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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 : jsr166 1.28 * <p>
22 :     * <table BORDER CELLPADDING=3 CELLSPACING=1>
23 : dl 1.23 * <tr>
24 : dl 1.24 * <td></td>
25 :     * <td ALIGN=CENTER><em>Throws exception</em></td>
26 :     * <td ALIGN=CENTER><em>Returns special value</em></td>
27 : dl 1.23 * </tr>
28 :     * <tr>
29 : dl 1.24 * <td><b>Insert</b></td>
30 :     * <td>{@link #add add(e)}</td>
31 :     * <td>{@link #offer offer(e)}</td>
32 : dl 1.23 * </tr>
33 :     * <tr>
34 : dl 1.24 * <td><b>Remove</b></td>
35 :     * <td>{@link #remove remove()}</td>
36 :     * <td>{@link #poll poll()}</td>
37 : dl 1.23 * </tr>
38 :     * <tr>
39 : dl 1.24 * <td><b>Examine</b></td>
40 :     * <td>{@link #element element()}</td>
41 :     * <td>{@link #peek peek()}</td>
42 : dl 1.23 * </tr>
43 : dl 1.24 * </table>
44 : dholmes 1.8 *
45 : tim 1.2 * <p>Queues typically, but do not necessarily, order elements in a
46 : dl 1.5 * FIFO (first-in-first-out) manner. Among the exceptions are
47 :     * priority queues, which order elements according to a supplied
48 : dholmes 1.8 * comparator, or the elements' natural ordering, and LIFO queues (or
49 :     * stacks) which order the elements LIFO (last-in-first-out).
50 : dl 1.17 * Whatever the ordering used, the <em>head</em> of the queue is that
51 :     * element which would be removed by a call to {@link #remove() } or
52 :     * {@link #poll()}. In a FIFO queue, all new elements are inserted at
53 :     * the <em> tail</em> of the queue. Other kinds of queues may use
54 :     * different placement rules. Every <tt>Queue</tt> implementation
55 :     * must specify its ordering properties.
56 :     *
57 :     * <p>The {@link #offer offer} method inserts an element if possible,
58 :     * otherwise returning <tt>false</tt>. This differs from the {@link
59 :     * java.util.Collection#add Collection.add} method, which can fail to
60 :     * add an element only by throwing an unchecked exception. The
61 :     * <tt>offer</tt> method is designed for use when failure is a normal,
62 :     * rather than exceptional occurrence, for example, in fixed-capacity
63 :     * (or &quot;bounded&quot;) queues.
64 : tim 1.9 *
65 : dl 1.7 * <p>The {@link #remove()} and {@link #poll()} methods remove and
66 : dholmes 1.8 * return the head of the queue.
67 :     * Exactly which element is removed from the queue is a
68 : dl 1.7 * function of the queue's ordering policy, which differs from
69 : dholmes 1.8 * implementation to implementation. The <tt>remove()</tt> and
70 : dl 1.7 * <tt>poll()</tt> methods differ only in their behavior when the
71 :     * queue is empty: the <tt>remove()</tt> method throws an exception,
72 :     * while the <tt>poll()</tt> method returns <tt>null</tt>.
73 : tim 1.1 *
74 : dholmes 1.8 * <p>The {@link #element()} and {@link #peek()} methods return, but do
75 : dholmes 1.10 * not remove, the head of the queue.
76 : tim 1.1 *
77 : tim 1.2 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
78 :     * methods</i>, which are common in concurrent programming. These methods,
79 :     * which wait for elements to appear or for space to become available, are
80 :     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
81 :     * extends this interface.
82 :     *
83 : dl 1.7 * <p><tt>Queue</tt> implementations generally do not allow insertion
84 :     * of <tt>null</tt> elements, although some implementations, such as
85 : brian 1.6 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
86 : dl 1.7 * Even in the implementations that permit it, <tt>null</tt> should
87 :     * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
88 :     * used as a special return value by the <tt>poll</tt> method to
89 :     * indicate that the queue contains no elements.
90 : tim 1.2 *
91 : dl 1.16 * <p><tt>Queue</tt> implementations generally do not define
92 :     * element-based versions of methods <tt>equals</tt> and
93 :     * <tt>hashCode</tt> but instead inherit the identity based versions
94 :     * from class <tt>Object</tt>, because element-based equality is not
95 :     * always well-defined for queues with the same elements but different
96 :     * ordering properties.
97 :     *
98 :     *
99 : tim 1.2 * <p>This interface is a member of the
100 :     * <a href="{@docRoot}/../guide/collections/index.html">
101 :     * Java Collections Framework</a>.
102 :     *
103 : tim 1.11 * @see java.util.Collection
104 : tim 1.2 * @see LinkedList
105 :     * @see PriorityQueue
106 : dholmes 1.15 * @see java.util.concurrent.LinkedBlockingQueue
107 : tim 1.2 * @see java.util.concurrent.BlockingQueue
108 :     * @see java.util.concurrent.ArrayBlockingQueue
109 :     * @see java.util.concurrent.LinkedBlockingQueue
110 :     * @see java.util.concurrent.PriorityBlockingQueue
111 : dl 1.7 * @since 1.5
112 :     * @author Doug Lea
113 : dl 1.21 * @param <E> the type of elements held in this collection
114 : tim 1.2 */
115 : tim 1.1 public interface Queue<E> extends Collection<E> {
116 : dholmes 1.8
117 : tim 1.1 /**
118 : dl 1.19 * Inserts the specified element into this queue, if possible. When
119 : dl 1.17 * using queues that may impose insertion restrictions (for
120 :     * example capacity bounds), method <tt>offer</tt> is generally
121 :     * preferable to method {@link Collection#add}, which can fail to
122 :     * insert an element only by throwing an exception.
123 : tim 1.2 *
124 : jsr166 1.27 * @param e the element to insert.
125 : tim 1.9 * @return <tt>true</tt> if it was possible to add the element to
126 : dholmes 1.10 * this queue, else <tt>false</tt>
127 : tim 1.2 */
128 : jsr166 1.27 boolean offer(E e);
129 : tim 1.1
130 :     /**
131 : dl 1.18 * Retrieves and removes the head of this queue, or <tt>null</tt>
132 :     * if this queue is empty.
133 : tim 1.2 *
134 : dholmes 1.8 * @return the head of this queue, or <tt>null</tt> if this
135 : tim 1.9 * queue is empty.
136 : tim 1.2 */
137 : dl 1.7 E poll();
138 : tim 1.1
139 :     /**
140 : dl 1.18 * Retrieves and removes the head of this queue. This method
141 : jsr166 1.26 * differs from the {@link #poll} method only in that it throws an
142 : dl 1.18 * exception if this queue is empty.
143 : tim 1.2 *
144 : dholmes 1.8 * @return the head of this queue.
145 :     * @throws NoSuchElementException if this queue is empty.
146 : tim 1.2 */
147 : tim 1.9 E remove();
148 : tim 1.1
149 :     /**
150 : dl 1.18 * Retrieves, but does not remove, the head of this queue,
151 :     * returning <tt>null</tt> if this queue is empty.
152 : tim 1.2 *
153 : dl 1.18 * @return the head of this queue, or <tt>null</tt> if this queue
154 :     * is empty.
155 : tim 1.2 */
156 : dl 1.7 E peek();
157 : tim 1.1
158 :     /**
159 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue. This method
160 : jsr166 1.26 * differs from the {@link #peek} method only in that it throws an
161 : dholmes 1.8 * exception if this queue is empty.
162 : tim 1.2 *
163 : dholmes 1.8 * @return the head of this queue.
164 :     * @throws NoSuchElementException if this queue is empty.
165 : tim 1.2 */
166 : tim 1.9 E element();
167 : tim 1.1 }

Doug Lea
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