    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     * Gets an empty {@code Queue}.
    042     *
    043     * @param <E> the type of the elements in the queue
    044     * @return an empty {@link Queue}
    045     */
    046    @SuppressWarnings("unchecked") // OK, empty queue is compatible with any type
    047    public static <E> Queue<E> emptyQueue() {
    048        return EMPTY_QUEUE;
    049    }
    050
    051    /**
    052     * Returns a predicated (validating) queue backed by the given queue.
    053     * <p>
    054     * Only objects that pass the test in the given predicate can be added to the queue.
    055     * Trying to add an invalid object results in an IllegalArgumentException.
    056     * It is important not to use the original queue after invoking this method,
    057     * as it is a backdoor for adding invalid objects.
    058     *
    059     * @param <E> the type of the elements in the queue
    060     * @param queue  the queue to predicate, must not be null
    061     * @param predicate  the predicate used to evaluate new elements, must not be null
    062     * @return a predicated queue
    063     * @throws NullPointerException if the queue or predicate is null
    064     */
    065    public static <E> Queue<E> predicatedQueue(final Queue<E> queue, final Predicate<? super E> predicate) {
    066        return PredicatedQueue.predicatedQueue(queue, predicate);
    067    }
    068
    069    /**
    070     * Returns a synchronized (thread-safe) queue backed by the given queue.
    071     * In order to guarantee serial access, it is critical that all access to the
    072     * backing queue is accomplished through the returned queue.
    073     * <p>
    074     * It is imperative that the user manually synchronize on the returned queue
    075     * when iterating over it:
    076     *
    077     * <pre>
    078     * Queue queue = QueueUtils.synchronizedQueue(new CircularFifoQueue());
    079     * ...
    080     * synchronized(queue) {
    081     *     Iterator i = queue.iterator(); // Must be in synchronized block
    082     *     while (i.hasNext())
    083     *         foo(i.next());
    084     *     }
    085     * }
    086     * </pre>
    087     *
    088     * Failure to follow this advice may result in non-deterministic behavior.
    089     *
    090     * @param <E> the element type
    091     * @param queue the queue to synchronize, must not be null
    092     * @return a synchronized queue backed by that queue
    093     * @throws NullPointerException if the queue is null
    094     * @since 4.2
    095     */
    096    public static <E> Queue<E> synchronizedQueue(final Queue<E> queue) {
    097        return SynchronizedQueue.synchronizedQueue(queue);
    098    }
    099
    100    /**
    101     * Returns a transformed queue backed by the given queue.
    102     * <p>
    103     * Each object is passed through the transformer as it is added to the
    104     * Queue. It is important not to use the original queue after invoking this
    105     * method, as it is a backdoor for adding untransformed objects.
    106     * <p>
    107     * Existing entries in the specified queue will not be transformed.
    108     * If you want that behavior, see {@link TransformedQueue#transformedQueue}.
    109     *
    110     * @param <E> the type of the elements in the queue
    111     * @param queue  the queue to predicate, must not be null
    112     * @param transformer  the transformer for the queue, must not be null
    113     * @return a transformed queue backed by the given queue
    114     * @throws NullPointerException if the queue or transformer is null
    115     */
    116    public static <E> Queue<E> transformingQueue(final Queue<E> queue,
    117                                                 final Transformer<? super E, ? extends E> transformer) {
    118        return TransformedQueue.transformingQueue(queue, transformer);
    119    }
    120
    121    /**
    122     * Returns an unmodifiable queue backed by the given queue.
    123     *
    124     * @param <E> the type of the elements in the queue
    125     * @param queue  the queue to make unmodifiable, must not be null
    126     * @return an unmodifiable queue backed by that queue
    127     * @throws NullPointerException if the queue is null
    128     */
    129    public static <E> Queue<E> unmodifiableQueue(final Queue<? extends E> queue) {
    130        return UnmodifiableQueue.unmodifiableQueue(queue);
    131    }
    132
    133    /**
    134     * Don't allow instances.
    135     */
    136    private QueueUtils() {}
    137}