/*

 * 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;