Don't obfuscate names of fields in objects - otherwise fields provided by two modules may clash
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31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
39 * A collection designed for holding elements prior to processing.
40 * Besides basic {@link java.util.Collection Collection} operations,
41 * queues provide additional insertion, extraction, and inspection
42 * operations. Each of these methods exists in two forms: one throws
43 * an exception if the operation fails, the other returns a special
44 * value (either <tt>null</tt> or <tt>false</tt>, depending on the
45 * operation). The latter form of the insert operation is designed
46 * specifically for use with capacity-restricted <tt>Queue</tt>
47 * implementations; in most implementations, insert operations cannot
51 * <table BORDER CELLPADDING=3 CELLSPACING=1>
54 * <td ALIGN=CENTER><em>Throws exception</em></td>
55 * <td ALIGN=CENTER><em>Returns special value</em></td>
58 * <td><b>Insert</b></td>
59 * <td>{@link #add add(e)}</td>
60 * <td>{@link #offer offer(e)}</td>
63 * <td><b>Remove</b></td>
64 * <td>{@link #remove remove()}</td>
65 * <td>{@link #poll poll()}</td>
68 * <td><b>Examine</b></td>
69 * <td>{@link #element element()}</td>
70 * <td>{@link #peek peek()}</td>
74 * <p>Queues typically, but do not necessarily, order elements in a
75 * FIFO (first-in-first-out) manner. Among the exceptions are
76 * priority queues, which order elements according to a supplied
77 * comparator, or the elements' natural ordering, and LIFO queues (or
78 * stacks) which order the elements LIFO (last-in-first-out).
79 * Whatever the ordering used, the <em>head</em> of the queue is that
80 * element which would be removed by a call to {@link #remove() } or
81 * {@link #poll()}. In a FIFO queue, all new elements are inserted at
82 * the <em> tail</em> of the queue. Other kinds of queues may use
83 * different placement rules. Every <tt>Queue</tt> implementation
84 * must specify its ordering properties.
86 * <p>The {@link #offer offer} method inserts an element if possible,
87 * otherwise returning <tt>false</tt>. This differs from the {@link
88 * java.util.Collection#add Collection.add} method, which can fail to
89 * add an element only by throwing an unchecked exception. The
90 * <tt>offer</tt> method is designed for use when failure is a normal,
91 * rather than exceptional occurrence, for example, in fixed-capacity
92 * (or "bounded") queues.
94 * <p>The {@link #remove()} and {@link #poll()} methods remove and
95 * return the head of the queue.
96 * Exactly which element is removed from the queue is a
97 * function of the queue's ordering policy, which differs from
98 * implementation to implementation. The <tt>remove()</tt> and
99 * <tt>poll()</tt> methods differ only in their behavior when the
100 * queue is empty: the <tt>remove()</tt> method throws an exception,
101 * while the <tt>poll()</tt> method returns <tt>null</tt>.
103 * <p>The {@link #element()} and {@link #peek()} methods return, but do
104 * not remove, the head of the queue.
106 * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
107 * methods</i>, which are common in concurrent programming. These methods,
108 * which wait for elements to appear or for space to become available, are
109 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
110 * extends this interface.
112 * <p><tt>Queue</tt> implementations generally do not allow insertion
113 * of <tt>null</tt> elements, although some implementations, such as
114 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
115 * Even in the implementations that permit it, <tt>null</tt> should
116 * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
117 * used as a special return value by the <tt>poll</tt> method to
118 * indicate that the queue contains no elements.
120 * <p><tt>Queue</tt> implementations generally do not define
121 * element-based versions of methods <tt>equals</tt> and
122 * <tt>hashCode</tt> but instead inherit the identity based versions
123 * from class <tt>Object</tt>, because element-based equality is not
124 * always well-defined for queues with the same elements but different
125 * ordering properties.
128 * <p>This interface is a member of the
129 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
130 * Java Collections Framework</a>.
132 * @see java.util.Collection
135 * @see java.util.concurrent.LinkedBlockingQueue
136 * @see java.util.concurrent.BlockingQueue
137 * @see java.util.concurrent.ArrayBlockingQueue
138 * @see java.util.concurrent.LinkedBlockingQueue
139 * @see java.util.concurrent.PriorityBlockingQueue
142 * @param <E> the type of elements held in this collection
144 public interface Queue<E> extends Collection<E> {
146 * Inserts the specified element into this queue if it is possible to do so
147 * immediately without violating capacity restrictions, returning
148 * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
149 * if no space is currently available.
151 * @param e the element to add
152 * @return <tt>true</tt> (as specified by {@link Collection#add})
153 * @throws IllegalStateException if the element cannot be added at this
154 * time due to capacity restrictions
155 * @throws ClassCastException if the class of the specified element
156 * prevents it from being added to this queue
157 * @throws NullPointerException if the specified element is null and
158 * this queue does not permit null elements
159 * @throws IllegalArgumentException if some property of this element
160 * prevents it from being added to this queue
165 * Inserts the specified element into this queue if it is possible to do
166 * so immediately without violating capacity restrictions.
167 * When using a capacity-restricted queue, this method is generally
168 * preferable to {@link #add}, which can fail to insert an element only
169 * by throwing an exception.
171 * @param e the element to add
172 * @return <tt>true</tt> if the element was added to this queue, else
174 * @throws ClassCastException if the class of the specified element
175 * prevents it from being added to this queue
176 * @throws NullPointerException if the specified element is null and
177 * this queue does not permit null elements
178 * @throws IllegalArgumentException if some property of this element
179 * prevents it from being added to this queue
184 * Retrieves and removes the head of this queue. This method differs
185 * from {@link #poll poll} only in that it throws an exception if this
188 * @return the head of this queue
189 * @throws NoSuchElementException if this queue is empty
194 * Retrieves and removes the head of this queue,
195 * or returns <tt>null</tt> if this queue is empty.
197 * @return the head of this queue, or <tt>null</tt> if this queue is empty
202 * Retrieves, but does not remove, the head of this queue. This method
203 * differs from {@link #peek peek} only in that it throws an exception
204 * if this queue is empty.
206 * @return the head of this queue
207 * @throws NoSuchElementException if this queue is empty
212 * Retrieves, but does not remove, the head of this queue,
213 * or returns <tt>null</tt> if this queue is empty.
215 * @return the head of this queue, or <tt>null</tt> if this queue is empty