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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain. Use, modify, and |
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* redistribute this code in any way without acknowledgement. |
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*/ |
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package java.util; |
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/** |
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* A Collection designed for holding elements prior to processing. |
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* Besides basic {@link Collection} operations, queues provide |
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* additional insertion, extraction, and inspection operations. |
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* |
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0 * |
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* <p>Queues typically, but do not necessarily, order elements in a |
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* FIFO (first-in-first-out) manner. Among the exceptions are priority |
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* queues, which order elements according to a supplied comparators, or |
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* the elements natural ordering. Every Queue implementation must specify |
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* its ordering guarantees. |
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* FIFO (first-in-first-out) manner. Among the exceptions are |
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* priority queues, which order elements according to a supplied |
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* comparator, or the elements' natural ordering. Every Queue |
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* implementation must specify its ordering guarantees. |
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* |
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* <p>The {@link #offer(E)} method adds an element if possible, otherwise |
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* returning <tt>false</tt>. This differs from the {@link |
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* failure. It is designed for use in collections in which failure to |
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* add is a normal, rather than exceptional occurrence, for example, |
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* in fixed-capacity (or “bounded”) queues. |
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* |
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* |
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* <p>The {@link #remove()} and {@link #poll()} methods remove and return an |
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* element in accord with the implementation's ordering policy. For example, |
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* in FIFO queues, they remove and return the oldest element in the queue. |
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* element in accord with the implementation's ordering policy. |
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* Exactly which element is removed from the queue is a function |
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* of the queue's ordering policy, which differs from implementation |
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* to implementation. Possible orderings include (but are not limited |
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* to) first-in-first-out (FIFO), last-in-first-out (LIFO), element priority, and arbitrary. |
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* The <tt>remove()</tt> and <tt>poll()</tt> methods differ only in their |
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* behavior when the queue is empty: the <tt>remove()</tt> method throws an |
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* exception, while the <tt>poll()</tt> method returns <tt>null</tt>. |
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* extends this interface. |
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* |
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* <p><tt>Queue</tt> implementations generally do not allow insertion of |
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* <tt>null</tt> elements. Even in the few implementations that permit it, |
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* it is a bad idea, as <tt>null</tt> is also used as a special return value |
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* <tt>null</tt> elements, although some implementations, such as |
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* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>. |
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* Even in the implementations that permit it, <tt>null</tt> should not be inserted into |
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* a <tt>Queue</tt>, as <tt>null</tt> is also used as a special return value |
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* by the <tt>poll</tt> method to indicate that the queue contains no |
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* elements. |
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* |
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* @see Collection |
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* @see LinkedList |
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* @see PriorityQueue |
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* @see LinkedQueue |
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* @see java.util.concurrent.LinkedQueue |
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* @see java.util.concurrent.BlockingQueue |
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* @see java.util.concurrent.ArrayBlockingQueue |
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* @see java.util.concurrent.LinkedBlockingQueue |
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/** |
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* Remove and return an element from the queue if one is available. |
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* Exactly which element is removed from the queue is a function |
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* of the queue's ordering policy, which differs from implementation |
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* to implementation. Possible orderings include (but are not limited |
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* to) first-in-first-out (FIFO), element priority, and arbitrary. |
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* |
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* @return an element previously on the queue, or <tt>null</tt> if the |
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* queue is empty. |
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* queue is empty. |
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*/ |
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public E poll(); |
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/** |
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* Remove and return an element from the queue. This method differs |
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* from the <tt>poll</tt> method in that it throws an exception if the |
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* queue is empty. |
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* queue is empty. |
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* |
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* @return an element previously on the queue. |
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* @throws NoSuchElementException if the queue is empty. |
