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| 1 | /* | ||
| 2 | * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. | ||
| 3 | * | ||
| 4 | * This code is free software; you can redistribute it and/or modify it | ||
| 5 | * under the terms of the GNU General Public License version 2 only, as | ||
| 6 | * published by the Free Software Foundation. Oracle designates this | ||
| 7 | * particular file as subject to the "Classpath" exception as provided | ||
| 8 | * by Oracle in the LICENSE file that accompanied this code. | ||
| 9 | * | ||
| 10 | * This code is distributed in the hope that it will be useful, but WITHOUT | ||
| 11 | * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | ||
| 12 | * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | ||
| 13 | * version 2 for more details (a copy is included in the LICENSE file that | ||
| 14 | * accompanied this code). | ||
| 15 | * | ||
| 16 | * You should have received a copy of the GNU General Public License version | ||
| 17 | * 2 along with this work; if not, write to the Free Software Foundation, | ||
| 18 | * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. | ||
| 19 | * | ||
| 20 | * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA | ||
| 21 | * or visit www.oracle.com if you need additional information or have any | ||
| 22 | * questions. | ||
| 23 | */ | ||
| 24 | |||
| 25 | /* | ||
| 26 | * This file is available under and governed by the GNU General Public | ||
| 27 | * License version 2 only, as published by the Free Software Foundation. | ||
| 28 | * However, the following notice accompanied the original version of this | ||
| 29 | * file: | ||
| 30 | * | ||
| 31 | * Written by Doug Lea with assistance from members of JCP JSR-166 | ||
| 32 | * Expert Group and released to the public domain, as explained at | ||
| 33 | * http://creativecommons.org/publicdomain/zero/1.0/ | ||
| 34 | */ | ||
| 35 | |||
| 36 | package java.util; | ||
| 37 | |||
| 38 | /** | ||
| 39 | * A collection designed for holding elements prior to processing. | ||
| 40 | * Besides basic {@link java.util.Collection Collection} operations, | ||
| 41 | * queues provide additional insertion, extraction, and inspection | ||
| 42 | * operations. Each of these methods exists in two forms: one throws | ||
| 43 | * an exception if the operation fails, the other returns a special | ||
| 44 | * value (either <tt>null</tt> or <tt>false</tt>, depending on the | ||
| 45 | * operation). The latter form of the insert operation is designed | ||
| 46 | * specifically for use with capacity-restricted <tt>Queue</tt> | ||
| 47 | * implementations; in most implementations, insert operations cannot | ||
| 48 | * fail. | ||
| 49 | * | ||
| 50 | * <p> | ||
| 51 | * <table BORDER CELLPADDING=3 CELLSPACING=1> | ||
| 52 | * <tr> | ||
| 53 | * <td></td> | ||
| 54 | * <td ALIGN=CENTER><em>Throws exception</em></td> | ||
| 55 | * <td ALIGN=CENTER><em>Returns special value</em></td> | ||
| 56 | * </tr> | ||
| 57 | * <tr> | ||
| 58 | * <td><b>Insert</b></td> | ||
| 59 | * <td>{@link #add add(e)}</td> | ||
| 60 | * <td>{@link #offer offer(e)}</td> | ||
| 61 | * </tr> | ||
| 62 | * <tr> | ||
| 63 | * <td><b>Remove</b></td> | ||
| 64 | * <td>{@link #remove remove()}</td> | ||
| 65 | * <td>{@link #poll poll()}</td> | ||
| 66 | * </tr> | ||
| 67 | * <tr> | ||
| 68 | * <td><b>Examine</b></td> | ||
| 69 | * <td>{@link #element element()}</td> | ||
| 70 | * <td>{@link #peek peek()}</td> | ||
| 71 | * </tr> | ||
| 72 | * </table> | ||
| 73 | * | ||
| 74 | * <p>Queues typically, but do not necessarily, order elements in a | ||
| 75 | * FIFO (first-in-first-out) manner. Among the exceptions are | ||
| 76 | * priority queues, which order elements according to a supplied | ||
| 77 | * comparator, or the elements' natural ordering, and LIFO queues (or | ||
| 78 | * stacks) which order the elements LIFO (last-in-first-out). | ||
| 79 | * Whatever the ordering used, the <em>head</em> of the queue is that | ||
| 80 | * element which would be removed by a call to {@link #remove() } or | ||
| 81 | * {@link #poll()}. In a FIFO queue, all new elements are inserted at | ||
| 82 | * the <em> tail</em> of the queue. Other kinds of queues may use | ||
| 83 | * different placement rules. Every <tt>Queue</tt> implementation | ||
| 84 | * must specify its ordering properties. | ||
| 85 | * | ||
| 86 | * <p>The {@link #offer offer} method inserts an element if possible, | ||
| 87 | * otherwise returning <tt>false</tt>. This differs from the {@link | ||
| 88 | * java.util.Collection#add Collection.add} method, which can fail to | ||
| 89 | * add an element only by throwing an unchecked exception. The | ||
| 90 | * <tt>offer</tt> method is designed for use when failure is a normal, | ||
| 91 | * rather than exceptional occurrence, for example, in fixed-capacity | ||
| 92 | * (or "bounded") queues. | ||
| 93 | * | ||
| 94 | * <p>The {@link #remove()} and {@link #poll()} methods remove and | ||
| 95 | * return the head of the queue. | ||
| 96 | * Exactly which element is removed from the queue is a | ||
| 97 | * function of the queue's ordering policy, which differs from | ||
| 98 | * implementation to implementation. The <tt>remove()</tt> and | ||
| 99 | * <tt>poll()</tt> methods differ only in their behavior when the | ||
| 100 | * queue is empty: the <tt>remove()</tt> method throws an exception, | ||
| 101 | * while the <tt>poll()</tt> method returns <tt>null</tt>. | ||
| 102 | * | ||
| 103 | * <p>The {@link #element()} and {@link #peek()} methods return, but do | ||
| 104 | * not remove, the head of the queue. | ||
| 105 | * | ||
| 106 | * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue | ||
| 107 | * methods</i>, which are common in concurrent programming. These methods, | ||
| 108 | * which wait for elements to appear or for space to become available, are | ||
| 109 | * defined in the {@link java.util.concurrent.BlockingQueue} interface, which | ||
| 110 | * extends this interface. | ||
| 111 | * | ||
| 112 | * <p><tt>Queue</tt> implementations generally do not allow insertion | ||
| 113 | * of <tt>null</tt> elements, although some implementations, such as | ||
| 114 | * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>. | ||
| 115 | * Even in the implementations that permit it, <tt>null</tt> should | ||
| 116 | * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also | ||
| 117 | * used as a special return value by the <tt>poll</tt> method to | ||
| 118 | * indicate that the queue contains no elements. | ||
| 119 | * | ||
| 120 | * <p><tt>Queue</tt> implementations generally do not define | ||
| 121 | * element-based versions of methods <tt>equals</tt> and | ||
| 122 | * <tt>hashCode</tt> but instead inherit the identity based versions | ||
| 123 | * from class <tt>Object</tt>, because element-based equality is not | ||
| 124 | * always well-defined for queues with the same elements but different | ||
| 125 | * ordering properties. | ||
| 126 | * | ||
| 127 | * | ||
| 128 | * <p>This interface is a member of the | ||
| 129 | * <a href="{@docRoot}/../technotes/guides/collections/index.html"> | ||
| 130 | * Java Collections Framework</a>. | ||
| 131 | * | ||
| 132 | * @see java.util.Collection | ||
| 133 | * @see LinkedList | ||
| 134 | * @see PriorityQueue | ||
| 135 | * @see java.util.concurrent.LinkedBlockingQueue | ||
| 136 | * @see java.util.concurrent.BlockingQueue | ||
| 137 | * @see java.util.concurrent.ArrayBlockingQueue | ||
| 138 | * @see java.util.concurrent.LinkedBlockingQueue | ||
| 139 | * @see java.util.concurrent.PriorityBlockingQueue | ||
| 140 | * @since 1.5 | ||
| 141 | * @author Doug Lea | ||
| 142 | * @param <E> the type of elements held in this collection | ||
| 143 | */ | ||
| 144 | public interface Queue<E> extends Collection<E> { | ||
| 145 | /** | ||
| 146 | * Inserts the specified element into this queue if it is possible to do so | ||
| 147 | * immediately without violating capacity restrictions, returning | ||
| 148 | * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt> | ||
| 149 | * if no space is currently available. | ||
| 150 | * | ||
| 151 | * @param e the element to add | ||
| 152 | * @return <tt>true</tt> (as specified by {@link Collection#add}) | ||
| 153 | * @throws IllegalStateException if the element cannot be added at this | ||
| 154 | * time due to capacity restrictions | ||
| 155 | * @throws ClassCastException if the class of the specified element | ||
| 156 | * prevents it from being added to this queue | ||
| 157 | * @throws NullPointerException if the specified element is null and | ||
| 158 | * this queue does not permit null elements | ||
| 159 | * @throws IllegalArgumentException if some property of this element | ||
| 160 | * prevents it from being added to this queue | ||
| 161 | */ | ||
| 162 | boolean add(E e); | ||
| 163 | |||
| 164 | /** | ||
| 165 | * Inserts the specified element into this queue if it is possible to do | ||
| 166 | * so immediately without violating capacity restrictions. | ||
| 167 | * When using a capacity-restricted queue, this method is generally | ||
| 168 | * preferable to {@link #add}, which can fail to insert an element only | ||
| 169 | * by throwing an exception. | ||
| 170 | * | ||
| 171 | * @param e the element to add | ||
| 172 | * @return <tt>true</tt> if the element was added to this queue, else | ||
| 173 | * <tt>false</tt> | ||
| 174 | * @throws ClassCastException if the class of the specified element | ||
| 175 | * prevents it from being added to this queue | ||
| 176 | * @throws NullPointerException if the specified element is null and | ||
| 177 | * this queue does not permit null elements | ||
| 178 | * @throws IllegalArgumentException if some property of this element | ||
| 179 | * prevents it from being added to this queue | ||
| 180 | */ | ||
| 181 | boolean offer(E e); | ||
| 182 | |||
| 183 | /** | ||
| 184 | * Retrieves and removes the head of this queue. This method differs | ||
| 185 | * from {@link #poll poll} only in that it throws an exception if this | ||
| 186 | * queue is empty. | ||
| 187 | * | ||
| 188 | * @return the head of this queue | ||
| 189 | * @throws NoSuchElementException if this queue is empty | ||
| 190 | */ | ||
| 191 | E remove(); | ||
| 192 | |||
| 193 | /** | ||
| 194 | * Retrieves and removes the head of this queue, | ||
| 195 | * or returns <tt>null</tt> if this queue is empty. | ||
| 196 | * | ||
| 197 | * @return the head of this queue, or <tt>null</tt> if this queue is empty | ||
| 198 | */ | ||
| 199 | E poll(); | ||
| 200 | |||
| 201 | /** | ||
| 202 | * Retrieves, but does not remove, the head of this queue. This method | ||
| 203 | * differs from {@link #peek peek} only in that it throws an exception | ||
| 204 | * if this queue is empty. | ||
| 205 | * | ||
| 206 | * @return the head of this queue | ||
| 207 | * @throws NoSuchElementException if this queue is empty | ||
| 208 | */ | ||
| 209 | E element(); | ||
| 210 | |||
| 211 | /** | ||
| 212 | * Retrieves, but does not remove, the head of this queue, | ||
| 213 | * or returns <tt>null</tt> if this queue is empty. | ||
| 214 | * | ||
| 215 | * @return the head of this queue, or <tt>null</tt> if this queue is empty | ||
| 216 | */ | ||
| 217 | E peek(); | ||
| 218 | } | ||