1
2 package net.sf.bddbddb.order;
3
4
5 /***
6 * A collection designed for holding elements prior to processing.
7 * Besides basic {@link java.util.Collection Collection} operations, queues provide
8 * additional insertion, extraction, and inspection operations.
9 *
10 * <p>Queues typically, but do not necessarily, order elements in a
11 * FIFO (first-in-first-out) manner. Among the exceptions are
12 * priority queues, which order elements according to a supplied
13 * comparator, or the elements' natural ordering, and LIFO queues (or
14 * stacks) which order the elements LIFO (last-in-first-out).
15 * Whatever the ordering used, the <em>head</em> of the queue is that
16 * element which would be removed by a call to {@link #remove() } or
17 * {@link #poll()}. In a FIFO queue, all new elements are inserted at
18 * the <em> tail</em> of the queue. Other kinds of queues may use
19 * different placement rules. Every <tt>Queue</tt> implementation
20 * must specify its ordering properties.
21 *
22 * <p>The {@link #offer offer} method inserts an element if possible,
23 * otherwise returning <tt>false</tt>. This differs from the {@link
24 * java.util.Collection#add Collection.add} method, which can fail to
25 * add an element only by throwing an unchecked exception. The
26 * <tt>offer</tt> method is designed for use when failure is a normal,
27 * rather than exceptional occurrence, for example, in fixed-capacity
28 * (or "bounded") queues.
29 *
30 * <p>The {@link #remove()} and {@link #poll()} methods remove and
31 * return the head of the queue.
32 * Exactly which element is removed from the queue is a
33 * function of the queue's ordering policy, which differs from
34 * implementation to implementation. The <tt>remove()</tt> and
35 * <tt>poll()</tt> methods differ only in their behavior when the
36 * queue is empty: the <tt>remove()</tt> method throws an exception,
37 * while the <tt>poll()</tt> method returns <tt>null</tt>.
38 *
39 * <p>The {@link #element()} and {@link #peek()} methods return, but do
40 * not remove, the head of the queue.
41 *
42 * <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
49 * of <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
52 * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
53 * used as a special return value by the <tt>poll</tt> method to
54 * indicate that the queue contains no elements.
55 *
56 * <p><tt>Queue</tt> implementations generally do not define
57 * element-based versions of methods <tt>equals</tt> and
58 * <tt>hashCode</tt> but instead inherit the identity based versions
59 * from class <tt>Object</tt>, because element-based equality is not
60 * always well-defined for queues with the same elements but different
61 * ordering properties.
62 *
63 *
64 * <p>This interface is a member of the
65 * <a href="{@docRoot}/../guide/collections/index.html">
66 * Java Collections Framework</a>.
67 *
68 * @see java.util.Collection
69 * @see LinkedList
70 * @see PriorityQueue
71 * @see java.util.concurrent.LinkedBlockingQueue
72 * @see java.util.concurrent.BlockingQueue
73 * @see java.util.concurrent.ArrayBlockingQueue
74 * @see java.util.concurrent.LinkedBlockingQueue
75 * @see java.util.concurrent.PriorityBlockingQueue
76 * @since 1.5
77 * @author Doug Lea
78 * @param <E> the type of elements held in this collection
79 */
80
81 import java.util.Collection;
82 import java.util.NoSuchElementException;
83
84
85 public interface Queue extends Collection {
86
87 /***
88 * Inserts the specified element into this queue, if possible. When
89 * using queues that may impose insertion restrictions (for
90 * example capacity bounds), method <tt>offer</tt> is generally
91 * preferable to method {@link Collection#add}, which can fail to
92 * insert an element only by throwing an exception.
93 *
94 * @param o the element to insert.
95 * @return <tt>true</tt> if it was possible to add the element to
96 * this queue, else <tt>false</tt>
97 */
98 boolean offer(Object o);
99
100 /***
101 * Retrieves and removes the head of this queue, or <tt>null</tt>
102 * if this queue is empty.
103 *
104 * @return the head of this queue, or <tt>null</tt> if this
105 * queue is empty.
106 */
107 Object poll();
108
109 /***
110 * Retrieves and removes the head of this queue. This method
111 * differs from the <tt>poll</tt> method in that it throws an
112 * exception if this queue is empty.
113 *
114 * @return the head of this queue.
115 * @throws NoSuchElementException if this queue is empty.
116 */
117 Object remove();
118
119 /***
120 * Retrieves, but does not remove, the head of this queue,
121 * returning <tt>null</tt> if this queue is empty.
122 *
123 * @return the head of this queue, or <tt>null</tt> if this queue
124 * is empty.
125 */
126 Object peek();
127
128 /***
129 * Retrieves, but does not remove, the head of this queue. This method
130 * differs from the <tt>peek</tt> method only in that it throws an
131 * exception if this queue is empty.
132 *
133 * @return the head of this queue.
134 * @throws NoSuchElementException if this queue is empty.
135 */
136 Object element();
137 }