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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. Sun designates this |
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7 * particular file as subject to the "Classpath" exception as provided |
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8 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
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21 * CA 95054 USA or visit www.sun.com if you need additional information or |
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22 * have any 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/licenses/publicdomain |
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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 java.util.Collection Collection} operations, |
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41 * queues provide additional insertion, extraction, and inspection |
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42 * operations. Each of these methods exists in two forms: one throws |
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43 * an exception if the operation fails, the other returns a special |
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44 * value (either <tt>null</tt> or <tt>false</tt>, depending on the |
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45 * operation). The latter form of the insert operation is designed |
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46 * specifically for use with capacity-restricted <tt>Queue</tt> |
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47 * implementations; in most implementations, insert operations cannot |
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48 * fail. |
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49 * |
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50 * <p> |
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51 * <table BORDER CELLPADDING=3 CELLSPACING=1> |
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52 * <tr> |
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53 * <td></td> |
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54 * <td ALIGN=CENTER><em>Throws exception</em></td> |
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55 * <td ALIGN=CENTER><em>Returns special value</em></td> |
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56 * </tr> |
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57 * <tr> |
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58 * <td><b>Insert</b></td> |
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59 * <td>{@link #add add(e)}</td> |
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60 * <td>{@link #offer offer(e)}</td> |
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61 * </tr> |
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62 * <tr> |
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63 * <td><b>Remove</b></td> |
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64 * <td>{@link #remove remove()}</td> |
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65 * <td>{@link #poll poll()}</td> |
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66 * </tr> |
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67 * <tr> |
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68 * <td><b>Examine</b></td> |
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69 * <td>{@link #element element()}</td> |
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70 * <td>{@link #peek peek()}</td> |
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71 * </tr> |
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72 * </table> |
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73 * |
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74 * <p>Queues typically, but do not necessarily, order elements in a |
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75 * FIFO (first-in-first-out) manner. Among the exceptions are |
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76 * priority queues, which order elements according to a supplied |
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77 * comparator, or the elements' natural ordering, and LIFO queues (or |
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78 * stacks) which order the elements LIFO (last-in-first-out). |
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79 * Whatever the ordering used, the <em>head</em> of the queue is that |
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80 * element which would be removed by a call to {@link #remove() } or |
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81 * {@link #poll()}. In a FIFO queue, all new elements are inserted at |
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82 * the <em> tail</em> of the queue. Other kinds of queues may use |
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83 * different placement rules. Every <tt>Queue</tt> implementation |
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84 * must specify its ordering properties. |
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85 * |
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86 * <p>The {@link #offer offer} method inserts an element if possible, |
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87 * otherwise returning <tt>false</tt>. This differs from the {@link |
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88 * java.util.Collection#add Collection.add} method, which can fail to |
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89 * add an element only by throwing an unchecked exception. The |
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90 * <tt>offer</tt> method is designed for use when failure is a normal, |
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91 * rather than exceptional occurrence, for example, in fixed-capacity |
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92 * (or "bounded") queues. |
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93 * |
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94 * <p>The {@link #remove()} and {@link #poll()} methods remove and |
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95 * return the head of the queue. |
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96 * Exactly which element is removed from the queue is a |
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97 * function of the queue's ordering policy, which differs from |
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98 * implementation to implementation. The <tt>remove()</tt> and |
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99 * <tt>poll()</tt> methods differ only in their behavior when the |
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100 * queue is empty: the <tt>remove()</tt> method throws an exception, |
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101 * while the <tt>poll()</tt> method returns <tt>null</tt>. |
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102 * |
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103 * <p>The {@link #element()} and {@link #peek()} methods return, but do |
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104 * not remove, the head of the queue. |
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105 * |
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106 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue |
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107 * methods</i>, which are common in concurrent programming. These methods, |
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108 * which wait for elements to appear or for space to become available, are |
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109 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which |
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110 * extends this interface. |
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111 * |
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112 * <p><tt>Queue</tt> implementations generally do not allow insertion |
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113 * of <tt>null</tt> elements, although some implementations, such as |
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114 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>. |
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115 * Even in the implementations that permit it, <tt>null</tt> should |
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116 * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also |
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117 * used as a special return value by the <tt>poll</tt> method to |
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118 * indicate that the queue contains no elements. |
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119 * |
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120 * <p><tt>Queue</tt> implementations generally do not define |
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121 * element-based versions of methods <tt>equals</tt> and |
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122 * <tt>hashCode</tt> but instead inherit the identity based versions |
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123 * from class <tt>Object</tt>, because element-based equality is not |
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124 * always well-defined for queues with the same elements but different |
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125 * ordering properties. |
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126 * |
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127 * |
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128 * <p>This interface is a member of the |
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129 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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130 * Java Collections Framework</a>. |
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131 * |
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132 * @see java.util.Collection |
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133 * @see LinkedList |
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134 * @see PriorityQueue |
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135 * @see java.util.concurrent.LinkedBlockingQueue |
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136 * @see java.util.concurrent.BlockingQueue |
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137 * @see java.util.concurrent.ArrayBlockingQueue |
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138 * @see java.util.concurrent.LinkedBlockingQueue |
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139 * @see java.util.concurrent.PriorityBlockingQueue |
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140 * @since 1.5 |
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141 * @author Doug Lea |
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142 * @param <E> the type of elements held in this collection |
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143 */ |
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144 public interface Queue<E> extends Collection<E> { |
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145 /** |
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146 * Inserts the specified element into this queue if it is possible to do so |
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147 * immediately without violating capacity restrictions, returning |
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148 * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt> |
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149 * if no space is currently available. |
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150 * |
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151 * @param e the element to add |
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152 * @return <tt>true</tt> (as specified by {@link Collection#add}) |
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153 * @throws IllegalStateException if the element cannot be added at this |
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154 * time due to capacity restrictions |
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155 * @throws ClassCastException if the class of the specified element |
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156 * prevents it from being added to this queue |
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157 * @throws NullPointerException if the specified element is null and |
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158 * this queue does not permit null elements |
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159 * @throws IllegalArgumentException if some property of this element |
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160 * prevents it from being added to this queue |
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161 */ |
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162 boolean add(E e); |
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163 |
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164 /** |
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165 * Inserts the specified element into this queue if it is possible to do |
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166 * so immediately without violating capacity restrictions. |
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167 * When using a capacity-restricted queue, this method is generally |
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168 * preferable to {@link #add}, which can fail to insert an element only |
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169 * by throwing an exception. |
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170 * |
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171 * @param e the element to add |
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172 * @return <tt>true</tt> if the element was added to this queue, else |
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173 * <tt>false</tt> |
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174 * @throws ClassCastException if the class of the specified element |
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175 * prevents it from being added to this queue |
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176 * @throws NullPointerException if the specified element is null and |
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177 * this queue does not permit null elements |
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178 * @throws IllegalArgumentException if some property of this element |
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179 * prevents it from being added to this queue |
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180 */ |
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181 boolean offer(E e); |
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182 |
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183 /** |
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184 * Retrieves and removes the head of this queue. This method differs |
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185 * from {@link #poll poll} only in that it throws an exception if this |
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186 * queue is empty. |
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187 * |
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188 * @return the head of this queue |
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189 * @throws NoSuchElementException if this queue is empty |
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190 */ |
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191 E remove(); |
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192 |
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193 /** |
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194 * Retrieves and removes the head of this queue, |
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195 * or returns <tt>null</tt> if this queue is empty. |
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196 * |
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197 * @return the head of this queue, or <tt>null</tt> if this queue is empty |
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198 */ |
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199 E poll(); |
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200 |
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201 /** |
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202 * Retrieves, but does not remove, the head of this queue. This method |
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203 * differs from {@link #peek peek} only in that it throws an exception |
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204 * if this queue is empty. |
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205 * |
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206 * @return the head of this queue |
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207 * @throws NoSuchElementException if this queue is empty |
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208 */ |
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209 E element(); |
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210 |
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211 /** |
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212 * Retrieves, but does not remove, the head of this queue, |
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213 * or returns <tt>null</tt> if this queue is empty. |
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214 * |
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215 * @return the head of this queue, or <tt>null</tt> if this queue is empty |
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216 */ |
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217 E peek(); |
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218 } |