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 // BEGIN android-note 39 // removed link to collections framework docs 40 // END android-note 41 42 /** 43 * A collection designed for holding elements prior to processing. 44 * Besides basic {@link java.util.Collection Collection} operations, 45 * queues provide additional insertion, extraction, and inspection 46 * operations. Each of these methods exists in two forms: one throws 47 * an exception if the operation fails, the other returns a special 48 * value (either {@code null} or {@code false}, depending on the 49 * operation). The latter form of the insert operation is designed 50 * specifically for use with capacity-restricted {@code Queue} 51 * implementations; in most implementations, insert operations cannot 52 * fail. 53 * 54 * <table BORDER CELLPADDING=3 CELLSPACING=1> 55 * <caption>Summary of Queue methods</caption> 56 * <tr> 57 * <td></td> 58 * <td ALIGN=CENTER><em>Throws exception</em></td> 59 * <td ALIGN=CENTER><em>Returns special value</em></td> 60 * </tr> 61 * <tr> 62 * <td><b>Insert</b></td> 63 * <td>{@link Queue#add add(e)}</td> 64 * <td>{@link Queue#offer offer(e)}</td> 65 * </tr> 66 * <tr> 67 * <td><b>Remove</b></td> 68 * <td>{@link Queue#remove remove()}</td> 69 * <td>{@link Queue#poll poll()}</td> 70 * </tr> 71 * <tr> 72 * <td><b>Examine</b></td> 73 * <td>{@link Queue#element element()}</td> 74 * <td>{@link Queue#peek peek()}</td> 75 * </tr> 76 * </table> 77 * 78 * <p>Queues typically, but do not necessarily, order elements in a 79 * FIFO (first-in-first-out) manner. Among the exceptions are 80 * priority queues, which order elements according to a supplied 81 * comparator, or the elements' natural ordering, and LIFO queues (or 82 * stacks) which order the elements LIFO (last-in-first-out). 83 * Whatever the ordering used, the <em>head</em> of the queue is that 84 * element which would be removed by a call to {@link #remove() } or 85 * {@link #poll()}. In a FIFO queue, all new elements are inserted at 86 * the <em>tail</em> of the queue. Other kinds of queues may use 87 * different placement rules. Every {@code Queue} implementation 88 * must specify its ordering properties. 89 * 90 * <p>The {@link #offer offer} method inserts an element if possible, 91 * otherwise returning {@code false}. This differs from the {@link 92 * java.util.Collection#add Collection.add} method, which can fail to 93 * add an element only by throwing an unchecked exception. The 94 * {@code offer} method is designed for use when failure is a normal, 95 * rather than exceptional occurrence, for example, in fixed-capacity 96 * (or "bounded") queues. 97 * 98 * <p>The {@link #remove()} and {@link #poll()} methods remove and 99 * return the head of the queue. 100 * Exactly which element is removed from the queue is a 101 * function of the queue's ordering policy, which differs from 102 * implementation to implementation. The {@code remove()} and 103 * {@code poll()} methods differ only in their behavior when the 104 * queue is empty: the {@code remove()} method throws an exception, 105 * while the {@code poll()} method returns {@code null}. 106 * 107 * <p>The {@link #element()} and {@link #peek()} methods return, but do 108 * not remove, the head of the queue. 109 * 110 * <p>The {@code Queue} interface does not define the <i>blocking queue 111 * methods</i>, which are common in concurrent programming. These methods, 112 * which wait for elements to appear or for space to become available, are 113 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which 114 * extends this interface. 115 * 116 * <p>{@code Queue} implementations generally do not allow insertion 117 * of {@code null} elements, although some implementations, such as 118 * {@link LinkedList}, do not prohibit insertion of {@code null}. 119 * Even in the implementations that permit it, {@code null} should 120 * not be inserted into a {@code Queue}, as {@code null} is also 121 * used as a special return value by the {@code poll} method to 122 * indicate that the queue contains no elements. 123 * 124 * <p>{@code Queue} implementations generally do not define 125 * element-based versions of methods {@code equals} and 126 * {@code hashCode} but instead inherit the identity based versions 127 * from class {@code Object}, because element-based equality is not 128 * always well-defined for queues with the same elements but different 129 * ordering properties. 130 * 131 * @since 1.5 132 * @author Doug Lea 133 * @param <E> the type of elements held in this queue 134 */ 135 public interface Queue<E> extends Collection<E> { 136 /** 137 * Inserts the specified element into this queue if it is possible to do so 138 * immediately without violating capacity restrictions, returning 139 * {@code true} upon success and throwing an {@code IllegalStateException} 140 * if no space is currently available. 141 * 142 * @param e the element to add 143 * @return {@code true} (as specified by {@link Collection#add}) 144 * @throws IllegalStateException if the element cannot be added at this 145 * time due to capacity restrictions 146 * @throws ClassCastException if the class of the specified element 147 * prevents it from being added to this queue 148 * @throws NullPointerException if the specified element is null and 149 * this queue does not permit null elements 150 * @throws IllegalArgumentException if some property of this element 151 * prevents it from being added to this queue 152 */ add(E e)153 boolean add(E e); 154 155 /** 156 * Inserts the specified element into this queue if it is possible to do 157 * so immediately without violating capacity restrictions. 158 * When using a capacity-restricted queue, this method is generally 159 * preferable to {@link #add}, which can fail to insert an element only 160 * by throwing an exception. 161 * 162 * @param e the element to add 163 * @return {@code true} if the element was added to this queue, else 164 * {@code false} 165 * @throws ClassCastException if the class of the specified element 166 * prevents it from being added to this queue 167 * @throws NullPointerException if the specified element is null and 168 * this queue does not permit null elements 169 * @throws IllegalArgumentException if some property of this element 170 * prevents it from being added to this queue 171 */ offer(E e)172 boolean offer(E e); 173 174 /** 175 * Retrieves and removes the head of this queue. This method differs 176 * from {@link #poll poll} only in that it throws an exception if this 177 * queue is empty. 178 * 179 * @return the head of this queue 180 * @throws NoSuchElementException if this queue is empty 181 */ remove()182 E remove(); 183 184 /** 185 * Retrieves and removes the head of this queue, 186 * or returns {@code null} if this queue is empty. 187 * 188 * @return the head of this queue, or {@code null} if this queue is empty 189 */ poll()190 E poll(); 191 192 /** 193 * Retrieves, but does not remove, the head of this queue. This method 194 * differs from {@link #peek peek} only in that it throws an exception 195 * if this queue is empty. 196 * 197 * @return the head of this queue 198 * @throws NoSuchElementException if this queue is empty 199 */ element()200 E element(); 201 202 /** 203 * Retrieves, but does not remove, the head of this queue, 204 * or returns {@code null} if this queue is empty. 205 * 206 * @return the head of this queue, or {@code null} if this queue is empty 207 */ peek()208 E peek(); 209 } 210