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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 : jsr166 1.29 * <td ALIGN=CENTER><em>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 :     /**
117 : jsr166 1.29 * Inserts the specified element into this queue if it is possible to do so
118 :     * immediately without violating capacity restrictions, returning
119 :     * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
120 :     * if no space is currently available.
121 :     *
122 :     * @param e the element to add
123 :     * @return <tt>true</tt> (as per the spec for {@link Collection#add})
124 :     * @throws IllegalStateException if the element cannot be added at this
125 :     * time due to capacity restrictions
126 : jsr166 1.30 * @throws ClassCastException if the class of the specified element
127 :     * prevents it from being added to this queue
128 : jsr166 1.29 * @throws NullPointerException if the specified element is null and
129 :     * this queue not permit null elements
130 :     * @throws IllegalArgumentException if some property of this element
131 :     * prevents it from being added to this queue
132 : tim 1.2 */
133 : jsr166 1.29 boolean add(E e);
134 : tim 1.1
135 :     /**
136 : jsr166 1.29 * Inserts the specified element into this queue if it is possible to do
137 :     * so immediately without violating capacity restrictions.
138 :     * When using a capacity-restricted deque, this method is generally
139 :     * preferable to {@link #add}, which can fail to insert an element only
140 :     * by throwing an exception.
141 : tim 1.2 *
142 : jsr166 1.29 * @param e the element to add
143 :     * @return <tt>true</tt> if the element was added to this queue, else
144 :     * <tt>false</tt>
145 : jsr166 1.30 * @throws ClassCastException if the class of the specified element
146 :     * prevents it from being added to this queue
147 : jsr166 1.29 * @throws NullPointerException if the specified element is null and
148 :     * this queue does not permit null elements
149 :     * @throws IllegalArgumentException if some property of this element
150 :     * prevents it from being added to this queue
151 : tim 1.2 */
152 : jsr166 1.29 boolean offer(E e);
153 : tim 1.1
154 :     /**
155 : jsr166 1.29 * Retrieves and removes the head of this queue. This method differs
156 :     * from {@link #poll} only in that it throws an exception if this queue
157 :     * is empty.
158 : tim 1.2 *
159 : jsr166 1.29 * @return the head of this queue
160 :     * @throws NoSuchElementException if this queue is empty
161 : tim 1.2 */
162 : tim 1.9 E remove();
163 : tim 1.1
164 :     /**
165 : jsr166 1.29 * Retrieves and removes the head of this queue,
166 :     * or returns <tt>null</tt> if this queue is empty.
167 : tim 1.2 *
168 : jsr166 1.29 * @return the head of this queue, or <tt>null</tt> if this queue is empty
169 : tim 1.2 */
170 : jsr166 1.29 E poll();
171 : tim 1.1
172 :     /**
173 : dholmes 1.14 * Retrieves, but does not remove, the head of this queue. This method
174 : jsr166 1.29 * differs from {@link #peek} only in that it throws an exception if
175 :     * this queue is empty.
176 : tim 1.2 *
177 : jsr166 1.29 * @return the head of this queue
178 :     * @throws NoSuchElementException if this queue is empty
179 : tim 1.2 */
180 : tim 1.9 E element();
181 : jsr166 1.29
182 :     /**
183 :     * Retrieves, but does not remove, the head of this queue,
184 :     * or returns <tt>null</tt> if this queue is empty.
185 :     *
186 :     * @return the head of this queue, or <tt>null</tt> if this queue is empty
187 :     */
188 :     E peek();
189 : tim 1.1 }

Doug Lea
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