content: /*
   * 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/licenses/publicdomain
   */
  
  package com.nostra13.universalimageloader.core.assist.deque;
  
  import java.util.AbstractQueue;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.concurrent.TimeUnit;
  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 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
       */
      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);
          }
      }
  
  }
  