].

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();
    }
}