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1 : tim 1.1 package java.util;
2 :    
3 :     /**
4 : tim 1.2 * A Collection designed for holding elements prior to processing.
5 :     * Besides basic {@link Collection} operations, queues provide
6 :     * additional insertion, extraction, and inspection operations.
7 :     *
8 :     * <p>Queues typically, but do not necessarily, order elements in a
9 :     * FIFO (first-in-first-out) manner. Among the exceptions are priority
10 :     * queues, which order elements according to a supplied comparators, or
11 :     * the elements natural ordering. Every Queue implementation must specify
12 :     * its ordering guarantees.
13 :     *
14 :     * <p>The {@link #offer(E)} method adds an element if possible, otherwise
15 :     * returning <tt>false</tt>. This differs from the {@link
16 :     * Collections#add(Object)} method, which throws an unchecked exception upon
17 : tim 1.1 * failure. It is designed for use in collections in which failure to
18 :     * add is a normal, rather than exceptional occurrence, for example,
19 : tim 1.2 * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.
20 : tim 1.1 *
21 : tim 1.2 * <p>The {@link #remove()} and {@link #poll()} methods remove and return an
22 :     * element in accord with the implementation's ordering policy. For example,
23 :     * in FIFO queues, they remove and return the oldest element in the queue.
24 :     * The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their
25 :     * behavior when the queue is empty: the <tt>remove()</tt> method throws an
26 :     * exception, while the <tt>poll()</tt> method returns <tt>null</tt>.
27 : tim 1.1 *
28 : tim 1.2 * <p>The {@link #element()} and {@link #peek()} methods return but do
29 : tim 1.1 * not delete the element that would be obtained by a call to
30 : tim 1.2 * the <tt>remove</tt> and <tt>poll</tt> methods respectively.
31 : tim 1.1 *
32 : tim 1.2 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
33 :     * methods</i>, which are common in concurrent programming. These methods,
34 :     * which wait for elements to appear or for space to become available, are
35 :     * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
36 :     * extends this interface.
37 :     *
38 :     * <p><tt>Queue</tt> implementations generally do not allow insertion of
39 :     * <tt>null</tt> elements. Even in the few implementations that permit it,
40 :     * it is a bad idea, as <tt>null</tt> is also used as a special return value
41 :     * by the <tt>poll</tt> method to indicate that the queue contains no
42 :     * elements.
43 :     *
44 :     * <p>This interface is a member of the
45 :     * <a href="{@docRoot}/../guide/collections/index.html">
46 :     * Java Collections Framework</a>.
47 :     *
48 :     * @see Collection
49 :     * @see LinkedList
50 :     * @see PriorityQueue
51 :     * @see LinkedQueue
52 :     * @see java.util.concurrent.BlockingQueue
53 :     * @see java.util.concurrent.ArrayBlockingQueue
54 :     * @see java.util.concurrent.LinkedBlockingQueue
55 :     * @see java.util.concurrent.PriorityBlockingQueue
56 :     */
57 : tim 1.1 public interface Queue<E> extends Collection<E> {
58 :     /**
59 : tim 1.2 * Add the specified element to this queue, if possible.
60 :     *
61 :     * @param element the element to add.
62 :     * @return true if it was possible to add the element to the queue.
63 :     */
64 :     public boolean offer(E element);
65 : tim 1.1
66 :     /**
67 : tim 1.2 * Remove and return an element from the queue if one is available.
68 :     * Exactly which element is removed from the queue is a function
69 :     * of the queue's ordering policy, which differs from implementation
70 :     * to implementation. Possible orderings include (but are not limited
71 :     * to) first-in-first-out (FIFO), element priority, and arbitrary.
72 :     *
73 :     * @return an element previously on the queue, or <tt>null</tt> if the
74 :     * queue is empty.
75 :     */
76 : tim 1.1 public E poll();
77 :    
78 :     /**
79 : tim 1.2 * Remove and return an element from the queue. This method differs
80 :     * from the <tt>poll</tt> method in that it throws an exception if the
81 :     * queue is empty.
82 :     *
83 :     * @return an element previously on the queue.
84 :     * @throws NoSuchElementException if the queue is empty.
85 :     */
86 : tim 1.1 public E remove() throws NoSuchElementException;
87 :    
88 :     /**
89 : tim 1.2 * Return, but do not remove, an element from the queue, or <tt>null</tt>
90 :     * if the queue is empty. This method returns the same object reference
91 :     * that would be returned by by the <tt>poll</tt> method. The two methods
92 :     * differ in that this method does not remove the element from the queue.
93 :     *
94 :     * @return an element on the queue, or <tt>null</tt> if the queue is empty.
95 :     */
96 : tim 1.1 public E peek();
97 :    
98 :     /**
99 : tim 1.2 * Return, but do not remove, an element from the queue. This method
100 :     * differs from the <tt>peek</tt> method in that it throws an exception if
101 :     * the queue is empty.
102 :     *
103 :     * @return an element on the queue.
104 :     * @throws NoSuchElementException if the queue is empty.
105 :     */
106 : tim 1.1 public E element() throws NoSuchElementException;
107 :     }

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