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jeremybenn |
/*
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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, as explained at
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* http://creativecommons.org/licenses/publicdomain
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*/
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package java.util;
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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 overridden
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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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* <p>This class is a member of the
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* <a href="{@docRoot}/../technotes/guides/collections/index.html">
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* Java Collections Framework</a>.
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*
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* @since 1.5
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* @author Doug Lea
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* @param <E> the type of elements held in this collection
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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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* 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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* Inserts the specified element into this queue if it is possible to do so
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* immediately without violating capacity restrictions, returning
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* <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
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* if no space is currently available.
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*
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* <p>This implementation returns <tt>true</tt> if <tt>offer</tt> succeeds,
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* else throws an <tt>IllegalStateException</tt>.
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*
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* @param e the element to add
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* @return <tt>true</tt> (as specified by {@link Collection#add})
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* @throws IllegalStateException if the element cannot be added at this
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* time due to capacity restrictions
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* @throws ClassCastException if the class of the specified element
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* prevents it from being added to this queue
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* @throws NullPointerException if the specified element is null and
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* this queue does not permit null elements
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* @throws IllegalArgumentException if some property of this element
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* prevents it from being added to this queue
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*/
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public boolean add(E e) {
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if (offer(e))
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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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* Retrieves and removes the head of this queue. This method differs
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* from {@link #poll poll} only in that it throws an exception if this
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* queue is empty.
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*
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* <p>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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* Retrieves, but does not remove, the head of this queue. This method
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* differs from {@link #peek peek} only in that it throws an exception if
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* this queue is empty.
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*
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* <p>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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* Removes all of the elements from this queue.
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* The queue will be empty after this call returns.
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*
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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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/**
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* Adds all of the elements in the specified collection to this
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* queue. Attempts to addAll of a queue to itself result in
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* <tt>IllegalArgumentException</tt>. Further, the behavior of
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* this operation is undefined if the specified collection is
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* modified while the operation is in progress.
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*
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* <p>This implementation iterates over the specified collection,
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* and adds each element returned by the iterator to this
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* queue, in turn. A runtime exception encountered while
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* trying to add an element (including, in particular, a
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* <tt>null</tt> element) may result in only some of the elements
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* having been successfully added when the associated exception is
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* thrown.
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*
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* @param c collection containing elements to be added to this queue
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* @return <tt>true</tt> if this queue changed as a result of the call
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* @throws ClassCastException if the class of an element of the specified
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* collection prevents it from being added to this queue
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* @throws NullPointerException if the specified collection contains a
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* null element and this queue does not permit null elements,
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* or if the specified collection is null
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* @throws IllegalArgumentException if some property of an element of the
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* specified collection prevents it from being added to this
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* queue, or if the specified collection is this queue
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* @throws IllegalStateException if not all the elements can be added at
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* this time due to insertion restrictions
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* @see #add(Object)
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*/
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public boolean addAll(Collection<? extends E> c) {
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if (c == null)
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throw new NullPointerException();
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if (c == this)
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throw new IllegalArgumentException();
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boolean modified = false;
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Iterator<? extends E> e = c.iterator();
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while (e.hasNext()) {
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if (add(e.next()))
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modified = true;
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}
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return modified;
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}
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}
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