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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 :     *
14 :     * <p>Queues typically, but do not necessarily, order elements in a
15 :     * FIFO (first-in-first-out) manner. Among the exceptions are priority
16 :     * queues, which order elements according to a supplied comparators, or
17 :     * the elements natural ordering. Every Queue implementation must specify
18 :     * its ordering guarantees.
19 :     *
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 1.3 *
27 : tim 1.2 * <p>The {@link #remove()} and {@link #poll()} methods remove and return an
28 :     * element in accord with the implementation's ordering policy. For example,
29 :     * in FIFO queues, they remove and return the oldest element in the queue.
30 :     * The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their
31 :     * behavior when the queue is empty: the <tt>remove()</tt> method throws an
32 :     * exception, while the <tt>poll()</tt> method returns <tt>null</tt>.
33 : tim 1.1 *
34 : tim 1.2 * <p>The {@link #element()} and {@link #peek()} methods return but do
35 : tim 1.1 * not delete the element that would be obtained by a call to
36 : tim 1.2 * the <tt>remove</tt> and <tt>poll</tt> methods respectively.
37 : tim 1.1 *
38 : tim 1.2 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
39 :     * methods</i>, which are common in concurrent programming. These methods,
40 :     * which wait for elements to appear or for space to become available, are
41 :     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
42 :     * extends this interface.
43 :     *
44 :     * <p><tt>Queue</tt> implementations generally do not allow insertion of
45 :     * <tt>null</tt> elements. Even in the few implementations that permit it,
46 :     * it is a bad idea, as <tt>null</tt> is also used as a special return value
47 :     * by the <tt>poll</tt> method to indicate that the queue contains no
48 :     * elements.
49 :     *
50 :     * <p>This interface is a member of the
51 :     * <a href="{@docRoot}/../guide/collections/index.html">
52 :     * Java Collections Framework</a>.
53 :     *
54 :     * @see Collection
55 :     * @see LinkedList
56 :     * @see PriorityQueue
57 :     * @see LinkedQueue
58 :     * @see java.util.concurrent.BlockingQueue
59 :     * @see java.util.concurrent.ArrayBlockingQueue
60 :     * @see java.util.concurrent.LinkedBlockingQueue
61 :     * @see java.util.concurrent.PriorityBlockingQueue
62 :     */
63 : tim 1.1 public interface Queue<E> extends Collection<E> {
64 :     /**
65 : tim 1.2 * Add the specified element to this queue, if possible.
66 :     *
67 :     * @param element the element to add.
68 :     * @return true if it was possible to add the element to the queue.
69 :     */
70 :     public boolean offer(E element);
71 : tim 1.1
72 :     /**
73 : tim 1.2 * Remove and return an element from the queue if one is available.
74 :     * Exactly which element is removed from the queue is a function
75 :     * of the queue's ordering policy, which differs from implementation
76 :     * to implementation. Possible orderings include (but are not limited
77 :     * to) first-in-first-out (FIFO), element priority, and arbitrary.
78 :     *
79 :     * @return an element previously on the queue, or <tt>null</tt> if the
80 : dl 1.3 * queue is empty.
81 : tim 1.2 */
82 : tim 1.1 public E poll();
83 :    
84 :     /**
85 : tim 1.2 * Remove and return an element from the queue. This method differs
86 :     * from the <tt>poll</tt> method in that it throws an exception if the
87 : dl 1.3 * queue is empty.
88 : tim 1.2 *
89 :     * @return an element previously on the queue.
90 :     * @throws NoSuchElementException if the queue is empty.
91 :     */
92 : tim 1.1 public E remove() throws NoSuchElementException;
93 :    
94 :     /**
95 : tim 1.2 * Return, but do not remove, an element from the queue, or <tt>null</tt>
96 :     * if the queue is empty. This method returns the same object reference
97 :     * that would be returned by by the <tt>poll</tt> method. The two methods
98 :     * differ in that this method does not remove the element from the queue.
99 :     *
100 :     * @return an element on the queue, or <tt>null</tt> if the queue is empty.
101 :     */
102 : tim 1.1 public E peek();
103 :    
104 :     /**
105 : tim 1.2 * Return, but do not remove, an element from the queue. This method
106 :     * differs from the <tt>peek</tt> method in that it throws an exception if
107 :     * the queue is empty.
108 :     *
109 :     * @return an element on the queue.
110 :     * @throws NoSuchElementException if the queue is empty.
111 :     */
112 : tim 1.1 public E element() throws NoSuchElementException;
113 :     }

Doug Lea
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