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 {@code null} or {@code false}, depending on the 45 * operation). The latter form of the insert operation is designed 46 * specifically for use with capacity-restricted {@code Queue} 47 * implementations; in most implementations, insert operations cannot 48 * fail. 49 * 50 * <table BORDER CELLPADDING=3 CELLSPACING=1> 51 * <caption>Summary of Queue methods</caption> 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 Queue#add add(e)}</td> 60 * <td>{@link Queue#offer offer(e)}</td> 61 * </tr> 62 * <tr> 63 * <td><b>Remove</b></td> 64 * <td>{@link Queue#remove remove()}</td> 65 * <td>{@link Queue#poll poll()}</td> 66 * </tr> 67 * <tr> 68 * <td><b>Examine</b></td> 69 * <td>{@link Queue#element element()}</td> 70 * <td>{@link Queue#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 {@code Queue} implementation 84 * must specify its ordering properties. 85 * 86 * <p>The {@link #offer offer} method inserts an element if possible, 87 * otherwise returning {@code false}. 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 * {@code offer} 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 {@code remove()} and 99 * {@code poll()} methods differ only in their behavior when the 100 * queue is empty: the {@code remove()} method throws an exception, 101 * while the {@code poll()} method returns {@code null}. 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 {@code Queue} 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>{@code Queue} implementations generally do not allow insertion 113 * of {@code null} elements, although some implementations, such as 114 * {@link LinkedList}, do not prohibit insertion of {@code null}. 115 * Even in the implementations that permit it, {@code null} should 116 * not be inserted into a {@code Queue}, as {@code null} is also 117 * used as a special return value by the {@code poll} method to 118 * indicate that the queue contains no elements. 119 * 120 * <p>{@code Queue} implementations generally do not define 121 * element-based versions of methods {@code equals} and 122 * {@code hashCode} but instead inherit the identity based versions 123 * from class {@code Object}, 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 * {@code true} upon success and throwing an {@code IllegalStateException} 149 * if no space is currently available. 150 * 151 * @param e the element to add 152 * @return {@code true} (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 */ add(E e)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 {@code true} if the element was added to this queue, else 173 * {@code false} 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 */ offer(E e)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 */ remove()191 E remove(); 192 193 /** 194 * Retrieves and removes the head of this queue, 195 * or returns {@code null} if this queue is empty. 196 * 197 * @return the head of this queue, or {@code null} if this queue is empty 198 */ poll()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 */ element()209 E element(); 210 211 /** 212 * Retrieves, but does not remove, the head of this queue, 213 * or returns {@code null} if this queue is empty. 214 * 215 * @return the head of this queue, or {@code null} if this queue is empty 216 */ peek()217 E peek(); 218 } 219