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  /*
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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   *
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   * This code is free software; you can redistribute it and/or modify it
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   * under the terms of the GNU General Public License version 2 only, as
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   * published by the Free Software Foundation.  Sun designates this
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   * particular file as subject to the "Classpath" exception as provided
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   * by Sun in the LICENSE file that accompanied this code.
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   *
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  * This code is distributed in the hope that it will be useful, but WITHOUT
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  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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  * version 2 for more details (a copy is included in the LICENSE file that
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  * accompanied this code).
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  *
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  * You should have received a copy of the GNU General Public License version
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  * 2 along with this work; if not, write to the Free Software Foundation,
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  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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  *
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  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
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  * CA 95054 USA or visit www.sun.com if you need additional information or
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  * have any questions.
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  */
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 /*
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  * This file is available under and governed by the GNU General Public
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  * License version 2 only, as published by the Free Software Foundation.
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  * However, the following notice accompanied the original version of this
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  * file:
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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, as explained at
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  * http://creativecommons.org/licenses/publicdomain
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  */
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 package java.util;

This class provides skeletal implementations of some Queue operations. The implementations in this class are appropriate when the base implementation does not allow null elements. Methods add, remove, and element are based on offer, Queue.poll(), and 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 Queue.offer(java.lang.Object) which does not permit insertion of null elements, along with methods Queue.peek(), Queue.poll(), Collection.size(), and a Collection.iterator() supporting Iterator.remove(). Typically, additional methods will be overridden as well. If these requirements cannot be met, consider instead subclassing AbstractCollection.

This class is a member of the Java Collections Framework.

Parameters:
<E> the type of elements held in this collection
Author(s):
Doug Lea
Since:
1.5
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 public abstract class AbstractQueue<E>
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     extends AbstractCollection<E>
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     implements Queue<E> {

    
Constructor for use by subclasses. 
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     protected AbstractQueue() {
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     }

    
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.

Parameters:
e the element to add
Returns:
true (as specified by Collection.add(java.lang.Object))
Throws:
java.lang.IllegalStateException if the element cannot be added at this time due to capacity restrictions
java.lang.ClassCastException if the class of the specified element prevents it from being added to this queue
java.lang.NullPointerException if the specified element is null and this queue does not permit null elements
java.lang.IllegalArgumentException if some property of this element prevents it from being added to this queue
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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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    }

    
Retrieves and removes the head of this queue. This method differs from 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.

Returns:
the head of this queue
Throws:
NoSuchElementException if this queue is empty
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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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    }

    
Retrieves, but does not remove, the head of this queue. This method differs from 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.

Returns:
the head of this queue
Throws:
NoSuchElementException if this queue is empty
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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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    }

    
Removes all of the elements from this queue. The queue will be empty after this call returns.

This implementation repeatedly invokes poll until it returns null. 

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    public void clear() {
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        while (poll() != null)
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            ;
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    }

    
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.

Parameters:
c collection containing elements to be added to this queue
Returns:
true if this queue changed as a result of the call
Throws:
java.lang.ClassCastException if the class of an element of the specified collection prevents it from being added to this queue
java.lang.NullPointerException if the specified collection contains a null element and this queue does not permit null elements, or if the specified collection is null
java.lang.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
java.lang.IllegalStateException if not all the elements can be added at this time due to insertion restrictions
See also:
add(java.lang.Object)
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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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