1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 package org.apache.commons.collections4; 18 19 import java.util.LinkedList; 20 import java.util.Queue; 21 22 import org.apache.commons.collections4.queue.PredicatedQueue; 23 import org.apache.commons.collections4.queue.SynchronizedQueue; 24 import org.apache.commons.collections4.queue.TransformedQueue; 25 import org.apache.commons.collections4.queue.UnmodifiableQueue; 26 27 /** 28 * Provides utility methods and decorators for {@link Queue} instances. 29 * 30 * @since 4.0 31 */ 32 public class QueueUtils { 33 34 /** 35 * An empty unmodifiable queue. 36 */ 37 @SuppressWarnings("rawtypes") // OK, empty queue is compatible with any type 38 public static final Queue EMPTY_QUEUE = UnmodifiableQueue.unmodifiableQueue(new LinkedList<>()); 39 40 /** 41 * Gets an empty {@code Queue}. 42 * 43 * @param <E> the type of the elements in the queue 44 * @return an empty {@link Queue} 45 */ 46 @SuppressWarnings("unchecked") // OK, empty queue is compatible with any type 47 public static <E> Queue<E> emptyQueue() { 48 return EMPTY_QUEUE; 49 } 50 51 /** 52 * Returns a predicated (validating) queue backed by the given queue. 53 * <p> 54 * Only objects that pass the test in the given predicate can be added to the queue. 55 * Trying to add an invalid object results in an IllegalArgumentException. 56 * It is important not to use the original queue after invoking this method, 57 * as it is a backdoor for adding invalid objects. 58 * 59 * @param <E> the type of the elements in the queue 60 * @param queue the queue to predicate, must not be null 61 * @param predicate the predicate used to evaluate new elements, must not be null 62 * @return a predicated queue 63 * @throws NullPointerException if the queue or predicate is null 64 */ 65 public static <E> Queue<E> predicatedQueue(final Queue<E> queue, final Predicate<? super E> predicate) { 66 return PredicatedQueue.predicatedQueue(queue, predicate); 67 } 68 69 /** 70 * Returns a synchronized (thread-safe) queue backed by the given queue. 71 * In order to guarantee serial access, it is critical that all access to the 72 * backing queue is accomplished through the returned queue. 73 * <p> 74 * It is imperative that the user manually synchronize on the returned queue 75 * when iterating over it: 76 * 77 * <pre> 78 * Queue queue = QueueUtils.synchronizedQueue(new CircularFifoQueue()); 79 * ... 80 * synchronized(queue) { 81 * Iterator i = queue.iterator(); // Must be in synchronized block 82 * while (i.hasNext()) 83 * foo(i.next()); 84 * } 85 * } 86 * </pre> 87 * 88 * Failure to follow this advice may result in non-deterministic behavior. 89 * 90 * @param <E> the element type 91 * @param queue the queue to synchronize, must not be null 92 * @return a synchronized queue backed by that queue 93 * @throws NullPointerException if the queue is null 94 * @since 4.2 95 */ 96 public static <E> Queue<E> synchronizedQueue(final Queue<E> queue) { 97 return SynchronizedQueue.synchronizedQueue(queue); 98 } 99 100 /** 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, final Transformer<? super E, ? extends E> transformer) { 117 return TransformedQueue.transformingQueue(queue, transformer); 118 } 119 120 /** 121 * Returns an unmodifiable queue backed by the given queue. 122 * 123 * @param <E> the type of the elements in the queue 124 * @param queue the queue to make unmodifiable, must not be null 125 * @return an unmodifiable queue backed by that queue 126 * @throws NullPointerException if the queue is null 127 */ 128 public static <E> Queue<E> unmodifiableQueue(final Queue<? extends E> queue) { 129 return UnmodifiableQueue.unmodifiableQueue(queue); 130 } 131 132 /** 133 * Don't allow instances. 134 */ 135 private QueueUtils() { 136 // empty 137 } 138 }