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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 java.util.Collection Collection} operations, queues provide |
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* additional insertion, extraction, and inspection operations. |
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* |
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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 |
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* priority queues, which order elements according to a supplied |
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* comparator, or the elements' natural ordering, and LIFO queues (or |
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* stacks) which order the elements LIFO (last-in-first-out). |
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* Whatever the ordering used, the <em>head</em> of the queue is that element |
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* which would be removed by a call to {@link #remove() } or {@link #poll()}. |
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* Every <tt>Queue</tt> implementation must specify its ordering guarantees. |
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* |
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* <p>The {@link #offer offer} method adds an element if possible, otherwise |
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* returning <tt>false</tt>. This differs from the |
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* {@link java.util.Collection#add Collection.add} |
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* method, which throws an unchecked exception upon |
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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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* <p>The {@link #remove()} and {@link #poll()} methods remove and |
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* return the head of the queue. |
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* Exactly which element is removed from the queue is a |
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* function of the queue's ordering policy, which differs from |
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* implementation to implementation. The <tt>remove()</tt> and |
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* <tt>poll()</tt> methods differ only in their behavior when the |
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* queue is empty: the <tt>remove()</tt> method throws an exception, |
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* while the <tt>poll()</tt> method returns <tt>null</tt>. |
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* |
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* <p>The {@link #element()} and {@link #peek()} methods return, but do |
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* not remove, the head of the queue. |
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* |
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* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue |
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* methods</i>, which are common in concurrent programming. These methods, |
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* which wait for elements to appear or for space to become available, are |
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* defined in the {@link java.util.concurrent.BlockingQueue} interface, which |
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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 |
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* of <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 |
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* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also |
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* used as a special return value by the <tt>poll</tt> method to |
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* indicate that the queue contains no elements. |
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* |
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* <p><tt>Queue</tt> implementations generally do not define |
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* element-based versions of methods <tt>equals</tt> and |
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* <tt>hashCode</tt> but instead inherit the identity based versions |
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* from class <tt>Object</tt>, because element-based equality is not |
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* always well-defined for queues with the same elements but different |
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* ordering properties. |
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* |
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* |
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* <p>This interface is a member of the |
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* <a href="{@docRoot}/../guide/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @see java.util.Collection |
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* @see LinkedList |
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* @see PriorityQueue |
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* @see java.util.concurrent.LinkedBlockingQueue |
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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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* @see java.util.concurrent.PriorityBlockingQueue |
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* @since 1.5 |
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* @author Doug Lea |
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*/ |
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public interface Queue<E> extends Collection<E> { |
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/** |
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* Adds the specified element to this queue, if possible. |
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* |
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* @param o the element to add. |
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* @return <tt>true</tt> if it was possible to add the element to |
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* this queue, else <tt>false</tt> |
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*/ |
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boolean offer(E o); |
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|
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/** |
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* Retrieves and removes the head of this queue, if it is available. |
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* |
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* @return the head of this queue, or <tt>null</tt> if this |
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* queue is empty. |
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*/ |
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E poll(); |
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|
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/** |
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* Retrieves and removes the head of this queue. |
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* This method differs |
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* from the <tt>poll</tt> method in that it throws an exception if this |
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* queue is empty. |
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* |
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* @return the head of this queue. |
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* @throws NoSuchElementException if this queue is empty. |
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*/ |
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E remove(); |
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/** |
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* Retrieves, but does not remove, the head of this queue. |
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* This method differs from the <tt>poll</tt> |
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* method only in that this method does not remove the head element from |
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* this queue. |
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* |
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* @return the head of this queue, or <tt>null</tt> if this queue is empty. |
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*/ |
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E peek(); |
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/** |
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* Retrieves, but does not remove, the head of this queue. This method |
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* differs from the <tt>peek</tt> method only in that it throws an |
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* exception if this queue is empty. |
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* |
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* @return the head of this queue. |
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* @throws NoSuchElementException if this queue is empty. |
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*/ |
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E element(); |
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} |
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