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package java.util; |
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/** |
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* <tt>AbstractQueue</tt> provides default implementations of |
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* {@link #add add}, {@link #remove remove}, and {@link #element element} |
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* based on |
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* {@link #offer offer}, {@link #poll poll}, and {@link #peek peek}, |
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* respectively but that |
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* throw exceptions instead of indicating failure via <tt>false</tt> or |
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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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* <p>The provided implementations all assume that the base implementation |
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* does <em>not</em> allow <tt>null</tt> elements. |
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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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extends AbstractCollection<E> |
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implements Queue<E> { |
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// note that optional methods are not optional here or in our subclasses, |
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// so we redefine each optional method to document that it is not optional |
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// We also inherit, or define, all necessary @throws comments |
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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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* Adds the specified element to this queue. |
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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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* |
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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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* <tt>Collection.add</tt>). |
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* |
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* @throws NullPointerException if the element is null |
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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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} |
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/** |
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* Adds all of the elements in the specified collection to this queue. |
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* The behavior of this operation is undefined if |
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* the specified collection is modified while the operation is in |
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* progress. (This implies that the behavior of this call is undefined if |
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* the specified collection is this queue, and this queue is nonempty.) |
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* <p> |
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* This implementation iterates over the specified collection, and adds |
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* each object returned by the iterator to this collection, in turn. |
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* |
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* @param c collection whose elements are to be added to this queue |
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* @return <tt>true</tt> if this collection changed as a result of the |
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* call. |
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* @throws NullPointerException if <tt>c</tt> is <tt>null</tt> |
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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 boolean addAll(Collection<? extends E> c) { |
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return super.addAll(c); |
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} |
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/** @throws NoSuchElementException {@inheritDoc} */ |
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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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throw new NoSuchElementException(); |
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} |
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/** @throws NoSuchElementException {@inheritDoc} */ |
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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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while (poll() != null) |
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; |
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} |
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// XXX Remove this redundant declaration, pending response from Neal Gafter. |
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public abstract Iterator<E> iterator(); |
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} |
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