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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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|
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package java.util; |
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|
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/** |
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* This class provides skeletal implementations of some {@link Queue} |
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* operations. The implementations in this class are appropriate when |
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* the base implementation does <em>not</em> allow <tt>null</tt> |
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* elements. Methods {@link #add add}, {@link #remove remove}, and |
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* {@link #element element} are based on {@link #offer offer}, {@link |
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* #poll poll}, and {@link #peek peek}, respectively but throw |
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* exceptions instead of indicating failure via <tt>false</tt> or |
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* <tt>null</tt> returns. |
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* |
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* <p> A <tt>Queue</tt> implementation that extends this class must |
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* minimally define a method {@link Queue#offer} which does not permit |
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* insertion of <tt>null</tt> elements, along with methods {@link |
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* Queue#peek}, {@link Queue#poll}, {@link Collection#size}, and a |
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* {@link Collection#iterator} supporting {@link |
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* Iterator#remove}. Typically, additional methods will be overriden |
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* as well. If these requirements cannot be met, consider instead |
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* subclassing {@link AbstractCollection}. |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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*/ |
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public abstract class AbstractQueue<E> |
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extends AbstractCollection<E> |
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implements Queue<E> { |
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|
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/** |
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* Constructor for use by subclasses. |
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*/ |
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protected AbstractQueue() { |
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} |
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|
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|
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/** |
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* Adds the specified element to this queue. This implementation |
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* returns <tt>true</tt> if <tt>offer</tt> succeeds, else |
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* throws an IllegalStateException. |
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* th |
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* @param o the element |
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* @return <tt>true</tt> (as per the general contract of |
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* <tt>Collection.add</tt>). |
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* |
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* @throws NullPointerException if the specified element is <tt>null</tt> |
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* @throws IllegalStateException if element cannot be added |
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*/ |
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public boolean add(E o) { |
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if (offer(o)) |
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return true; |
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else |
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throw new IllegalStateException("Queue full"); |
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} |
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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 implementation returns the result of <tt>poll</tt> |
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* unless the 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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public E remove() { |
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E x = poll(); |
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if (x != null) |
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return x; |
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else |
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throw new NoSuchElementException(); |
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} |
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|
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|
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/** |
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* Retrieves, but does not remove, the head of this queue. |
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* This implementation returns the result of <tt>peek</tt> |
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* unless the 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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public E element() { |
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E x = peek(); |
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if (x != null) |
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return x; |
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else |
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throw new NoSuchElementException(); |
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} |
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|
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/** |
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* Removes all of the elements from this collection. |
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* The collection will be empty after this call returns. |
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* <p>This implementation repeatedly invokes {@link #poll poll} until it |
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* returns <tt>null</tt>. |
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*/ |
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public void clear() { |
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while (poll() != null) |
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; |
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} |
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} |
