    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.collections4;
    018
    019import java.util.LinkedList;
    020import java.util.Queue;
    021
    022import org.apache.commons.collections4.queue.PredicatedQueue;
    023import org.apache.commons.collections4.queue.SynchronizedQueue;
    024import org.apache.commons.collections4.queue.TransformedQueue;
    025import org.apache.commons.collections4.queue.UnmodifiableQueue;
    026
    027/**
    028 * Provides utility methods and decorators for {@link Queue} instances.
    029 *
    030 * @since 4.0
    031 */
    032public class QueueUtils {
    033
    034    /**
    035     * An empty unmodifiable queue.
    036     */
    037    @SuppressWarnings("rawtypes") // OK, empty queue is compatible with any type
    038    public static final Queue EMPTY_QUEUE = UnmodifiableQueue.unmodifiableQueue(new LinkedList<>());
    039
    040    /**
    041     * <code>QueueUtils</code> should not normally be instantiated.
    042     */
    043    private QueueUtils() {}
    044
    045    //-----------------------------------------------------------------------
    046
    047    /**
    048     * Returns a synchronized (thread-safe) queue backed by the given queue.
    049     * In order to guarantee serial access, it is critical that all access to the
    050     * backing queue is accomplished through the returned queue.
    051     * <p>
    052     * It is imperative that the user manually synchronize on the returned queue
    053     * when iterating over it:
    054     *
    055     * <pre>
    056     * Queue queue = QueueUtils.synchronizedQueue(new CircularFifoQueue());
    057     * ...
    058     * synchronized(queue) {
    059     *     Iterator i = queue.iterator(); // Must be in synchronized block
    060     *     while (i.hasNext())
    061     *         foo(i.next());
    062     *     }
    063     * }
    064     * </pre>
    065     *
    066     * Failure to follow this advice may result in non-deterministic behavior.
    067     *
    068     * @param <E> the element type
    069     * @param queue the queue to synchronize, must not be null
    070     * @return a synchronized queue backed by that queue
    071     * @throws NullPointerException if the queue is null
    072     * @since 4.2
    073     */
    074    public static <E> Queue<E> synchronizedQueue(final Queue<E> queue) {
    075        return SynchronizedQueue.synchronizedQueue(queue);
    076    }
    077
    078    /**
    079     * Returns an unmodifiable queue backed by the given queue.
    080     *
    081     * @param <E> the type of the elements in the queue
    082     * @param queue  the queue to make unmodifiable, must not be null
    083     * @return an unmodifiable queue backed by that queue
    084     * @throws NullPointerException if the queue is null
    085     */
    086    public static <E> Queue<E> unmodifiableQueue(final Queue<? extends E> queue) {
    087        return UnmodifiableQueue.unmodifiableQueue(queue);
    088    }
    089
    090    /**
    091     * Returns a predicated (validating) queue backed by the given queue.
    092     * <p>
    093     * Only objects that pass the test in the given predicate can be added to the queue.
    094     * Trying to add an invalid object results in an IllegalArgumentException.
    095     * It is important not to use the original queue after invoking this method,
    096     * as it is a backdoor for adding invalid objects.
    097     *
    098     * @param <E> the type of the elements in the queue
    099     * @param queue  the queue to predicate, must not be null
    100     * @param predicate  the predicate used to evaluate new elements, must not be null
    101     * @return a predicated queue
    102     * @throws NullPointerException if the queue or predicate is null
    103     */
    104    public static <E> Queue<E> predicatedQueue(final Queue<E> queue, final Predicate<? super E> predicate) {
    105        return PredicatedQueue.predicatedQueue(queue, predicate);
    106    }
    107
    108    /**
    109     * Returns a transformed queue backed by the given queue.
    110     * <p>
    111     * Each object is passed through the transformer as it is added to the
    112     * Queue. It is important not to use the original queue after invoking this
    113     * method, as it is a backdoor for adding untransformed objects.
    114     * <p>
    115     * Existing entries in the specified queue will not be transformed.
    116     * If you want that behaviour, see {@link TransformedQueue#transformedQueue}.
    117     *
    118     * @param <E> the type of the elements in the queue
    119     * @param queue  the queue to predicate, must not be null
    120     * @param transformer  the transformer for the queue, must not be null
    121     * @return a transformed queue backed by the given queue
    122     * @throws NullPointerException if the queue or transformer is null
    123     */
    124    public static <E> Queue<E> transformingQueue(final Queue<E> queue,
    125                                                 final Transformer<? super E, ? extends E> transformer) {
    126        return TransformedQueue.transformingQueue(queue, transformer);
    127    }
    128
    129    /**
    130     * Get an empty <code>Queue</code>.
    131     *
    132     * @param <E> the type of the elements in the queue
    133     * @return an empty {@link Queue}
    134     */
    135    @SuppressWarnings("unchecked") // OK, empty queue is compatible with any type
    136    public static <E> Queue<E> emptyQueue() {
    137        return EMPTY_QUEUE;
    138    }
    139}