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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.collections4.queue;

import java.util.Queue;

import org.apache.commons.collections4.collection.SynchronizedCollection;

/**
 * Decorates another {@link Queue} to synchronize its behaviour for a multi-threaded environment.
 * 
 * Methods are synchronized, then forwarded to the decorated queue. Iterators must be separately synchronized around the
 * loop.
 *
 * @param  the type of the elements in the collection
 * @since 4.2
 */
public class SynchronizedQueue extends SynchronizedCollection implements Queue {

    /** Serialization version */
    private static final long serialVersionUID = 1L;

    /**
     * Factory method to create a synchronized queue.
     *
     * @param 
     *            the type of the elements in the queue
     * @param queue
     *            the queue to decorate, must not be null
     * @return a new synchronized Queue
     * @throws NullPointerException
     *             if queue is null
     */
    public static  SynchronizedQueue synchronizedQueue(final Queue queue) {
        return new SynchronizedQueue<>(queue);
    }

    // -----------------------------------------------------------------------
    /**
     * Constructor that wraps (not copies).
     *
     * @param queue
     *            the queue to decorate, must not be null
     * @throws NullPointerException
     *             if queue is null
     */
    protected SynchronizedQueue(final Queue queue) {
        super(queue);
    }

    /**
     * Constructor that wraps (not copies).
     *
     * @param queue
     *            the queue to decorate, must not be null
     * @param lock
     *            the lock to use, must not be null
     * @throws NullPointerException
     *             if queue or lock is null
     */
    protected SynchronizedQueue(final Queue queue, final Object lock) {
        super(queue, lock);
    }

    /**
     * Gets the queue being decorated.
     *
     * @return the decorated queue
     */
    @Override
    protected Queue decorated() {
        return (Queue) super.decorated();
    }

    @Override
    public E element() {
        synchronized (lock) {
            return decorated().element();
        }
    }

    @Override
    public boolean equals(final Object object) {
        if (object == this) {
            return true;
        }
        synchronized (lock) {
            return decorated().equals(object);
        }
    }

    // -----------------------------------------------------------------------

    @Override
    public int hashCode() {
        synchronized (lock) {
            return decorated().hashCode();
        }
    }

    @Override
    public boolean offer(final E e) {
        synchronized (lock) {
            return decorated().offer(e);
        }
    }

    @Override
    public E peek() {
        synchronized (lock) {
            return decorated().peek();
        }
    }

    @Override
    public E poll() {
        synchronized (lock) {
            return decorated().poll();
        }
    }

    @Override
    public E remove() {
        synchronized (lock) {
            return decorated().remove();
        }
    }

}