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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;

import java.util.LinkedList;
import java.util.Queue;

import org.apache.commons.collections4.queue.PredicatedQueue;
import org.apache.commons.collections4.queue.SynchronizedQueue;
import org.apache.commons.collections4.queue.TransformedQueue;
import org.apache.commons.collections4.queue.UnmodifiableQueue;

/**
 * Provides utility methods and decorators for {@link Queue} instances.
 *
 * @since 4.0
 */
public class QueueUtils {

    /**
     * An empty unmodifiable queue.
     */
    @SuppressWarnings("rawtypes") // OK, empty queue is compatible with any type
    public static final Queue EMPTY_QUEUE = UnmodifiableQueue.unmodifiableQueue(new LinkedList<>());

    /**
     * QueueUtils should not normally be instantiated.
     */
    private QueueUtils() {}

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

    /**
     * Returns a synchronized (thread-safe) queue backed by the given queue.
     * In order to guarantee serial access, it is critical that all access to the
     * backing queue is accomplished through the returned queue.
     * 

* It is imperative that the user manually synchronize on the returned queue * when iterating over it: * *

     * Queue queue = QueueUtils.synchronizedQueue(new CircularFifoQueue());
     * ...
     * synchronized(queue) {
     *     Iterator i = queue.iterator(); // Must be in synchronized block
     *     while (i.hasNext())
     *         foo(i.next());
     *     }
     * }
     * 
* * Failure to follow this advice may result in non-deterministic behavior. * * @param the element type * @param queue the queue to synchronize, must not be null * @return a synchronized queue backed by that queue * @throws NullPointerException if the queue is null * @since 4.2 */ public static Queue synchronizedQueue(final Queue queue) { return SynchronizedQueue.synchronizedQueue(queue); } /** * Returns an unmodifiable queue backed by the given queue. * * @param the type of the elements in the queue * @param queue the queue to make unmodifiable, must not be null * @return an unmodifiable queue backed by that queue * @throws NullPointerException if the queue is null */ public static Queue unmodifiableQueue(final Queue queue) { return UnmodifiableQueue.unmodifiableQueue(queue); } /** * Returns a predicated (validating) queue backed by the given queue. *

* Only objects that pass the test in the given predicate can be added to the queue. * Trying to add an invalid object results in an IllegalArgumentException. * It is important not to use the original queue after invoking this method, * as it is a backdoor for adding invalid objects. * * @param the type of the elements in the queue * @param queue the queue to predicate, must not be null * @param predicate the predicate used to evaluate new elements, must not be null * @return a predicated queue * @throws NullPointerException if the queue or predicate is null */ public static Queue predicatedQueue(final Queue queue, final Predicate predicate) { return PredicatedQueue.predicatedQueue(queue, predicate); } /** * Returns a transformed queue backed by the given queue. *

* Each object is passed through the transformer as it is added to the * Queue. It is important not to use the original queue after invoking this * method, as it is a backdoor for adding untransformed objects. *

* Existing entries in the specified queue will not be transformed. * If you want that behaviour, see {@link TransformedQueue#transformedQueue}. * * @param the type of the elements in the queue * @param queue the queue to predicate, must not be null * @param transformer the transformer for the queue, must not be null * @return a transformed queue backed by the given queue * @throws NullPointerException if the queue or transformer is null */ public static Queue transformingQueue(final Queue queue, final Transformer transformer) { return TransformedQueue.transformingQueue(queue, transformer); } /** * Get an empty Queue. * * @param the type of the elements in the queue * @return an empty {@link Queue} */ @SuppressWarnings("unchecked") // OK, empty queue is compatible with any type public static Queue emptyQueue() { return EMPTY_QUEUE; } }





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