| 1 |
|
/* |
| 2 |
|
* Written by Doug Lea with assistance from members of JCP JSR-166 |
| 3 |
< |
* Expert Group and released to the public domain. Use, modify, and |
| 4 |
< |
* redistribute this code in any way without acknowledgement. |
| 3 |
> |
* Expert Group and released to the public domain, as explained at |
| 4 |
> |
* http://creativecommons.org/licenses/publicdomain |
| 5 |
|
*/ |
| 6 |
|
|
| 7 |
|
package java.util; |
| 8 |
|
|
| 9 |
|
/** |
| 10 |
< |
* A Collection designed for holding elements prior to processing. |
| 11 |
< |
* Besides basic {@link Collection} operations, queues provide |
| 12 |
< |
* additional insertion, extraction, and inspection operations. |
| 10 |
> |
* A collection designed for holding elements prior to processing. |
| 11 |
> |
* Besides basic {@link java.util.Collection Collection} operations, |
| 12 |
> |
* 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><table BORDER CELLPADDING=3 CELLSPACING=1> |
| 22 |
> |
* <tr> |
| 23 |
> |
* <td></td> |
| 24 |
> |
* <td ALIGN=CENTER><em>Throws exception</em></td> |
| 25 |
> |
* <td ALIGN=CENTER><em>Returns special value</em></td> |
| 26 |
> |
* </tr> |
| 27 |
> |
* <tr> |
| 28 |
> |
* <td><b>Insert</b></td> |
| 29 |
> |
* <td>{@link #add add(e)}</td> |
| 30 |
> |
* <td>{@link #offer offer(e)}</td> |
| 31 |
> |
* </tr> |
| 32 |
> |
* <tr> |
| 33 |
> |
* <td><b>Remove</b></td> |
| 34 |
> |
* <td>{@link #remove remove()}</td> |
| 35 |
> |
* <td>{@link #poll poll()}</td> |
| 36 |
> |
* </tr> |
| 37 |
> |
* <tr> |
| 38 |
> |
* <td><b>Examine</b></td> |
| 39 |
> |
* <td>{@link #element element()}</td> |
| 40 |
> |
* <td>{@link #peek peek()}</td> |
| 41 |
> |
* </tr> |
| 42 |
> |
* </table> |
| 43 |
|
* |
| 44 |
|
* <p>Queues typically, but do not necessarily, order elements in a |
| 45 |
< |
* FIFO (first-in-first-out) manner. Among the exceptions are priority |
| 46 |
< |
* queues, which order elements according to a supplied comparators, or |
| 47 |
< |
* the elements natural ordering. Every Queue implementation must specify |
| 48 |
< |
* its ordering guarantees. |
| 49 |
< |
* |
| 50 |
< |
* <p>The {@link #offer(E)} method adds an element if possible, otherwise |
| 51 |
< |
* returning <tt>false</tt>. This differs from the {@link |
| 52 |
< |
* Collections#add(Object)} method, which throws an unchecked exception upon |
| 53 |
< |
* failure. It is designed for use in collections in which failure to |
| 54 |
< |
* add is a normal, rather than exceptional occurrence, for example, |
| 55 |
< |
* in fixed-capacity (or “bounded”) queues. |
| 56 |
< |
|
| 57 |
< |
* |
| 58 |
< |
* <p>The {@link #remove()} and {@link #poll()} methods remove and return an |
| 59 |
< |
* element in accord with the implementation's ordering policy. |
| 60 |
< |
* Exactly which element is removed from the queue is a function |
| 61 |
< |
* of the queue's ordering policy, which differs from implementation |
| 62 |
< |
* to implementation. Possible orderings include (but are not limited |
| 63 |
< |
* to) first-in-first-out (FIFO), element priority, and arbitrary. |
| 64 |
< |
* The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their |
| 65 |
< |
* behavior when the queue is empty: the <tt>remove()</tt> method throws an |
| 66 |
< |
* exception, while the <tt>poll()</tt> method returns <tt>null</tt>. |
| 67 |
< |
* |
| 68 |
< |
* <p>The {@link #element()} and {@link #peek()} methods return but do |
| 69 |
< |
* not delete the element that would be obtained by a call to |
| 70 |
< |
* the <tt>remove</tt> and <tt>poll</tt> methods respectively. |
| 45 |
> |
* FIFO (first-in-first-out) manner. Among the exceptions are |
| 46 |
> |
* priority queues, which order elements according to a supplied |
| 47 |
> |
* comparator, or the elements' natural ordering, and LIFO queues (or |
| 48 |
> |
* stacks) which order the elements LIFO (last-in-first-out). |
| 49 |
> |
* Whatever the ordering used, the <em>head</em> of the queue is that |
| 50 |
> |
* element which would be removed by a call to {@link #remove() } or |
| 51 |
> |
* {@link #poll()}. In a FIFO queue, all new elements are inserted at |
| 52 |
> |
* the <em> tail</em> of the queue. Other kinds of queues may use |
| 53 |
> |
* different placement rules. Every <tt>Queue</tt> implementation |
| 54 |
> |
* must specify its ordering properties. |
| 55 |
> |
* |
| 56 |
> |
* <p>The {@link #offer offer} method inserts an element if possible, |
| 57 |
> |
* otherwise returning <tt>false</tt>. This differs from the {@link |
| 58 |
> |
* java.util.Collection#add Collection.add} method, which can fail to |
| 59 |
> |
* add an element only by throwing an unchecked exception. The |
| 60 |
> |
* <tt>offer</tt> method is designed for use when failure is a normal, |
| 61 |
> |
* rather than exceptional occurrence, for example, in fixed-capacity |
| 62 |
> |
* (or "bounded") queues. |
| 63 |
> |
* |
| 64 |
> |
* <p>The {@link #remove()} and {@link #poll()} methods remove and |
| 65 |
> |
* return the head of the queue. |
| 66 |
> |
* Exactly which element is removed from the queue is a |
| 67 |
> |
* function of the queue's ordering policy, which differs from |
| 68 |
> |
* implementation to implementation. The <tt>remove()</tt> and |
| 69 |
> |
* <tt>poll()</tt> methods differ only in their behavior when the |
| 70 |
> |
* queue is empty: the <tt>remove()</tt> method throws an exception, |
| 71 |
> |
* while the <tt>poll()</tt> method returns <tt>null</tt>. |
| 72 |
> |
* |
| 73 |
> |
* <p>The {@link #element()} and {@link #peek()} methods return, but do |
| 74 |
> |
* not remove, the head of the queue. |
| 75 |
|
* |
| 76 |
|
* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue |
| 77 |
|
* methods</i>, which are common in concurrent programming. These methods, |
| 79 |
|
* defined in the {@link java.util.concurrent.BlockingQueue} interface, which |
| 80 |
|
* extends this interface. |
| 81 |
|
* |
| 82 |
< |
* <p><tt>Queue</tt> implementations generally do not allow insertion of |
| 83 |
< |
* <tt>null</tt> elements. Even in the few implementations that permit it, |
| 84 |
< |
* it is a bad idea, as <tt>null</tt> is also used as a special return value |
| 85 |
< |
* by the <tt>poll</tt> method to indicate that the queue contains no |
| 86 |
< |
* elements. |
| 82 |
> |
* <p><tt>Queue</tt> implementations generally do not allow insertion |
| 83 |
> |
* of <tt>null</tt> elements, although some implementations, such as |
| 84 |
> |
* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>. |
| 85 |
> |
* Even in the implementations that permit it, <tt>null</tt> should |
| 86 |
> |
* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also |
| 87 |
> |
* used as a special return value by the <tt>poll</tt> method to |
| 88 |
> |
* indicate that the queue contains no elements. |
| 89 |
> |
* |
| 90 |
> |
* <p><tt>Queue</tt> implementations generally do not define |
| 91 |
> |
* element-based versions of methods <tt>equals</tt> and |
| 92 |
> |
* <tt>hashCode</tt> but instead inherit the identity based versions |
| 93 |
> |
* from class <tt>Object</tt>, because element-based equality is not |
| 94 |
> |
* always well-defined for queues with the same elements but different |
| 95 |
> |
* ordering properties. |
| 96 |
> |
* |
| 97 |
|
* |
| 98 |
|
* <p>This interface is a member of the |
| 99 |
|
* <a href="{@docRoot}/../guide/collections/index.html"> |
| 100 |
|
* Java Collections Framework</a>. |
| 101 |
|
* |
| 102 |
< |
* @see Collection |
| 102 |
> |
* @see java.util.Collection |
| 103 |
|
* @see LinkedList |
| 104 |
|
* @see PriorityQueue |
| 105 |
< |
* @see LinkedQueue |
| 105 |
> |
* @see java.util.concurrent.LinkedBlockingQueue |
| 106 |
|
* @see java.util.concurrent.BlockingQueue |
| 107 |
|
* @see java.util.concurrent.ArrayBlockingQueue |
| 108 |
|
* @see java.util.concurrent.LinkedBlockingQueue |
| 109 |
|
* @see java.util.concurrent.PriorityBlockingQueue |
| 110 |
+ |
* @since 1.5 |
| 111 |
+ |
* @author Doug Lea |
| 112 |
+ |
* @param <E> the type of elements held in this collection |
| 113 |
|
*/ |
| 114 |
|
public interface Queue<E> extends Collection<E> { |
| 115 |
+ |
|
| 116 |
|
/** |
| 117 |
< |
* Add the specified element to this queue, if possible. |
| 117 |
> |
* Inserts the specified element into this queue, if possible. When |
| 118 |
> |
* using queues that may impose insertion restrictions (for |
| 119 |
> |
* example capacity bounds), method <tt>offer</tt> is generally |
| 120 |
> |
* preferable to method {@link Collection#add}, which can fail to |
| 121 |
> |
* insert an element only by throwing an exception. |
| 122 |
|
* |
| 123 |
< |
* @param element the element to add. |
| 124 |
< |
* @return true if it was possible to add the element to the queue. |
| 123 |
> |
* @param o the element to insert. |
| 124 |
> |
* @return <tt>true</tt> if it was possible to add the element to |
| 125 |
> |
* this queue, else <tt>false</tt> |
| 126 |
|
*/ |
| 127 |
< |
public boolean offer(E element); |
| 127 |
> |
boolean offer(E o); |
| 128 |
|
|
| 129 |
|
/** |
| 130 |
< |
* Remove and return an element from the queue if one is available. |
| 130 |
> |
* Retrieves and removes the head of this queue, or <tt>null</tt> |
| 131 |
> |
* if this queue is empty. |
| 132 |
|
* |
| 133 |
< |
* @return an element previously on the queue, or <tt>null</tt> if the |
| 134 |
< |
* queue is empty. |
| 133 |
> |
* @return the head of this queue, or <tt>null</tt> if this |
| 134 |
> |
* queue is empty. |
| 135 |
|
*/ |
| 136 |
< |
public E poll(); |
| 136 |
> |
E poll(); |
| 137 |
|
|
| 138 |
|
/** |
| 139 |
< |
* Remove and return an element from the queue. This method differs |
| 140 |
< |
* from the <tt>poll</tt> method in that it throws an exception if the |
| 141 |
< |
* queue is empty. |
| 139 |
> |
* Retrieves and removes the head of this queue. This method |
| 140 |
> |
* differs from the {@link #poll} method only in that it throws an |
| 141 |
> |
* exception if this queue is empty. |
| 142 |
|
* |
| 143 |
< |
* @return an element previously on the queue. |
| 144 |
< |
* @throws NoSuchElementException if the queue is empty. |
| 143 |
> |
* @return the head of this queue. |
| 144 |
> |
* @throws NoSuchElementException if this queue is empty. |
| 145 |
|
*/ |
| 146 |
< |
public E remove() throws NoSuchElementException; |
| 146 |
> |
E remove(); |
| 147 |
|
|
| 148 |
|
/** |
| 149 |
< |
* Return, but do not remove, an element from the queue, or <tt>null</tt> |
| 150 |
< |
* if the queue is empty. This method returns the same object reference |
| 97 |
< |
* that would be returned by by the <tt>poll</tt> method. The two methods |
| 98 |
< |
* differ in that this method does not remove the element from the queue. |
| 149 |
> |
* Retrieves, but does not remove, the head of this queue, |
| 150 |
> |
* returning <tt>null</tt> if this queue is empty. |
| 151 |
|
* |
| 152 |
< |
* @return an element on the queue, or <tt>null</tt> if the queue is empty. |
| 152 |
> |
* @return the head of this queue, or <tt>null</tt> if this queue |
| 153 |
> |
* is empty. |
| 154 |
|
*/ |
| 155 |
< |
public E peek(); |
| 155 |
> |
E peek(); |
| 156 |
|
|
| 157 |
|
/** |
| 158 |
< |
* Return, but do not remove, an element from the queue. This method |
| 159 |
< |
* differs from the <tt>peek</tt> method in that it throws an exception if |
| 160 |
< |
* the queue is empty. |
| 158 |
> |
* Retrieves, but does not remove, the head of this queue. This method |
| 159 |
> |
* differs from the {@link #peek} method only in that it throws an |
| 160 |
> |
* exception if this queue is empty. |
| 161 |
|
* |
| 162 |
< |
* @return an element on the queue. |
| 163 |
< |
* @throws NoSuchElementException if the queue is empty. |
| 162 |
> |
* @return the head of this queue. |
| 163 |
> |
* @throws NoSuchElementException if this queue is empty. |
| 164 |
|
*/ |
| 165 |
< |
public E element() throws NoSuchElementException; |
| 165 |
> |
E element(); |
| 166 |
|
} |
