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1 : dl 1.3 /*
2 :     * Written by Doug Lea with assistance from members of JCP JSR-166
3 :     * Expert Group and released to the public domain. Use, modify, and
4 :     * redistribute this code in any way without acknowledgement.
5 :     */
6 :    
7 : tim 1.1 package java.util;
8 :    
9 :     /**
10 : tim 1.2 * A Collection designed for holding elements prior to processing.
11 :     * Besides basic {@link Collection} operations, queues provide
12 :     * additional insertion, extraction, and inspection operations.
13 : brian 1.6 0 *
14 : tim 1.2 * <p>Queues typically, but do not necessarily, order elements in a
15 : dl 1.5 * FIFO (first-in-first-out) manner. Among the exceptions are
16 :     * priority queues, which order elements according to a supplied
17 :     * comparator, or the elements' natural ordering. Every Queue
18 :     * implementation must specify its ordering guarantees.
19 : tim 1.2 *
20 :     * <p>The {@link #offer(E)} method adds an element if possible, otherwise
21 :     * returning <tt>false</tt>. This differs from the {@link
22 :     * Collections#add(Object)} method, which throws an unchecked exception upon
23 : tim 1.1 * failure. It is designed for use in collections in which failure to
24 :     * add is a normal, rather than exceptional occurrence, for example,
25 : tim 1.2 * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.
26 : dl 1.4
27 : dl 1.3 *
28 : tim 1.2 * <p>The {@link #remove()} and {@link #poll()} methods remove and return an
29 : dl 1.4 * element in accord with the implementation's ordering policy.
30 :     * Exactly which element is removed from the queue is a function
31 :     * of the queue's ordering policy, which differs from implementation
32 :     * to implementation. Possible orderings include (but are not limited
33 : brian 1.6 * to) first-in-first-out (FIFO), last-in-first-out (LIFO), element priority, and arbitrary.
34 : tim 1.2 * The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their
35 :     * behavior when the queue is empty: the <tt>remove()</tt> method throws an
36 :     * exception, while the <tt>poll()</tt> method returns <tt>null</tt>.
37 : tim 1.1 *
38 : tim 1.2 * <p>The {@link #element()} and {@link #peek()} methods return but do
39 : tim 1.1 * not delete the element that would be obtained by a call to
40 : tim 1.2 * the <tt>remove</tt> and <tt>poll</tt> methods respectively.
41 : tim 1.1 *
42 : tim 1.2 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
43 :     * methods</i>, which are common in concurrent programming. These methods,
44 :     * which wait for elements to appear or for space to become available, are
45 :     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
46 :     * extends this interface.
47 :     *
48 :     * <p><tt>Queue</tt> implementations generally do not allow insertion of
49 : brian 1.6 * <tt>null</tt> elements, although some implementations, such as
50 :     * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
51 :     * Even in the implementations that permit it, <tt>null</tt> should not be inserted into
52 :     * a <tt>Queue</tt>, as <tt>null</tt> is also used as a special return value
53 : tim 1.2 * by the <tt>poll</tt> method to indicate that the queue contains no
54 :     * elements.
55 :     *
56 :     * <p>This interface is a member of the
57 :     * <a href="{@docRoot}/../guide/collections/index.html">
58 :     * Java Collections Framework</a>.
59 :     *
60 :     * @see Collection
61 :     * @see LinkedList
62 :     * @see PriorityQueue
63 : brian 1.6 * @see java.util.concurrent.LinkedQueue
64 : tim 1.2 * @see java.util.concurrent.BlockingQueue
65 :     * @see java.util.concurrent.ArrayBlockingQueue
66 :     * @see java.util.concurrent.LinkedBlockingQueue
67 :     * @see java.util.concurrent.PriorityBlockingQueue
68 :     */
69 : tim 1.1 public interface Queue<E> extends Collection<E> {
70 :     /**
71 : tim 1.2 * Add the specified element to this queue, if possible.
72 :     *
73 :     * @param element the element to add.
74 :     * @return true if it was possible to add the element to the queue.
75 :     */
76 :     public boolean offer(E element);
77 : tim 1.1
78 :     /**
79 : tim 1.2 * Remove and return an element from the queue if one is available.
80 :     *
81 :     * @return an element previously on the queue, or <tt>null</tt> if the
82 : dl 1.3 * queue is empty.
83 : tim 1.2 */
84 : tim 1.1 public E poll();
85 :    
86 :     /**
87 : tim 1.2 * Remove and return an element from the queue. This method differs
88 :     * from the <tt>poll</tt> method in that it throws an exception if the
89 : dl 1.3 * queue is empty.
90 : tim 1.2 *
91 :     * @return an element previously on the queue.
92 :     * @throws NoSuchElementException if the queue is empty.
93 :     */
94 : tim 1.1 public E remove() throws NoSuchElementException;
95 :    
96 :     /**
97 : tim 1.2 * Return, but do not remove, an element from the queue, or <tt>null</tt>
98 :     * if the queue is empty. This method returns the same object reference
99 :     * that would be returned by by the <tt>poll</tt> method. The two methods
100 :     * differ in that this method does not remove the element from the queue.
101 :     *
102 :     * @return an element on the queue, or <tt>null</tt> if the queue is empty.
103 :     */
104 : tim 1.1 public E peek();
105 :    
106 :     /**
107 : tim 1.2 * Return, but do not remove, an element from the queue. This method
108 :     * differs from the <tt>peek</tt> method in that it throws an exception if
109 :     * the queue is empty.
110 :     *
111 :     * @return an element on the queue.
112 :     * @throws NoSuchElementException if the queue is empty.
113 :     */
114 : tim 1.1 public E element() throws NoSuchElementException;
115 :     }

Doug Lea
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