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