/*

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  */

 /*

  * This file is available under and governed by the GNU General Public

  * License version 2 only, as published by the Free Software Foundation.

  * However, the following notice accompanied the original version of this

  * file:

  *

  * 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.concurrent;

 import java.util.AbstractQueue;

 import java.util.Collection;

 import java.util.Iterator;

 import java.util.NoSuchElementException;

 import java.util.concurrent.locks.Condition;

 import java.util.concurrent.locks.ReentrantLock;

 /**

  * An optionally-bounded {@linkplain BlockingDeque blocking deque} based on

  * linked nodes.

  *

  * <p> The optional capacity bound constructor argument serves as a

  * way to prevent excessive expansion. The capacity, if unspecified,

  * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are

  * dynamically created upon each insertion unless this would bring the

  * deque above capacity.

  *

  * <p>Most operations run in constant time (ignoring time spent

  * blocking).  Exceptions include {@link #remove(Object) remove},

  * {@link #removeFirstOccurrence removeFirstOccurrence}, {@link

  * #removeLastOccurrence removeLastOccurrence}, {@link #contains

  * contains}, {@link #iterator iterator.remove()}, and the bulk

  * operations, all of which run in linear time.

  *

  * <p>This class and its iterator implement all of the

  * <em>optional</em> methods of the {@link Collection} and {@link

  * Iterator} interfaces.

  *

  * <p>This class is a member of the

  * <a href="{@docRoot}/../technotes/guides/collections/index.html">

  * Java Collections Framework</a>.

  *

  * @since 1.6

  * @author  Doug Lea

  * @param <E> the type of elements held in this collection

  */

 public class LinkedBlockingDeque<E>

     extends AbstractQueue<E>

     implements BlockingDeque<E>,  java.io.Serializable {

     /*

      * Implemented as a simple doubly-linked list protected by a

      * single lock and using conditions to manage blocking.

      *

      * To implement weakly consistent iterators, it appears we need to

      * keep all Nodes GC-reachable from a predecessor dequeued Node.

      * That would cause two problems:

      * - allow a rogue Iterator to cause unbounded memory retention

      * - cause cross-generational linking of old Nodes to new Nodes if

      *   a Node was tenured while live, which generational GCs have a

      *   hard time dealing with, causing repeated major collections.

      * However, only non-deleted Nodes need to be reachable from

      * dequeued Nodes, and reachability does not necessarily have to

      * be of the kind understood by the GC.  We use the trick of

      * linking a Node that has just been dequeued to itself.  Such a

      * self-link implicitly means to jump to "first" (for next links)

      * or "last" (for prev links).

      */

     /*

      * We have "diamond" multiple interface/abstract class inheritance

      * here, and that introduces ambiguities. Often we want the

      * BlockingDeque javadoc combined with the AbstractQueue

      * implementation, so a lot of method specs are duplicated here.

      */

     private static final long serialVersionUID = -387911632671998426L;

     /** Doubly-linked list node class */

     static final class Node<E> {

         /**

          * The item, or null if this node has been removed.

          */

         E item;

         /**

          * One of:

          * - the real predecessor Node

          * - this Node, meaning the predecessor is tail

          * - null, meaning there is no predecessor

          */

         Node<E> prev;

         /**

          * One of:

          * - the real successor Node

          * - this Node, meaning the successor is head

          * - null, meaning there is no successor

          */

         Node<E> next;

         Node(E x) {

             item = x;

         }

     }

     /**

      * Pointer to first node.

      * Invariant: (first == null && last == null) ||

      *            (first.prev == null && first.item != null)

      */

     transient Node<E> first;

     /**

      * Pointer to last node.

      * Invariant: (first == null && last == null) ||

      *            (last.next == null && last.item != null)

      */

     transient Node<E> last;

     /** Number of items in the deque */

     private transient int count;

     /** Maximum number of items in the deque */

     private final int capacity;

     /** Main lock guarding all access */

     final ReentrantLock lock = new ReentrantLock();

     /** Condition for waiting takes */

     private final Condition notEmpty = lock.newCondition();

     /** Condition for waiting puts */

     private final Condition notFull = lock.newCondition();

     /**

      * Creates a {@code LinkedBlockingDeque} with a capacity of

      * {@link Integer#MAX_VALUE}.

      */

     public LinkedBlockingDeque() {

         this(Integer.MAX_VALUE);

     }

     /**

      * Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.

      *

      * @param capacity the capacity of this deque

      * @throws IllegalArgumentException if {@code capacity} is less than 1

      */

     public LinkedBlockingDeque(int capacity) {

         if (capacity <= 0) throw new IllegalArgumentException();

         this.capacity = capacity;

     }

     /**

      * Creates a {@code LinkedBlockingDeque} with a capacity of

      * {@link Integer#MAX_VALUE}, initially containing the elements of

      * the given collection, added in traversal order of the

      * collection's iterator.

      *

      * @param c the collection of elements to initially contain

      * @throws NullPointerException if the specified collection or any

      *         of its elements are null

      */

     public LinkedBlockingDeque(Collection<? extends E> c) {

         this(Integer.MAX_VALUE);

         final ReentrantLock lock = this.lock;

         lock.lock(); // Never contended, but necessary for visibility

         try {

             for (E e : c) {

                 if (e == null)

                     throw new NullPointerException();

                 if (!linkLast(new Node<E>(e)))

                     throw new IllegalStateException("Deque full");

             }

         } finally {

             lock.unlock();

         }

     }

     // Basic linking and unlinking operations, called only while holding lock

     /**

      * Links node as first element, or returns false if full.

      */

     private boolean linkFirst(Node<E> node) {

         // assert lock.isHeldByCurrentThread();

         if (count >= capacity)

             return false;

         Node<E> f = first;

         node.next = f;

         first = node;

         if (last == null)

             last = node;

         else

             f.prev = node;

         ++count;

         notEmpty.signal();

         return true;

     }

     /**

      * Links node as last element, or returns false if full.

      */

     private boolean linkLast(Node<E> node) {

         // assert lock.isHeldByCurrentThread();

         if (count >= capacity)

             return false;

         Node<E> l = last;

         node.prev = l;

         last = node;

         if (first == null)

             first = node;

         else

             l.next = node;

         ++count;

         notEmpty.signal();

         return true;

     }

     /**

      * Removes and returns first element, or null if empty.

      */

     private E unlinkFirst() {

         // assert lock.isHeldByCurrentThread();

         Node<E> f = first;

         if (f == null)

             return null;

         Node<E> n = f.next;

         E item = f.item;

         f.item = null;

         f.next = f; // help GC

         first = n;

         if (n == null)

             last = null;

         else

             n.prev = null;

         --count;

         notFull.signal();

         return item;

     }

     /**

      * Removes and returns last element, or null if empty.

      */

     private E unlinkLast() {

         // assert lock.isHeldByCurrentThread();

         Node<E> l = last;

         if (l == null)

             return null;

         Node<E> p = l.prev;

         E item = l.item;

         l.item = null;

         l.prev = l; // help GC

         last = p;

         if (p == null)

             first = null;

         else

             p.next = null;

         --count;

         notFull.signal();

         return item;

     }

     /**

      * Unlinks x.

      */

     void unlink(Node<E> x) {

         // assert lock.isHeldByCurrentThread();

         Node<E> p = x.prev;

         Node<E> n = x.next;

         if (p == null) {

             unlinkFirst();

         } else if (n == null) {

             unlinkLast();

         } else {

             p.next = n;

             n.prev = p;

             x.item = null;

             // Don't mess with x's links.  They may still be in use by

             // an iterator.

             --count;

             notFull.signal();

         }

     }

     // BlockingDeque methods

     /**

      * @throws IllegalStateException {@inheritDoc}

      * @throws NullPointerException  {@inheritDoc}

      */

     public void addFirst(E e) {

         if (!offerFirst(e))

             throw new IllegalStateException("Deque full");

     }

     /**

      * @throws IllegalStateException {@inheritDoc}

      * @throws NullPointerException  {@inheritDoc}

      */

     public void addLast(E e) {

         if (!offerLast(e))

             throw new IllegalStateException("Deque full");

     }

     /**

      * @throws NullPointerException {@inheritDoc}

      */

     public boolean offerFirst(E e) {

         if (e == null) throw new NullPointerException();

         Node<E> node = new Node<E>(e);

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             return linkFirst(node);

         } finally {

             lock.unlock();

         }

     }

     /**

      * @throws NullPointerException {@inheritDoc}

      */

     public boolean offerLast(E e) {

         if (e == null) throw new NullPointerException();

         Node<E> node = new Node<E>(e);

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             return linkLast(node);

         } finally {

             lock.unlock();

         }

     }

     /**

      * @throws NullPointerException {@inheritDoc}

      * @throws InterruptedException {@inheritDoc}

      */

     public void putFirst(E e) throws InterruptedException {

         if (e == null) throw new NullPointerException();

         Node<E> node = new Node<E>(e);

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             while (!linkFirst(node))

                 notFull.await();

         } finally {

             lock.unlock();

         }

     }

     /**

      * @throws NullPointerException {@inheritDoc}

      * @throws InterruptedException {@inheritDoc}

      */

     public void putLast(E e) throws InterruptedException {

         if (e == null) throw new NullPointerException();

         Node<E> node = new Node<E>(e);

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             while (!linkLast(node))

                 notFull.await();

         } finally {

             lock.unlock();

         }

     }

     /**

      * @throws NullPointerException {@inheritDoc}

      * @throws InterruptedException {@inheritDoc}

      */

     public boolean offerFirst(E e, long timeout, TimeUnit unit)

         throws InterruptedException {

         if (e == null) throw new NullPointerException();

         Node<E> node = new Node<E>(e);

         long nanos = unit.toNanos(timeout);

         final ReentrantLock lock = this.lock;

         lock.lockInterruptibly();

         try {

             while (!linkFirst(node)) {

                 if (nanos <= 0)

                     return false;

                 nanos = notFull.awaitNanos(nanos);

             }

             return true;

         } finally {

             lock.unlock();

         }

     }

     /**

      * @throws NullPointerException {@inheritDoc}

      * @throws InterruptedException {@inheritDoc}

      */

     public boolean offerLast(E e, long timeout, TimeUnit unit)

         throws InterruptedException {

         if (e == null) throw new NullPointerException();

         Node<E> node = new Node<E>(e);

         long nanos = unit.toNanos(timeout);

         final ReentrantLock lock = this.lock;

         lock.lockInterruptibly();

         try {

             while (!linkLast(node)) {

                 if (nanos <= 0)

                     return false;

                 nanos = notFull.awaitNanos(nanos);

             }

             return true;

         } finally {

             lock.unlock();

         }

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E removeFirst() {

         E x = pollFirst();

         if (x == null) throw new NoSuchElementException();

         return x;

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E removeLast() {

         E x = pollLast();

         if (x == null) throw new NoSuchElementException();

         return x;

     }

     public E pollFirst() {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             return unlinkFirst();

         } finally {

             lock.unlock();

         }

     }

     public E pollLast() {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             return unlinkLast();

         } finally {

             lock.unlock();

         }

     }

     public E takeFirst() throws InterruptedException {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             E x;

             while ( (x = unlinkFirst()) == null)

                 notEmpty.await();

             return x;

         } finally {

             lock.unlock();

         }

     }

     public E takeLast() throws InterruptedException {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             E x;

             while ( (x = unlinkLast()) == null)

                 notEmpty.await();

             return x;

         } finally {

             lock.unlock();

         }

     }

     public E pollFirst(long timeout, TimeUnit unit)

         throws InterruptedException {

         long nanos = unit.toNanos(timeout);

         final ReentrantLock lock = this.lock;

         lock.lockInterruptibly();

         try {

             E x;

             while ( (x = unlinkFirst()) == null) {

                 if (nanos <= 0)

                     return null;

                 nanos = notEmpty.awaitNanos(nanos);

             }

             return x;

         } finally {

             lock.unlock();

         }

     }

     public E pollLast(long timeout, TimeUnit unit)

         throws InterruptedException {

         long nanos = unit.toNanos(timeout);

         final ReentrantLock lock = this.lock;

         lock.lockInterruptibly();

         try {

             E x;

             while ( (x = unlinkLast()) == null) {

                 if (nanos <= 0)

                     return null;

                 nanos = notEmpty.awaitNanos(nanos);

             }

             return x;

         } finally {

             lock.unlock();

         }

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E getFirst() {

         E x = peekFirst();

         if (x == null) throw new NoSuchElementException();

         return x;

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E getLast() {

         E x = peekLast();

         if (x == null) throw new NoSuchElementException();

         return x;

     }

     public E peekFirst() {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             return (first == null) ? null : first.item;

         } finally {

             lock.unlock();

         }

     }

     public E peekLast() {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             return (last == null) ? null : last.item;

         } finally {

             lock.unlock();

         }

     }

     public boolean removeFirstOccurrence(Object o) {

         if (o == null) return false;

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             for (Node<E> p = first; p != null; p = p.next) {

                 if (o.equals(p.item)) {

                     unlink(p);

                     return true;

                 }

             }

             return false;

         } finally {

             lock.unlock();

         }

     }

     public boolean removeLastOccurrence(Object o) {

         if (o == null) return false;

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             for (Node<E> p = last; p != null; p = p.prev) {

                 if (o.equals(p.item)) {

                     unlink(p);

                     return true;

                 }

             }

             return false;

         } finally {

             lock.unlock();

         }

     }

     // BlockingQueue methods

     /**

      * Inserts the specified element at the end of this deque unless it would

      * violate capacity restrictions.  When using a capacity-restricted deque,

      * it is generally preferable to use method {@link #offer(Object) offer}.

      *

      * <p>This method is equivalent to {@link #addLast}.

      *

      * @throws IllegalStateException if the element cannot be added at this

      *         time due to capacity restrictions

      * @throws NullPointerException if the specified element is null

      */

     public boolean add(E e) {

         addLast(e);

         return true;

     }

     /**

      * @throws NullPointerException if the specified element is null

      */

     public boolean offer(E e) {

         return offerLast(e);

     }

     /**

      * @throws NullPointerException {@inheritDoc}

      * @throws InterruptedException {@inheritDoc}

      */

     public void put(E e) throws InterruptedException {

         putLast(e);

     }

     /**

      * @throws NullPointerException {@inheritDoc}

      * @throws InterruptedException {@inheritDoc}

      */

     public boolean offer(E e, long timeout, TimeUnit unit)

         throws InterruptedException {

         return offerLast(e, timeout, unit);

     }

     /**

      * Retrieves and removes the head of the queue represented by this deque.

      * This method differs from {@link #poll poll} only in that it throws an

      * exception if this deque is empty.

      *

      * <p>This method is equivalent to {@link #removeFirst() removeFirst}.

      *

      * @return the head of the queue represented by this deque

      * @throws NoSuchElementException if this deque is empty

      */

     public E remove() {

         return removeFirst();

     }

     public E poll() {

         return pollFirst();

     }

     public E take() throws InterruptedException {

         return takeFirst();

     }

     public E poll(long timeout, TimeUnit unit) throws InterruptedException {

         return pollFirst(timeout, unit);

     }

     /**

      * Retrieves, but does not remove, the head of the queue represented by

      * this deque.  This method differs from {@link #peek peek} only in that

      * it throws an exception if this deque is empty.

      *

      * <p>This method is equivalent to {@link #getFirst() getFirst}.

      *

      * @return the head of the queue represented by this deque

      * @throws NoSuchElementException if this deque is empty

      */

     public E element() {

         return getFirst();

     }

     public E peek() {

         return peekFirst();

     }

     /**

      * Returns the number of additional elements that this deque can ideally

      * (in the absence of memory or resource constraints) accept without

      * blocking. This is always equal to the initial capacity of this deque

      * less the current {@code size} of this deque.

      *

      * <p>Note that you <em>cannot</em> always tell if an attempt to insert

      * an element will succeed by inspecting {@code remainingCapacity}

      * because it may be the case that another thread is about to

      * insert or remove an element.

      */

     public int remainingCapacity() {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             return capacity - count;

         } finally {

             lock.unlock();

         }

     }

     /**

      * @throws UnsupportedOperationException {@inheritDoc}

      * @throws ClassCastException            {@inheritDoc}

      * @throws NullPointerException          {@inheritDoc}

      * @throws IllegalArgumentException      {@inheritDoc}

      */

     public int drainTo(Collection<? super E> c) {

         return drainTo(c, Integer.MAX_VALUE);

     }

     /**

      * @throws UnsupportedOperationException {@inheritDoc}

      * @throws ClassCastException            {@inheritDoc}

      * @throws NullPointerException          {@inheritDoc}

      * @throws IllegalArgumentException      {@inheritDoc}

      */

     public int drainTo(Collection<? super E> c, int maxElements) {

         if (c == null)

             throw new NullPointerException();

         if (c == this)

             throw new IllegalArgumentException();

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             int n = Math.min(maxElements, count);

             for (int i = 0; i < n; i++) {

                 c.add(first.item);   // In this order, in case add() throws.

                 unlinkFirst();

             }

             return n;

         } finally {

             lock.unlock();

         }

     }

     // Stack methods

     /**

      * @throws IllegalStateException {@inheritDoc}

      * @throws NullPointerException  {@inheritDoc}

      */

     public void push(E e) {

         addFirst(e);

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E pop() {

         return removeFirst();

     }

     // Collection methods

     /**

      * Removes the first occurrence of the specified element from this deque.

      * If the deque does not contain the element, it is unchanged.

      * More formally, removes the first element {@code e} such that

      * {@code o.equals(e)} (if such an element exists).

      * Returns {@code true} if this deque contained the specified element

      * (or equivalently, if this deque changed as a result of the call).

      *

      * <p>This method is equivalent to

      * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.

      *

      * @param o element to be removed from this deque, if present

      * @return {@code true} if this deque changed as a result of the call

      */

     public boolean remove(Object o) {

         return removeFirstOccurrence(o);

     }

     /**

      * Returns the number of elements in this deque.

      *

      * @return the number of elements in this deque

      */

     public int size() {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             return count;

         } finally {

             lock.unlock();

         }

     }

     /**

      * Returns {@code true} if this deque contains the specified element.

      * More formally, returns {@code true} if and only if this deque contains

      * at least one element {@code e} such that {@code o.equals(e)}.

      *

      * @param o object to be checked for containment in this deque

      * @return {@code true} if this deque contains the specified element

      */

     public boolean contains(Object o) {

         if (o == null) return false;

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             for (Node<E> p = first; p != null; p = p.next)

                 if (o.equals(p.item))

                     return true;

             return false;

         } finally {

             lock.unlock();

         }

     }

     /*

      * TODO: Add support for more efficient bulk operations.

      *

      * We don't want to acquire the lock for every iteration, but we

      * also want other threads a chance to interact with the

      * collection, especially when count is close to capacity.

      */

 //     /**

 //      * Adds all of the elements in the specified collection to this

 //      * queue.  Attempts to addAll of a queue to itself result in

 //      * {@code IllegalArgumentException}. Further, the behavior of

 //      * this operation is undefined if the specified collection is

 //      * modified while the operation is in progress.

 //      *

 //      * @param c collection containing elements to be added to this queue

 //      * @return {@code true} if this queue changed as a result of the call

 //      * @throws ClassCastException            {@inheritDoc}

 //      * @throws NullPointerException          {@inheritDoc}

 //      * @throws IllegalArgumentException      {@inheritDoc}

 //      * @throws IllegalStateException         {@inheritDoc}

 //      * @see #add(Object)

 //      */

 //     public boolean addAll(Collection<? extends E> c) {

 //         if (c == null)

 //             throw new NullPointerException();

 //         if (c == this)

 //             throw new IllegalArgumentException();

 //         final ReentrantLock lock = this.lock;

 //         lock.lock();

 //         try {

 //             boolean modified = false;

 //             for (E e : c)

 //                 if (linkLast(e))

 //                     modified = true;

 //             return modified;

 //         } finally {

 //             lock.unlock();

 //         }

 //     }

     /**

      * Returns an array containing all of the elements in this deque, in

      * proper sequence (from first to last element).

      *

      * <p>The returned array will be "safe" in that no references to it are

      * maintained by this deque.  (In other words, this method must allocate

      * a new array).  The caller is thus free to modify the returned array.

      *

      * <p>This method acts as bridge between array-based and collection-based

      * APIs.

      *

      * @return an array containing all of the elements in this deque

      */

     @SuppressWarnings("unchecked")

     public Object[] toArray() {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             Object[] a = new Object[count];

             int k = 0;

             for (Node<E> p = first; p != null; p = p.next)

                 a[k++] = p.item;

             return a;

         } finally {

             lock.unlock();

         }

     }

     /**

      * Returns an array containing all of the elements in this deque, in

      * proper sequence; the runtime type of the returned array is that of

      * the specified array.  If the deque fits in the specified array, it

      * is returned therein.  Otherwise, a new array is allocated with the

      * runtime type of the specified array and the size of this deque.

      *

      * <p>If this deque fits in the specified array with room to spare

      * (i.e., the array has more elements than this deque), the element in

      * the array immediately following the end of the deque is set to

      * {@code null}.

      *

      * <p>Like the {@link #toArray()} method, this method acts as bridge between

      * array-based and collection-based APIs.  Further, this method allows

      * precise control over the runtime type of the output array, and may,

      * under certain circumstances, be used to save allocation costs.

      *

      * <p>Suppose {@code x} is a deque known to contain only strings.

      * The following code can be used to dump the deque into a newly

      * allocated array of {@code String}:

      *

      * <pre>

      *     String[] y = x.toArray(new String[0]);</pre>

      *

      * Note that {@code toArray(new Object[0])} is identical in function to

      * {@code toArray()}.

      *

      * @param a the array into which the elements of the deque are to

      *          be stored, if it is big enough; otherwise, a new array of the

      *          same runtime type is allocated for this purpose

      * @return an array containing all of the elements in this deque

      * @throws ArrayStoreException if the runtime type of the specified array

      *         is not a supertype of the runtime type of every element in

      *         this deque

      * @throws NullPointerException if the specified array is null

      */

     @SuppressWarnings("unchecked")

     public <T> T[] toArray(T[] a) {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             if (a.length < count)

                 a = (T[])java.lang.reflect.Array.newInstance

                     (a.getClass().getComponentType(), count);

             int k = 0;

             for (Node<E> p = first; p != null; p = p.next)

                 a[k++] = (T)p.item;

             if (a.length > k)

                 a[k] = null;

             return a;

         } finally {

             lock.unlock();

         }

     }

     public String toString() {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             Node<E> p = first;

             if (p == null)

                 return "[]";

             StringBuilder sb = new StringBuilder();

             sb.append('[');

             for (;;) {

                 E e = p.item;

                 sb.append(e == this ? "(this Collection)" : e);

                 p = p.next;

                 if (p == null)

                     return sb.append(']').toString();

                 sb.append(',').append(' ');

             }

         } finally {

             lock.unlock();

         }

     }

     /**

      * Atomically removes all of the elements from this deque.

      * The deque will be empty after this call returns.

      */

     public void clear() {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             for (Node<E> f = first; f != null; ) {

                 f.item = null;

                 Node<E> n = f.next;

                 f.prev = null;

                 f.next = null;

                 f = n;

             }

             first = last = null;

             count = 0;

             notFull.signalAll();

         } finally {

             lock.unlock();

         }

     }

     /**

      * Returns an iterator over the elements in this deque in proper sequence.

      * The elements will be returned in order from first (head) to last (tail).

      *

      * <p>The returned iterator is a "weakly consistent" iterator that

      * will never throw {@link java.util.ConcurrentModificationException

      * ConcurrentModificationException}, and guarantees to traverse

      * elements as they existed upon construction of the iterator, and

      * may (but is not guaranteed to) reflect any modifications

      * subsequent to construction.

      *

      * @return an iterator over the elements in this deque in proper sequence

      */

     public Iterator<E> iterator() {

         return new Itr();

     }

     /**

      * Returns an iterator over the elements in this deque in reverse

      * sequential order.  The elements will be returned in order from

      * last (tail) to first (head).

      *

      * <p>The returned iterator is a "weakly consistent" iterator that

      * will never throw {@link java.util.ConcurrentModificationException

      * ConcurrentModificationException}, and guarantees to traverse

      * elements as they existed upon construction of the iterator, and

      * may (but is not guaranteed to) reflect any modifications

      * subsequent to construction.

      *

      * @return an iterator over the elements in this deque in reverse order

      */

     public Iterator<E> descendingIterator() {

         return new DescendingItr();

     }

     /**

      * Base class for Iterators for LinkedBlockingDeque

      */

     private abstract class AbstractItr implements Iterator<E> {

         /**

          * The next node to return in next()

          */

          Node<E> next;

         /**

          * nextItem holds on to item fields because once we claim that

          * an element exists in hasNext(), we must return item read

          * under lock (in advance()) even if it was in the process of

          * being removed when hasNext() was called.

          */

         E nextItem;

         /**

          * Node returned by most recent call to next. Needed by remove.

          * Reset to null if this element is deleted by a call to remove.

          */

         private Node<E> lastRet;

         abstract Node<E> firstNode();

         abstract Node<E> nextNode(Node<E> n);

         AbstractItr() {

             // set to initial position

             final ReentrantLock lock = LinkedBlockingDeque.this.lock;

             lock.lock();

             try {

                 next = firstNode();

                 nextItem = (next == null) ? null : next.item;

             } finally {

                 lock.unlock();

             }

         }

         /**

          * Returns the successor node of the given non-null, but

          * possibly previously deleted, node.

          */

         private Node<E> succ(Node<E> n) {

             // Chains of deleted nodes ending in null or self-links

             // are possible if multiple interior nodes are removed.

             for (;;) {

                 Node<E> s = nextNode(n);

                 if (s == null)

                     return null;

                 else if (s.item != null)

                     return s;

                 else if (s == n)

                     return firstNode();

                 else

                     n = s;

             }

         }

         /**

          * Advances next.

          */

         void advance() {

             final ReentrantLock lock = LinkedBlockingDeque.this.lock;

             lock.lock();

             try {

                 // assert next != null;

                 next = succ(next);

                 nextItem = (next == null) ? null : next.item;

             } finally {

                 lock.unlock();

             }

         }

         public boolean hasNext() {

             return next != null;

         }

         public E next() {

             if (next == null)

                 throw new NoSuchElementException();

             lastRet = next;

             E x = nextItem;

             advance();

             return x;

         }

         public void remove() {

             Node<E> n = lastRet;

             if (n == null)

                 throw new IllegalStateException();

             lastRet = null;

             final ReentrantLock lock = LinkedBlockingDeque.this.lock;

             lock.lock();

             try {

                 if (n.item != null)

                     unlink(n);

             } finally {

                 lock.unlock();

             }

         }

     }

     /** Forward iterator */

     private class Itr extends AbstractItr {

         Node<E> firstNode() { return first; }

         Node<E> nextNode(Node<E> n) { return n.next; }

     }

     /** Descending iterator */

     private class DescendingItr extends AbstractItr {

         Node<E> firstNode() { return last; }

         Node<E> nextNode(Node<E> n) { return n.prev; }

     }

     /**

      * Save the state of this deque to a stream (that is, serialize it).

      *

      * @serialData The capacity (int), followed by elements (each an

      * {@code Object}) in the proper order, followed by a null

      * @param s the stream

      */

     private void writeObject(java.io.ObjectOutputStream s)

         throws java.io.IOException {

         final ReentrantLock lock = this.lock;

         lock.lock();

         try {

             // Write out capacity and any hidden stuff

             s.defaultWriteObject();

             // Write out all elements in the proper order.

             for (Node<E> p = first; p != null; p = p.next)

                 s.writeObject(p.item);

             // Use trailing null as sentinel

             s.writeObject(null);

         } finally {

             lock.unlock();

         }

     }

     /**

      * Reconstitute this deque from a stream (that is,

      * deserialize it).

      * @param s the stream

      */

     private void readObject(java.io.ObjectInputStream s)

         throws java.io.IOException, ClassNotFoundException {

         s.defaultReadObject();

         count = 0;

         first = null;

         last = null;

         // Read in all elements and place in queue

         for (;;) {

             @SuppressWarnings("unchecked")

             E item = (E)s.readObject();

             if (item == null)

                 break;

             add(item);

         }

     }

 }