| 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. |
| 13 |
< |
* |
| 13 |
> |
0 * |
| 14 |
|
* <p>Queues typically, but do not necessarily, order elements in a |
| 15 |
< |
* FIFO (first-in-first-out) manner. Among the exceptions are priority |
| 16 |
< |
* queues, which order elements according to a supplied comparators, or |
| 17 |
< |
* the elements natural ordering. Every Queue implementation must specify |
| 18 |
< |
* its ordering guarantees. |
| 15 |
> |
* FIFO (first-in-first-out) manner. Among the exceptions are |
| 16 |
> |
* priority queues, which order elements according to a supplied |
| 17 |
> |
* comparator, or the elements' natural ordering. Every Queue |
| 18 |
> |
* implementation must specify its ordering guarantees. |
| 19 |
|
* |
| 20 |
|
* <p>The {@link #offer(E)} method adds an element if possible, otherwise |
| 21 |
|
* returning <tt>false</tt>. This differs from the {@link |
| 25 |
|
* in fixed-capacity (or “bounded”) queues. |
| 26 |
|
|
| 27 |
|
* |
| 28 |
< |
* <p>The {@link #remove()} and {@link #poll()} methods remove and return an |
| 29 |
< |
* element in accord with the implementation's ordering policy. |
| 30 |
< |
* Exactly which element is removed from the queue is a function |
| 31 |
< |
* of the queue's ordering policy, which differs from implementation |
| 32 |
< |
* to implementation. Possible orderings include (but are not limited |
| 33 |
< |
* to) first-in-first-out (FIFO), element priority, and arbitrary. |
| 34 |
< |
* The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their |
| 35 |
< |
* behavior when the queue is empty: the <tt>remove()</tt> method throws an |
| 36 |
< |
* exception, while the <tt>poll()</tt> method returns <tt>null</tt>. |
| 28 |
> |
* <p>The {@link #remove()} and {@link #poll()} methods remove and |
| 29 |
> |
* return an element in accord with the implementation's ordering |
| 30 |
> |
* policy. Exactly which element is removed from the queue is a |
| 31 |
> |
* function of the queue's ordering policy, which differs from |
| 32 |
> |
* implementation to implementation. Possible orderings include (but |
| 33 |
> |
* are not limited to) first-in-first-out (FIFO), last-in-first-out |
| 34 |
> |
* (LIFO), element priority, and arbitrary. The <tt>remove()</tt> and |
| 35 |
> |
* <tt>poll()</tt> methods differ only in their behavior when the |
| 36 |
> |
* queue is empty: the <tt>remove()</tt> method throws an exception, |
| 37 |
> |
* while the <tt>poll()</tt> method returns <tt>null</tt>. |
| 38 |
|
* |
| 39 |
|
* <p>The {@link #element()} and {@link #peek()} methods return but do |
| 40 |
|
* not delete the element that would be obtained by a call to |
| 46 |
|
* defined in the {@link java.util.concurrent.BlockingQueue} interface, which |
| 47 |
|
* extends this interface. |
| 48 |
|
* |
| 49 |
< |
* <p><tt>Queue</tt> implementations generally do not allow insertion of |
| 50 |
< |
* <tt>null</tt> elements. Even in the few implementations that permit it, |
| 51 |
< |
* it is a bad idea, as <tt>null</tt> is also used as a special return value |
| 52 |
< |
* by the <tt>poll</tt> method to indicate that the queue contains no |
| 53 |
< |
* elements. |
| 49 |
> |
* <p><tt>Queue</tt> implementations generally do not allow insertion |
| 50 |
> |
* of <tt>null</tt> elements, although some implementations, such as |
| 51 |
> |
* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>. |
| 52 |
> |
* Even in the implementations that permit it, <tt>null</tt> should |
| 53 |
> |
* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also |
| 54 |
> |
* used as a special return value by the <tt>poll</tt> method to |
| 55 |
> |
* indicate that the queue contains no elements. |
| 56 |
|
* |
| 57 |
|
* <p>This interface is a member of the |
| 58 |
|
* <a href="{@docRoot}/../guide/collections/index.html"> |
| 61 |
|
* @see Collection |
| 62 |
|
* @see LinkedList |
| 63 |
|
* @see PriorityQueue |
| 64 |
< |
* @see LinkedQueue |
| 64 |
> |
* @see java.util.concurrent.LinkedQueue |
| 65 |
|
* @see java.util.concurrent.BlockingQueue |
| 66 |
|
* @see java.util.concurrent.ArrayBlockingQueue |
| 67 |
|
* @see java.util.concurrent.LinkedBlockingQueue |
| 68 |
|
* @see java.util.concurrent.PriorityBlockingQueue |
| 69 |
+ |
* @since 1.5 |
| 70 |
+ |
* @author Doug Lea |
| 71 |
|
*/ |
| 72 |
|
public interface Queue<E> extends Collection<E> { |
| 73 |
|
/** |
| 76 |
|
* @param element the element to add. |
| 77 |
|
* @return true if it was possible to add the element to the queue. |
| 78 |
|
*/ |
| 79 |
< |
public boolean offer(E element); |
| 79 |
> |
boolean offer(E element); |
| 80 |
|
|
| 81 |
|
/** |
| 82 |
|
* Remove and return an element from the queue if one is available. |
| 84 |
|
* @return an element previously on the queue, or <tt>null</tt> if the |
| 85 |
|
* queue is empty. |
| 86 |
|
*/ |
| 87 |
< |
public E poll(); |
| 87 |
> |
E poll(); |
| 88 |
|
|
| 89 |
|
/** |
| 90 |
|
* Remove and return an element from the queue. This method differs |
| 94 |
|
* @return an element previously on the queue. |
| 95 |
|
* @throws NoSuchElementException if the queue is empty. |
| 96 |
|
*/ |
| 97 |
< |
public E remove() throws NoSuchElementException; |
| 97 |
> |
E remove() throws NoSuchElementException; |
| 98 |
|
|
| 99 |
|
/** |
| 100 |
|
* Return, but do not remove, an element from the queue, or <tt>null</tt> |
| 104 |
|
* |
| 105 |
|
* @return an element on the queue, or <tt>null</tt> if the queue is empty. |
| 106 |
|
*/ |
| 107 |
< |
public E peek(); |
| 107 |
> |
E peek(); |
| 108 |
|
|
| 109 |
|
/** |
| 110 |
|
* Return, but do not remove, an element from the queue. This method |
| 114 |
|
* @return an element on the queue. |
| 115 |
|
* @throws NoSuchElementException if the queue is empty. |
| 116 |
|
*/ |
| 117 |
< |
public E element() throws NoSuchElementException; |
| 117 |
> |
E element() throws NoSuchElementException; |
| 118 |
|
} |
