001: /*
002: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
003: *
004: * This code is free software; you can redistribute it and/or modify it
005: * under the terms of the GNU General Public License version 2 only, as
006: * published by the Free Software Foundation. Sun designates this
007: * particular file as subject to the "Classpath" exception as provided
008: * by Sun in the LICENSE file that accompanied this code.
009: *
010: * This code is distributed in the hope that it will be useful, but WITHOUT
011: * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
012: * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
013: * version 2 for more details (a copy is included in the LICENSE file that
014: * accompanied this code).
015: *
016: * You should have received a copy of the GNU General Public License version
017: * 2 along with this work; if not, write to the Free Software Foundation,
018: * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
019: *
020: * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
021: * CA 95054 USA or visit www.sun.com if you need additional information or
022: * have any questions.
023: */
024:
025: /*
026: * This file is available under and governed by the GNU General Public
027: * License version 2 only, as published by the Free Software Foundation.
028: * However, the following notice accompanied the original version of this
029: * file:
030: *
031: * Written by Doug Lea with assistance from members of JCP JSR-166
032: * Expert Group and released to the public domain, as explained at
033: * http://creativecommons.org/licenses/publicdomain
034: */
035:
036: package java.util;
037:
038: /**
039: * A collection designed for holding elements prior to processing.
040: * Besides basic {@link java.util.Collection Collection} operations,
041: * queues provide additional insertion, extraction, and inspection
042: * operations. Each of these methods exists in two forms: one throws
043: * an exception if the operation fails, the other returns a special
044: * value (either <tt>null</tt> or <tt>false</tt>, depending on the
045: * operation). The latter form of the insert operation is designed
046: * specifically for use with capacity-restricted <tt>Queue</tt>
047: * implementations; in most implementations, insert operations cannot
048: * fail.
049: *
050: * <p>
051: * <table BORDER CELLPADDING=3 CELLSPACING=1>
052: * <tr>
053: * <td></td>
054: * <td ALIGN=CENTER><em>Throws exception</em></td>
055: * <td ALIGN=CENTER><em>Returns special value</em></td>
056: * </tr>
057: * <tr>
058: * <td><b>Insert</b></td>
059: * <td>{@link #add add(e)}</td>
060: * <td>{@link #offer offer(e)}</td>
061: * </tr>
062: * <tr>
063: * <td><b>Remove</b></td>
064: * <td>{@link #remove remove()}</td>
065: * <td>{@link #poll poll()}</td>
066: * </tr>
067: * <tr>
068: * <td><b>Examine</b></td>
069: * <td>{@link #element element()}</td>
070: * <td>{@link #peek peek()}</td>
071: * </tr>
072: * </table>
073: *
074: * <p>Queues typically, but do not necessarily, order elements in a
075: * FIFO (first-in-first-out) manner. Among the exceptions are
076: * priority queues, which order elements according to a supplied
077: * comparator, or the elements' natural ordering, and LIFO queues (or
078: * stacks) which order the elements LIFO (last-in-first-out).
079: * Whatever the ordering used, the <em>head</em> of the queue is that
080: * element which would be removed by a call to {@link #remove() } or
081: * {@link #poll()}. In a FIFO queue, all new elements are inserted at
082: * the <em> tail</em> of the queue. Other kinds of queues may use
083: * different placement rules. Every <tt>Queue</tt> implementation
084: * must specify its ordering properties.
085: *
086: * <p>The {@link #offer offer} method inserts an element if possible,
087: * otherwise returning <tt>false</tt>. This differs from the {@link
088: * java.util.Collection#add Collection.add} method, which can fail to
089: * add an element only by throwing an unchecked exception. The
090: * <tt>offer</tt> method is designed for use when failure is a normal,
091: * rather than exceptional occurrence, for example, in fixed-capacity
092: * (or "bounded") queues.
093: *
094: * <p>The {@link #remove()} and {@link #poll()} methods remove and
095: * return the head of the queue.
096: * Exactly which element is removed from the queue is a
097: * function of the queue's ordering policy, which differs from
098: * implementation to implementation. The <tt>remove()</tt> and
099: * <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>.
102: *
103: * <p>The {@link #element()} and {@link #peek()} methods return, but do
104: * not remove, the head of the queue.
105: *
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.
111: *
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.
119: *
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.
126: *
127: *
128: * <p>This interface is a member of the
129: * <a href="{@docRoot}/../technotes/guides/collections/index.html">
130: * Java Collections Framework</a>.
131: *
132: * @see java.util.Collection
133: * @see LinkedList
134: * @see PriorityQueue
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
140: * @since 1.5
141: * @author Doug Lea
142: * @param <E> the type of elements held in this collection
143: */
144: public interface Queue<E> extends Collection<E> {
145: /**
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.
150: *
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
161: */
162: boolean add(E e);
163:
164: /**
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.
170: *
171: * @param e the element to add
172: * @return <tt>true</tt> if the element was added to this queue, else
173: * <tt>false</tt>
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
180: */
181: boolean offer(E e);
182:
183: /**
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
186: * queue is empty.
187: *
188: * @return the head of this queue
189: * @throws NoSuchElementException if this queue is empty
190: */
191: E remove();
192:
193: /**
194: * Retrieves and removes the head of this queue,
195: * or returns <tt>null</tt> if this queue is empty.
196: *
197: * @return the head of this queue, or <tt>null</tt> if this queue is empty
198: */
199: E poll();
200:
201: /**
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.
205: *
206: * @return the head of this queue
207: * @throws NoSuchElementException if this queue is empty
208: */
209: E element();
210:
211: /**
212: * Retrieves, but does not remove, the head of this queue,
213: * or returns <tt>null</tt> if this queue is empty.
214: *
215: * @return the head of this queue, or <tt>null</tt> if this queue is empty
216: */
217: E peek();
218: }
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