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revision 1.7, Tue Jun 24 14:34:30 2003 UTC revision 1.8, Mon Jul 28 04:11:54 2003 UTC
# Line 7  Line 7 
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 Collection} operations, queues provide
12   * additional insertion, extraction, and inspection operations.   * additional insertion, extraction, and inspection operations.
13  0 *   *
14   * <p>Queues typically, but do not necessarily, order elements in a   * <p>Queues typically, but do not necessarily, order elements in a
15   * FIFO (first-in-first-out) manner.  Among the exceptions are   * FIFO (first-in-first-out) manner.  Among the exceptions are
16   * priority queues, which order elements according to a supplied   * priority queues, which order elements according to a supplied
17   * comparator, or the elements' natural ordering.  Every Queue   * comparator, or the elements' natural ordering, and LIFO queues (or
18   * implementation must specify its ordering guarantees.   * stacks) which order the elements LIFO (last-in-first-out).
19     * Whatever the ordering used, the <em>head</em> of the queue is that element
20     * which would be removed by a call to {@link #remove() } or {@link #poll()}.
21     * Every Queue implementation must specify its ordering guarantees.
22   *   *
23   * <p>The {@link #offer(E)} method adds an element if possible, otherwise   * <p>The {@link #offer(E)} method adds an element if possible, otherwise
24   * returning <tt>false</tt>.  This differs from the {@link   * returning <tt>false</tt>.  This differs from the {@link
25   * Collections#add(Object)} method, which throws an unchecked exception upon   * Collections#add(Object)} method, which throws an unchecked exception upon
26   * failure. It is designed for use in collections in which failure to   * failure. It is designed for use in collections in which failure to
27   * add is a normal, rather than exceptional occurrence, for example,   * add is a normal, rather than exceptional occurrence, for example,
28   * in fixed-capacity (or &ldquo;bounded&rdquo;) queues.   * in fixed-capacity (or &quot;bounded&quot;) queues.
   
29   *   *
30   * <p>The {@link #remove()} and {@link #poll()} methods remove and   * <p>The {@link #remove()} and {@link #poll()} methods remove and
31   * return an element in accord with the implementation's ordering   * return the head of the queue.
32   * policy.  Exactly which element is removed from the queue is a   * Exactly which element is removed from the queue is a
33   * function of the queue's ordering policy, which differs from   * function of the queue's ordering policy, which differs from
34   * implementation to implementation.  Possible orderings include (but   * implementation to implementation. The <tt>remove()</tt> and
  * are not limited to) first-in-first-out (FIFO), last-in-first-out  
  * (LIFO), element priority, and arbitrary.  The <tt>remove()</tt> and  
35   * <tt>poll()</tt> methods differ only in their behavior when the   * <tt>poll()</tt> methods differ only in their behavior when the
36   * queue is empty: the <tt>remove()</tt> method throws an exception,   * queue is empty: the <tt>remove()</tt> method throws an exception,
37   * while the <tt>poll()</tt> method returns <tt>null</tt>.   * while the <tt>poll()</tt> method returns <tt>null</tt>.
38   *   *
39   * <p>The {@link #element()} and {@link #peek()} methods return but do   * <p>The {@link #element()} and {@link #peek()} methods return, but do
40   * not delete the element that would be obtained by a call to   * not delete, the head of the queue.
  * the <tt>remove</tt> and <tt>poll</tt> methods respectively.  
41   *   *
42   * <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
43   * methods</i>, which are common in concurrent programming.  These methods,   * methods</i>, which are common in concurrent programming.  These methods,
# Line 70  Line 69 
69   * @author Doug Lea   * @author Doug Lea
70   */   */
71  public interface Queue<E> extends Collection<E> {  public interface Queue<E> extends Collection<E> {
72    
73      /**      /**
74       * Add the specified element to this queue, if possible.       * Add the specified element to this queue, if possible.
75       *       *
76       * @param element the element to add.       * @param element the element to add.
77       * @return true if it was possible to add the element to the queue.       * @return <tt>true</tt> if it was possible to add the element to
78         * this queue.
79       */       */
80      boolean offer(E element);      boolean offer(E element);
81    
82      /**      /**
83       * Remove and return an element from the queue if one is available.       * Retrieve and remove the head of this queue, if it is available.
84       *       *
85       * @return an element previously on the queue, or <tt>null</tt> if the       * @return the head of this queue, or <tt>null</tt> if this
86       *         queue is empty.       *         queue is empty.
87       */       */
88      E poll();      E poll();
89    
90      /**      /**
91       * Remove and return an element from the queue.  This method differs       * Retrieve and remove the head of this queue.
92         * This method differs
93       * from the <tt>poll</tt> method in that it throws an exception if the       * from the <tt>poll</tt> method in that it throws an exception if the
94       * queue is empty.       * queue is empty.
95       *       *
96       * @return an element previously on the queue.       * @return the head of this queue.
97       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
98       */       */
99      E remove() throws NoSuchElementException;      E remove() throws NoSuchElementException;
100    
101      /**      /**
102       * Return, but do not remove, an element from the queue, or <tt>null</tt>       * Retrieve, but do not remove, the head of this queue, or <tt>null</tt>
103       * if the queue is empty.  This method returns the same object reference       * if this queue is empty.  This method differs from the <tt>poll</tt>
104       * that would be returned by by the <tt>poll</tt> method.  The two methods       * method only in that this method does not remove the element from
105       * differ in that this method does not remove the element from the queue.       * this queue.
106       *       *
107       * @return an element on the queue, or <tt>null</tt> if the queue is empty.       * @return the head of this queue, or <tt>null</tt> if this queue is empty.
108       */       */
109      E peek();      E peek();
110    
111      /**      /**
112       * Return, but do not remove, an element from the queue.  This method       * Retrieve, but do not remove, the head of this queue.  This method
113       * differs from the <tt>peek</tt> method in that it throws an exception if       * differs from the <tt>peek</tt> method only in that it throws an
114       * the queue is empty.       * exception if this queue is empty.
115       *       *
116       * @return an element on the queue.       * @return the head of this queue.
117       * @throws NoSuchElementException if the queue is empty.       * @throws NoSuchElementException if this queue is empty.
118       */       */
119      E element() throws NoSuchElementException;      E element() throws NoSuchElementException;
120  }  }
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