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1 : dl 1.3 /*
2 :     * Written by Doug Lea with assistance from members of JCP JSR-166
3 : dl 1.22 * Expert Group and released to the public domain, as explained at
4 :     * http://creativecommons.org/licenses/publicdomain
5 : dl 1.3 */
6 :    
7 : tim 1.1 package java.util;
8 :    
9 :     /**
10 : dholmes 1.8 * A collection designed for holding elements prior to processing.
11 : dl 1.24 * Besides basic {@link java.util.Collection Collection} operations,
12 :     * queues provide additional insertion, extraction, and inspection
13 :     * operations. Each of these methods exists in two forms: one throws
14 :     * an exception if the operation fails, the other returns a special
15 :     * value (either <tt>null</tt> or <tt>false</tt>, depending on the
16 :     * operation). The latter form of the insert operation is designed
17 :     * specifically for use with capacity-restricted <tt>Queue</tt>
18 :     * implementations; in most implementations, insert operations cannot
19 :     * fail.
20 : dl 1.23 *
21 : dl 1.24 * <table BORDER CELLPADDING=3 CELLSPACING=1>
22 : dl 1.23 * <tr>
23 : dl 1.24 * <td></td>
24 :     * <td ALIGN=CENTER><em>Throws exception</em></td>
25 :     * <td ALIGN=CENTER><em>Returns special value</em></td>
26 : dl 1.23 * </tr>
27 :     * <tr>
28 : dl 1.24 * <td><b>Insert</b></td>
29 :     * <td>{@link #add add(e)}</td>
30 :     * <td>{@link #offer offer(e)}</td>
31 : dl 1.23 * </tr>
32 :     * <tr>
33 : dl 1.24 * <td><b>Remove</b></td>
34 :     * <td>{@link #remove remove()}</td>
35 :     * <td>{@link #poll poll()}</td>
36 : dl 1.23 * </tr>
37 :     * <tr>
38 : dl 1.24 * <td><b>Examine</b></td>
39 :     * <td>{@link #element element()}</td>
40 :     * <td>{@link #peek peek()}</td>
41 : dl 1.23 * </tr>
42 : dl 1.24 * </table>
43 : dholmes 1.8 *
44 : tim 1.2 * <p>Queues typically, but do not necessarily, order elements in a
45 : dl 1.5 * FIFO (first-in-first-out) manner. Among the exceptions are
46 :     * priority queues, which order elements according to a supplied
47 : dholmes 1.8 * comparator, or the elements' natural ordering, and LIFO queues (or
48 :     * stacks) which order the elements LIFO (last-in-first-out).
49 : dl 1.17 * Whatever the ordering used, the <em>head</em> of the queue is that
50 :     * element which would be removed by a call to {@link #remove() } or
51 :     * {@link #poll()}. In a FIFO queue, all new elements are inserted at
52 :     * the <em> tail</em> of the queue. Other kinds of queues may use
53 :     * different placement rules. Every <tt>Queue</tt> implementation
54 :     * must specify its ordering properties.
55 :     *
56 :     * <p>The {@link #offer offer} method inserts an element if possible,
57 :     * otherwise returning <tt>false</tt>. This differs from the {@link
58 :     * java.util.Collection#add Collection.add} method, which can fail to
59 :     * add an element only by throwing an unchecked exception. The
60 :     * <tt>offer</tt> method is designed for use when failure is a normal,
61 :     * rather than exceptional occurrence, for example, in fixed-capacity
62 :     * (or &quot;bounded&quot;) queues.
63 : tim 1.9 *
64 : dl 1.7 * <p>The {@link #remove()} and {@link #poll()} methods remove and
65 : dholmes 1.8 * return the head of the queue.
66 :     * Exactly which element is removed from the queue is a
67 : dl 1.7 * function of the queue's ordering policy, which differs from
68 : dholmes 1.8 * implementation to implementation. The <tt>remove()</tt> and
69 : dl 1.7 * <tt>poll()</tt> methods differ only in their behavior when the
70 :     * queue is empty: the <tt>remove()</tt> method throws an exception,
71 :     * while the <tt>poll()</tt> method returns <tt>null</tt>.
72 : tim 1.1 *
73 : dholmes 1.8 * <p>The {@link #element()} and {@link #peek()} methods return, but do
74 : dholmes 1.10 * not remove, the head of the queue.
75 : tim 1.1 *
76 : tim 1.2 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
77 :     * methods</i>, which are common in concurrent programming. These methods,
78 :     * which wait for elements to appear or for space to become available, are
79 :     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
80 :     * extends this interface.
81 :     *
82 : dl 1.7 * <p><tt>Queue</tt> implementations generally do not allow insertion
83 :     * of <tt>null</tt> elements, although some implementations, such as
84 : brian 1.6 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
85 : dl 1.7 * Even in the implementations that permit it, <tt>null</tt> should
86 :     * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
87 :     * used as a special return value by the <tt>poll</tt> method to
88 :     * indicate that the queue contains no elements.
89 : tim 1.2 *
90 : dl 1.16 * <p><tt>Queue</tt> implementations generally do not define
91 :     * element-based versions of methods <tt>equals</tt> and
92 :     * <tt>hashCode</tt> but instead inherit the identity based versions
93 :     * from class <tt>Object</tt>, because element-based equality is not
94 :     * always well-defined for queues with the same elements but different
95 :     * ordering properties.
96 :     *
97 :     *
98 : tim 1.2 * <p>This interface is a member of the
99 :     * <a href="{@docRoot}/../guide/collections/index.html">
100 :     * Java Collections Framework</a>.
101 :     *
102 : tim 1.11 * @see java.util.Collection
103 : tim 1.2 * @see LinkedList
104 :     * @see PriorityQueue
105 : dholmes 1.15 * @see java.util.concurrent.LinkedBlockingQueue
106 : tim 1.2 * @see java.util.concurrent.BlockingQueue
107 :     * @see java.util.concurrent.ArrayBlockingQueue
108 :     * @see java.util.concurrent.LinkedBlockingQueue
109 :     * @see java.util.concurrent.PriorityBlockingQueue
110 : dl 1.7 * @since 1.5
111 :     * @author Doug Lea
112 : dl 1.21 * @param <E> the type of elements held in this collection
113 : tim 1.2 */
114 : tim 1.1 public interface Queue<E> extends Collection<E> {
115 : dholmes 1.8
116 : tim 1.1 /**
117 : dl 1.19 * Inserts the specified element into this queue, if possible. When
118 : dl 1.17 * using queues that may impose insertion restrictions (for
119 :     * example capacity bounds), method <tt>offer</tt> is generally
120 :     * preferable to method {@link Collection#add}, which can fail to
121 :     * insert an element only by throwing an exception.
122 : tim 1.2 *
123 : dl 1.17 * @param o the element to insert.
124 : tim 1.9 * @return <tt>true</tt> if it was possible to add the element to
125 : dholmes 1.10 * this queue, else <tt>false</tt>
126 : tim 1.2 */
127 : dholmes 1.10 boolean offer(E o);
128 : tim 1.1
129 :     /**
130 : dl 1.18 * Retrieves and removes the head of this queue, or <tt>null</tt>
131 :     * if this queue is empty.
132 : tim 1.2 *
133 : dholmes 1.8 * @return the head of this queue, or <tt>null</tt> if this
134 : tim 1.9 * queue is empty.
135 : tim 1.2 */
136 : dl 1.7 E poll();
137 : tim 1.1
138 :     /**
139 : dl 1.18 * Retrieves and removes the head of this queue. This method
140 :     * differs from the <tt>poll</tt> method in that it throws an
141 :     * exception if this queue is empty.
142 : tim 1.2 *
143 : dholmes 1.8 * @return the head of this queue.
144 :     * @throws NoSuchElementException if this queue is empty.
145 : tim 1.2 */
146 : tim 1.9 E remove();
147 : tim 1.1
148 :     /**
149 : dl 1.18 * Retrieves, but does not remove, the head of this queue,
150 :     * returning <tt>null</tt> if this queue is empty.
151 : tim 1.2 *
152 : dl 1.18 * @return the head of this queue, or <tt>null</tt> if this queue
153 :     * is empty.
154 : tim 1.2 */
155 : dl 1.7 E peek();
156 : tim 1.1
157 :     /**
158 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue. This method
159 : tim 1.9 * differs from the <tt>peek</tt> method only in that it throws an
160 : dholmes 1.8 * exception if this queue is empty.
161 : tim 1.2 *
162 : dholmes 1.8 * @return the head of this queue.
163 :     * @throws NoSuchElementException if this queue is empty.
164 : tim 1.2 */
165 : tim 1.9 E element();
166 : tim 1.1 }

Doug Lea
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