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/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util;

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/**
 * This class provides skeletal implementations of some {@link Queue}
 * operations. The implementations in this class are appropriate when
 * the base implementation does not allow null
 * elements.  Methods {@link #add add}, {@link #remove remove}, and
 * {@link #element element} are based on {@link #offer offer}, {@link
 * #poll poll}, and {@link #peek peek}, respectively, but throw
 * exceptions instead of indicating failure via false or
 * null returns.
 *
 * 

A Queue implementation that extends this class must * minimally define a method {@link Queue#offer} which does not permit * insertion of null elements, along with methods {@link * Queue#peek}, {@link Queue#poll}, {@link Collection#size}, and * {@link Collection#iterator}. Typically, additional methods will be * overridden as well. If these requirements cannot be met, consider * instead subclassing {@link AbstractCollection}. * * @since 1.5 * @author Doug Lea * @param the type of elements held in this collection */ public abstract class AbstractQueue extends AbstractCollection implements Queue { /** * Constructor for use by subclasses. */ protected AbstractQueue() { } /** * Inserts the specified element into this queue if it is possible to do so * immediately without violating capacity restrictions, returning * true upon success and throwing an IllegalStateException * if no space is currently available. * *

This implementation returns true if offer succeeds, * else throws an IllegalStateException. * * @param e the element to add * @return true (as specified by {@link Collection#add}) * @throws IllegalStateException if the element cannot be added at this * time due to capacity restrictions * @throws ClassCastException if the class of the specified element * prevents it from being added to this queue * @throws NullPointerException if the specified element is null and * this queue does not permit null elements * @throws IllegalArgumentException if some property of this element * prevents it from being added to this queue */ public boolean add(E e) { if (offer(e)) return true; else throw new IllegalStateException("Queue full"); } /** * Retrieves and removes the head of this queue. This method differs * from {@link #poll poll} only in that it throws an exception if this * queue is empty. * *

This implementation returns the result of poll * unless the queue is empty. * * @return the head of this queue * @throws NoSuchElementException if this queue is empty */ public E remove() { E x = poll(); if (x != null) return x; else throw new NoSuchElementException(); } /** * Retrieves, but does not remove, the head of this queue. This method * differs from {@link #peek peek} only in that it throws an exception if * this queue is empty. * *

This implementation returns the result of peek * unless the queue is empty. * * @return the head of this queue * @throws NoSuchElementException if this queue is empty */ public E element() { E x = peek(); if (x != null) return x; else throw new NoSuchElementException(); } /** * Removes all of the elements from this queue. * The queue will be empty after this call returns. * *

This implementation repeatedly invokes {@link #poll poll} until it * returns null. */ public void clear() { while (poll() != null) ; } /** * Adds all of the elements in the specified collection to this * queue. Attempts to addAll of a queue to itself result in * IllegalArgumentException. Further, the behavior of * this operation is undefined if the specified collection is * modified while the operation is in progress. * *

This implementation iterates over the specified collection, * and adds each element returned by the iterator to this * queue, in turn. A runtime exception encountered while * trying to add an element (including, in particular, a * null element) may result in only some of the elements * having been successfully added when the associated exception is * thrown. * * @param c collection containing elements to be added to this queue * @return true if this queue changed as a result of the call * @throws ClassCastException if the class of an element of the specified * collection prevents it from being added to this queue * @throws NullPointerException if the specified collection contains a * null element and this queue does not permit null elements, * or if the specified collection is null * @throws IllegalArgumentException if some property of an element of the * specified collection prevents it from being added to this * queue, or if the specified collection is this queue * @throws IllegalStateException if not all the elements can be added at * this time due to insertion restrictions * @see #add(Object) */ public boolean addAll(Collection c) { if (c == null) throw new NullPointerException("c == null"); if (c == this) throw new IllegalArgumentException("c == this"); boolean modified = false; for (E e : c) if (add(e)) modified = true; return modified; } }





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