[cvs] / jsr166 / src / main / java / util / Queue.java Repository:
ViewVC logotype

Diff of /jsr166/src/main/java/util/Queue.java

Parent Directory Parent Directory | Revision Log Revision Log | View Patch Patch

revision 1.2, Sun May 18 18:10:02 2003 UTC revision 1.29, Sat May 14 02:11:52 2005 UTC
# Line 1  Line 1 
1    /*
2     * Written by Doug Lea with assistance from members of JCP JSR-166
3     * Expert Group and released to the public domain, as explained at
4     * http://creativecommons.org/licenses/publicdomain
5     */
6    
7  package java.util;  package java.util;
8    
9  /**  /**
10   * A Collection designed for holding elements prior to processing.   * A collection designed for holding elements prior to processing.
11   * Besides basic {@link Collection} operations, queues provide   * Besides basic {@link java.util.Collection Collection} operations,
12   * additional insertion, extraction, and inspection operations.   * queues provide additional insertion, extraction, and inspection
13     * operations.  Each of these methods exists in two forms: one throws
14     * an exception if the operation fails, the other returns a special
15     * value (either <tt>null</tt> or <tt>false</tt>, depending on the
16     * operation).  The latter form of the insert operation is designed
17     * specifically for use with capacity-restricted <tt>Queue</tt>
18     * implementations; in most implementations, insert operations cannot
19     * fail.
20     *
21     * <p>
22     * <table BORDER CELLPADDING=3 CELLSPACING=1>
23     *  <tr>
24     *    <td></td>
25     *    <td ALIGN=CENTER><em>Throws exception</em></td>
26     *    <td ALIGN=CENTER><em>Special value</em></td>
27     *  </tr>
28     *  <tr>
29     *    <td><b>Insert</b></td>
30     *    <td>{@link #add add(e)}</td>
31     *    <td>{@link #offer offer(e)}</td>
32     *  </tr>
33     *  <tr>
34     *    <td><b>Remove</b></td>
35     *    <td>{@link #remove remove()}</td>
36     *    <td>{@link #poll poll()}</td>
37     *  </tr>
38     *  <tr>
39     *    <td><b>Examine</b></td>
40     *    <td>{@link #element element()}</td>
41     *    <td>{@link #peek peek()}</td>
42     *  </tr>
43     * </table>
44   *   *
45   * <p>Queues typically, but do not necessarily, order elements in a   * <p>Queues typically, but do not necessarily, order elements in a
46   * FIFO (first-in-first-out) manner.  Among the exceptions are priority   * FIFO (first-in-first-out) manner.  Among the exceptions are
47   * queues, which order elements according to a supplied comparators, or   * priority queues, which order elements according to a supplied
48   * the elements natural ordering.  Every Queue implementation must specify   * comparator, or the elements' natural ordering, and LIFO queues (or
49   * its ordering guarantees.   * stacks) which order the elements LIFO (last-in-first-out).
50   *   * Whatever the ordering used, the <em>head</em> of the queue is that
51   * <p>The {@link #offer(E)} method adds an element if possible, otherwise   * element which would be removed by a call to {@link #remove() } or
52   * returning <tt>false</tt>.  This differs from the {@link   * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
53   * Collections#add(Object)} method, which throws an unchecked exception upon   * the <em> tail</em> of the queue. Other kinds of queues may use
54   * failure. It is designed for use in collections in which failure to   * different placement rules.  Every <tt>Queue</tt> implementation
55   * add is a normal, rather than exceptional occurrence, for example,   * must specify its ordering properties.
56   * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.   *
57   *   * <p>The {@link #offer offer} method inserts an element if possible,
58   * <p>The {@link #remove()} and {@link #poll()} methods remove and return an   * otherwise returning <tt>false</tt>.  This differs from the {@link
59   * element in accord with the implementation's ordering policy. For example,   * java.util.Collection#add Collection.add} method, which can fail to
60   * in FIFO queues, they remove and return the oldest element in the queue.   * add an element only by throwing an unchecked exception.  The
61   * The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their   * <tt>offer</tt> method is designed for use when failure is a normal,
62   * behavior when the queue is empty: the <tt>remove()</tt> method throws an   * rather than exceptional occurrence, for example, in fixed-capacity
63   * exception, while the <tt>poll()</tt> method returns <tt>null</tt>.   * (or &quot;bounded&quot;) queues.
64   *   *
65   * <p>The {@link #element()} and {@link #peek()} methods return but do   * <p>The {@link #remove()} and {@link #poll()} methods remove and
66   * not delete the element that would be obtained by a call to   * return the head of the queue.
67   * the <tt>remove</tt> and <tt>poll</tt> methods respectively.   * Exactly which element is removed from the queue is a
68     * function of the queue's ordering policy, which differs from
69     * implementation to implementation. The <tt>remove()</tt> and
70     * <tt>poll()</tt> methods differ only in their behavior when the
71     * queue is empty: the <tt>remove()</tt> method throws an exception,
72     * while the <tt>poll()</tt> method returns <tt>null</tt>.
73     *
74     * <p>The {@link #element()} and {@link #peek()} methods return, but do
75     * not remove, the head of the queue.
76   *   *
77   * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue   * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
78   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
# Line 35  Line 80 
80   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which   * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
81   * extends this interface.   * extends this interface.
82   *   *
83   * <p><tt>Queue</tt> implementations generally do not allow insertion of   * <p><tt>Queue</tt> implementations generally do not allow insertion
84   * <tt>null</tt> elements.  Even in the few implementations that permit it,   * of <tt>null</tt> elements, although some implementations, such as
85   * it is a bad idea, as <tt>null</tt> is also used as a special return value   * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
86   * by the <tt>poll</tt> method to indicate that the queue contains no   * Even in the implementations that permit it, <tt>null</tt> should
87   * elements.   * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
88     * used as a special return value by the <tt>poll</tt> method to
89     * indicate that the queue contains no elements.
90     *
91     * <p><tt>Queue</tt> implementations generally do not define
92     * element-based versions of methods <tt>equals</tt> and
93     * <tt>hashCode</tt> but instead inherit the identity based versions
94     * from class <tt>Object</tt>, because element-based equality is not
95     * always well-defined for queues with the same elements but different
96     * ordering properties.
97     *
98   *   *
99   * <p>This interface is a member of the   * <p>This interface is a member of the
100   * <a href="{@docRoot}/../guide/collections/index.html">   * <a href="{@docRoot}/../guide/collections/index.html">
101   * Java Collections Framework</a>.   * Java Collections Framework</a>.
102   *   *
103   * @see Collection   * @see java.util.Collection
104   * @see LinkedList   * @see LinkedList
105   * @see PriorityQueue   * @see PriorityQueue
106   * @see LinkedQueue   * @see java.util.concurrent.LinkedBlockingQueue
107   * @see java.util.concurrent.BlockingQueue   * @see java.util.concurrent.BlockingQueue
108   * @see java.util.concurrent.ArrayBlockingQueue   * @see java.util.concurrent.ArrayBlockingQueue
109   * @see java.util.concurrent.LinkedBlockingQueue   * @see java.util.concurrent.LinkedBlockingQueue
110   * @see java.util.concurrent.PriorityBlockingQueue   * @see java.util.concurrent.PriorityBlockingQueue
111     * @since 1.5
112     * @author Doug Lea
113     * @param <E> the type of elements held in this collection
114   */   */
115  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
116      /**      /**
117       * Add the specified element to this queue, if possible.       * Inserts the specified element into this queue if it is possible to do so
118       *       * immediately without violating capacity restrictions, returning
119       * @param element the element to add.       * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
120       * @return true if it was possible to add the element to the queue.       * if no space is currently available.
121         *
122         * @param e the element to add
123         * @return <tt>true</tt> (as per the spec for {@link Collection#add})
124         * @throws IllegalStateException if the element cannot be added at this
125         *         time due to capacity restrictions
126         * @throws NullPointerException if the specified element is null and
127         *         this queue not permit null elements
128         * @throws ClassCastException if the class of the specified element
129         *         prevents it from being added to this queue
130         * @throws IllegalArgumentException if some property of this element
131         *         prevents it from being added to this queue
132         */
133        boolean add(E e);
134    
135        /**
136         * Inserts the specified element into this queue if it is possible to do
137         * so immediately without violating capacity restrictions.
138         * When using a capacity-restricted deque, this method is generally
139         * preferable to {@link #add}, which can fail to insert an element only
140         * by throwing an exception.
141         *
142         * @param e the element to add
143         * @return <tt>true</tt> if the element was added to this queue, else
144         *         <tt>false</tt>
145         * @throws NullPointerException if the specified element is null and
146         *         this queue does not permit null elements
147         * @throws ClassCastException if the class of the specified element
148         *         prevents it from being added to this queue
149         * @throws IllegalArgumentException if some property of this element
150         *         prevents it from being added to this queue
151       */       */
152      public boolean offer(E element);      boolean offer(E e);
153    
154      /**      /**
155       * Remove and return an element from the queue if one is available.       * Retrieves and removes the head of this queue.  This method differs
156       * Exactly which element is removed from the queue is a function       * from {@link #poll} only in that it throws an exception if this queue
157       * of the queue's ordering policy, which differs from implementation       * is empty.
      * to implementation.  Possible orderings include (but are not limited  
      * to) first-in-first-out (FIFO), element priority, and arbitrary.  
158       *       *
159       * @return an element previously on the queue, or <tt>null</tt> if the       * @return the head of this queue
160       *         queue is empty.       * @throws NoSuchElementException if this queue is empty
161       */       */
162      public E poll();      E remove();
163    
164      /**      /**
165       * Remove and return an element from the queue.  This method differs       * Retrieves and removes the head of this queue,
166       * from the <tt>poll</tt> method in that it throws an exception if the       * or returns <tt>null</tt> if this queue is empty.
      * queue is empty.  
167       *       *
168       * @return an element previously on the queue.       * @return the head of this queue, or <tt>null</tt> if this queue is empty
      * @throws NoSuchElementException if the queue is empty.  
169       */       */
170      public E remove() throws NoSuchElementException;      E poll();
171    
172      /**      /**
173       * Return, but do not remove, an element from the queue, or <tt>null</tt>       * Retrieves, but does not remove, the head of this queue.  This method
174       * if the queue is empty.  This method returns the same object reference       * differs from {@link #peek} only in that it throws an exception if
175       * that would be returned by by the <tt>poll</tt> method.  The two methods       * this queue is empty.
      * differ in that this method does not remove the element from the queue.  
176       *       *
177       * @return an element on the queue, or <tt>null</tt> if the queue is empty.       * @return the head of this queue
178         * @throws NoSuchElementException if this queue is empty
179       */       */
180      public E peek();      E element();
181    
182      /**      /**
183       * Return, but do not remove, an element from the queue.  This method       * Retrieves, but does not remove, the head of this queue,
184       * differs from the <tt>peek</tt> method in that it throws an exception if       * or returns <tt>null</tt> if this queue is empty.
      * the queue is empty.  
185       *       *
186       * @return an element on the queue.       * @return the head of this queue, or <tt>null</tt> if this queue is empty
      * @throws NoSuchElementException if the queue is empty.  
187       */       */
188      public E element() throws NoSuchElementException;      E peek();
189  }  }

Legend:
Removed from v.1.2  
changed lines
  Added in v.1.29

Doug Lea
ViewVC Help
Powered by ViewVC 1.0.8