/*

  * 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 and Martin Buchholz 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.ArrayList;

 import java.util.Collection;

 import java.util.Iterator;

 import java.util.NoSuchElementException;

 import java.util.Queue;

 /**

  * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.

  * This queue orders elements FIFO (first-in-first-out).

  * The <em>head</em> of the queue is that element that has been on the

  * queue the longest time.

  * The <em>tail</em> of the queue is that element that has been on the

  * queue the shortest time. New elements

  * are inserted at the tail of the queue, and the queue retrieval

  * operations obtain elements at the head of the queue.

  * A {@code ConcurrentLinkedQueue} is an appropriate choice when

  * many threads will share access to a common collection.

  * Like most other concurrent collection implementations, this class

  * does not permit the use of {@code null} elements.

  *

  * <p>This implementation employs an efficient &quot;wait-free&quot;

  * algorithm based on one described in <a

  * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,

  * Fast, and Practical Non-Blocking and Blocking Concurrent Queue

  * Algorithms</a> by Maged M. Michael and Michael L. Scott.

  *

  * <p>Iterators are <i>weakly consistent</i>, returning elements

  * reflecting the state of the queue at some point at or since the

  * creation of the iterator.  They do <em>not</em> throw {@link

  * java.util.ConcurrentModificationException}, and may proceed concurrently

  * with other operations.  Elements contained in the queue since the creation

  * of the iterator will be returned exactly once.

  *

  * <p>Beware that, unlike in most collections, the {@code size} method

  * is <em>NOT</em> a constant-time operation. Because of the

  * asynchronous nature of these queues, determining the current number

  * of elements requires a traversal of the elements, and so may report

  * inaccurate results if this collection is modified during traversal.

  * Additionally, the bulk operations {@code addAll},

  * {@code removeAll}, {@code retainAll}, {@code containsAll},

  * {@code equals}, and {@code toArray} are <em>not</em> guaranteed

  * to be performed atomically. For example, an iterator operating

  * concurrently with an {@code addAll} operation might view only some

  * of the added elements.

  *

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

  * methods of the {@link Queue} and {@link Iterator} interfaces.

  *

  * <p>Memory consistency effects: As with other concurrent

  * collections, actions in a thread prior to placing an object into a

  * {@code ConcurrentLinkedQueue}

  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>

  * actions subsequent to the access or removal of that element from

  * the {@code ConcurrentLinkedQueue} in another thread.

  *

  * <p>This class is a member of the

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

  * Java Collections Framework</a>.

  *

  * @since 1.5

  * @author Doug Lea

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

  *

  */

 public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>

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

     private static final long serialVersionUID = 196745693267521676L;

     /*

      * This is a modification of the Michael & Scott algorithm,

      * adapted for a garbage-collected environment, with support for

      * interior node deletion (to support remove(Object)).  For

      * explanation, read the paper.

      *

      * Note that like most non-blocking algorithms in this package,

      * this implementation relies on the fact that in garbage

      * collected systems, there is no possibility of ABA problems due

      * to recycled nodes, so there is no need to use "counted

      * pointers" or related techniques seen in versions used in

      * non-GC'ed settings.

      *

      * The fundamental invariants are:

      * - There is exactly one (last) Node with a null next reference,

      *   which is CASed when enqueueing.  This last Node can be

      *   reached in O(1) time from tail, but tail is merely an

      *   optimization - it can always be reached in O(N) time from

      *   head as well.

      * - The elements contained in the queue are the non-null items in

      *   Nodes that are reachable from head.  CASing the item

      *   reference of a Node to null atomically removes it from the

      *   queue.  Reachability of all elements from head must remain

      *   true even in the case of concurrent modifications that cause

      *   head to advance.  A dequeued Node may remain in use

      *   indefinitely due to creation of an Iterator or simply a

      *   poll() that has lost its time slice.

      *

      * The above might appear to imply that all Nodes are 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 advance to head.

      *

      * Both head and tail are permitted to lag.  In fact, failing to

      * update them every time one could is a significant optimization

      * (fewer CASes). As with LinkedTransferQueue (see the internal

      * documentation for that class), we use a slack threshold of two;

      * that is, we update head/tail when the current pointer appears

      * to be two or more steps away from the first/last node.

      *

      * Since head and tail are updated concurrently and independently,

      * it is possible for tail to lag behind head (why not)?

      *

      * CASing a Node's item reference to null atomically removes the

      * element from the queue.  Iterators skip over Nodes with null

      * items.  Prior implementations of this class had a race between

      * poll() and remove(Object) where the same element would appear

      * to be successfully removed by two concurrent operations.  The

      * method remove(Object) also lazily unlinks deleted Nodes, but

      * this is merely an optimization.

      *

      * When constructing a Node (before enqueuing it) we avoid paying

      * for a volatile write to item by using Unsafe.putObject instead

      * of a normal write.  This allows the cost of enqueue to be

      * "one-and-a-half" CASes.

      *

      * Both head and tail may or may not point to a Node with a

      * non-null item.  If the queue is empty, all items must of course

      * be null.  Upon creation, both head and tail refer to a dummy

      * Node with null item.  Both head and tail are only updated using

      * CAS, so they never regress, although again this is merely an

      * optimization.

      */

     private static class Node<E> {

         volatile E item;

         volatile Node<E> next;

         /**

          * Constructs a new node.  Uses relaxed write because item can

          * only be seen after publication via casNext.

          */

         Node(E item) {

             UNSAFE.putObject(this, itemOffset, item);

         }

         boolean casItem(E cmp, E val) {

             return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);

         }

         void lazySetNext(Node<E> val) {

             UNSAFE.putOrderedObject(this, nextOffset, val);

         }

         boolean casNext(Node<E> cmp, Node<E> val) {

             return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);

         }

         // Unsafe mechanics

         private static final sun.misc.Unsafe UNSAFE;

         private static final long itemOffset;

         private static final long nextOffset;

         static {

             try {

                 UNSAFE = sun.misc.Unsafe.getUnsafe();

                 Class k = Node.class;

                 itemOffset = UNSAFE.objectFieldOffset

                     (k.getDeclaredField("item"));

                 nextOffset = UNSAFE.objectFieldOffset

                     (k.getDeclaredField("next"));

             } catch (Exception e) {

                 throw new Error(e);

             }

         }

     }

     /**

      * A node from which the first live (non-deleted) node (if any)

      * can be reached in O(1) time.

      * Invariants:

      * - all live nodes are reachable from head via succ()

      * - head != null

      * - (tmp = head).next != tmp || tmp != head

      * Non-invariants:

      * - head.item may or may not be null.

      * - it is permitted for tail to lag behind head, that is, for tail

      *   to not be reachable from head!

      */

     private transient volatile Node<E> head;

     /**

      * A node from which the last node on list (that is, the unique

      * node with node.next == null) can be reached in O(1) time.

      * Invariants:

      * - the last node is always reachable from tail via succ()

      * - tail != null

      * Non-invariants:

      * - tail.item may or may not be null.

      * - it is permitted for tail to lag behind head, that is, for tail

      *   to not be reachable from head!

      * - tail.next may or may not be self-pointing to tail.

      */

     private transient volatile Node<E> tail;

     /**

      * Creates a {@code ConcurrentLinkedQueue} that is initially empty.

      */

     public ConcurrentLinkedQueue() {

         head = tail = new Node<E>(null);

     }

     /**

      * Creates a {@code ConcurrentLinkedQueue}

      * 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 ConcurrentLinkedQueue(Collection<? extends E> c) {

         Node<E> h = null, t = null;

         for (E e : c) {

             checkNotNull(e);

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

             if (h == null)

                 h = t = newNode;

             else {

                 t.lazySetNext(newNode);

                 t = newNode;

             }

         }

         if (h == null)

             h = t = new Node<E>(null);

         head = h;

         tail = t;

     }

     // Have to override just to update the javadoc

     /**

      * Inserts the specified element at the tail of this queue.

      * As the queue is unbounded, this method will never throw

      * {@link IllegalStateException} or return {@code false}.

      *

      * @return {@code true} (as specified by {@link Collection#add})

      * @throws NullPointerException if the specified element is null

      */

     public boolean add(E e) {

         return offer(e);

     }

     /**

      * Try to CAS head to p. If successful, repoint old head to itself

      * as sentinel for succ(), below.

      */

     final void updateHead(Node<E> h, Node<E> p) {

         if (h != p && casHead(h, p))

             h.lazySetNext(h);

     }

     /**

      * Returns the successor of p, or the head node if p.next has been

      * linked to self, which will only be true if traversing with a

      * stale pointer that is now off the list.

      */

     final Node<E> succ(Node<E> p) {

         Node<E> next = p.next;

         return (p == next) ? head : next;

     }

     /**

      * Inserts the specified element at the tail of this queue.

      * As the queue is unbounded, this method will never return {@code false}.

      *

      * @return {@code true} (as specified by {@link Queue#offer})

      * @throws NullPointerException if the specified element is null

      */

     public boolean offer(E e) {

         checkNotNull(e);

         final Node<E> newNode = new Node<E>(e);

         for (Node<E> t = tail, p = t;;) {

             Node<E> q = p.next;

             if (q == null) {

                 // p is last node

                 if (p.casNext(null, newNode)) {

                     // Successful CAS is the linearization point

                     // for e to become an element of this queue,

                     // and for newNode to become "live".

                     if (p != t) // hop two nodes at a time

                         casTail(t, newNode);  // Failure is OK.

                     return true;

                 }

                 // Lost CAS race to another thread; re-read next

             }

             else if (p == q)

                 // We have fallen off list.  If tail is unchanged, it

                 // will also be off-list, in which case we need to

                 // jump to head, from which all live nodes are always

                 // reachable.  Else the new tail is a better bet.

                 p = (t != (t = tail)) ? t : head;

             else

                 // Check for tail updates after two hops.

                 p = (p != t && t != (t = tail)) ? t : q;

         }

     }

     public E poll() {

         restartFromHead:

         for (;;) {

             for (Node<E> h = head, p = h, q;;) {

                 E item = p.item;

                 if (item != null && p.casItem(item, null)) {

                     // Successful CAS is the linearization point

                     // for item to be removed from this queue.

                     if (p != h) // hop two nodes at a time

                         updateHead(h, ((q = p.next) != null) ? q : p);

                     return item;

                 }

                 else if ((q = p.next) == null) {

                     updateHead(h, p);

                     return null;

                 }

                 else if (p == q)

                     continue restartFromHead;

                 else

                     p = q;

             }

         }

     }

     public E peek() {

         restartFromHead:

         for (;;) {

             for (Node<E> h = head, p = h, q;;) {

                 E item = p.item;

                 if (item != null || (q = p.next) == null) {

                     updateHead(h, p);

                     return item;

                 }

                 else if (p == q)

                     continue restartFromHead;

                 else

                     p = q;

             }

         }

     }

     /**

      * Returns the first live (non-deleted) node on list, or null if none.

      * This is yet another variant of poll/peek; here returning the

      * first node, not element.  We could make peek() a wrapper around

      * first(), but that would cost an extra volatile read of item,

      * and the need to add a retry loop to deal with the possibility

      * of losing a race to a concurrent poll().

      */

     Node<E> first() {

         restartFromHead:

         for (;;) {

             for (Node<E> h = head, p = h, q;;) {

                 boolean hasItem = (p.item != null);

                 if (hasItem || (q = p.next) == null) {

                     updateHead(h, p);

                     return hasItem ? p : null;

                 }

                 else if (p == q)

                     continue restartFromHead;

                 else

                     p = q;

             }

         }

     }

     /**

      * Returns {@code true} if this queue contains no elements.

      *

      * @return {@code true} if this queue contains no elements

      */

     public boolean isEmpty() {

         return first() == null;

     }

     /**

      * Returns the number of elements in this queue.  If this queue

      * contains more than {@code Integer.MAX_VALUE} elements, returns

      * {@code Integer.MAX_VALUE}.

      *

      * <p>Beware that, unlike in most collections, this method is

      * <em>NOT</em> a constant-time operation. Because of the

      * asynchronous nature of these queues, determining the current

      * number of elements requires an O(n) traversal.

      * Additionally, if elements are added or removed during execution

      * of this method, the returned result may be inaccurate.  Thus,

      * this method is typically not very useful in concurrent

      * applications.

      *

      * @return the number of elements in this queue

      */

     public int size() {

         int count = 0;

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

             if (p.item != null)

                 // Collection.size() spec says to max out

                 if (++count == Integer.MAX_VALUE)

                     break;

         return count;

     }

     /**

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

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

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

      *

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

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

      */

     public boolean contains(Object o) {

         if (o == null) return false;

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

             E item = p.item;

             if (item != null && o.equals(item))

                 return true;

         }

         return false;

     }

     /**

      * Removes a single instance of the specified element from this queue,

      * if it is present.  More formally, removes an element {@code e} such

      * that {@code o.equals(e)}, if this queue contains one or more such

      * elements.

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

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

      *

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

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

      */

     public boolean remove(Object o) {

         if (o == null) return false;

         Node<E> pred = null;

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

             E item = p.item;

             if (item != null &&

                 o.equals(item) &&

                 p.casItem(item, null)) {

                 Node<E> next = succ(p);

                 if (pred != null && next != null)

                     pred.casNext(p, next);

                 return true;

             }

             pred = p;

         }

         return false;

     }

     /**

      * Appends all of the elements in the specified collection to the end of

      * this queue, in the order that they are returned by the specified

      * collection's iterator.  Attempts to {@code addAll} of a queue to

      * itself result in {@code IllegalArgumentException}.

      *

      * @param c the elements to be inserted into this queue

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

      * @throws NullPointerException if the specified collection or any

      *         of its elements are null

      * @throws IllegalArgumentException if the collection is this queue

      */

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

         if (c == this)

             // As historically specified in AbstractQueue#addAll

             throw new IllegalArgumentException();

         // Copy c into a private chain of Nodes

         Node<E> beginningOfTheEnd = null, last = null;

         for (E e : c) {

             checkNotNull(e);

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

             if (beginningOfTheEnd == null)

                 beginningOfTheEnd = last = newNode;

             else {

                 last.lazySetNext(newNode);

                 last = newNode;

             }

         }

         if (beginningOfTheEnd == null)

             return false;

         // Atomically append the chain at the tail of this collection

         for (Node<E> t = tail, p = t;;) {

             Node<E> q = p.next;

             if (q == null) {

                 // p is last node

                 if (p.casNext(null, beginningOfTheEnd)) {

                     // Successful CAS is the linearization point

                     // for all elements to be added to this queue.

                     if (!casTail(t, last)) {

                         // Try a little harder to update tail,

                         // since we may be adding many elements.

                         t = tail;

                         if (last.next == null)

                             casTail(t, last);

                     }

                     return true;

                 }

                 // Lost CAS race to another thread; re-read next

             }

             else if (p == q)

                 // We have fallen off list.  If tail is unchanged, it

                 // will also be off-list, in which case we need to

                 // jump to head, from which all live nodes are always

                 // reachable.  Else the new tail is a better bet.

                 p = (t != (t = tail)) ? t : head;

             else

                 // Check for tail updates after two hops.

                 p = (p != t && t != (t = tail)) ? t : q;

         }

     }

     /**

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

      * proper sequence.

      *

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

      * maintained by this queue.  (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 queue

      */

     public Object[] toArray() {

         // Use ArrayList to deal with resizing.

         ArrayList<E> al = new ArrayList<E>();

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

             E item = p.item;

             if (item != null)

                 al.add(item);

         }

         return al.toArray();

     }

     /**

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

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

      * the specified array.  If the queue 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 queue.

      *

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

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

      * the array immediately following the end of the queue 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 queue known to contain only strings.

      * The following code can be used to dump the queue 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 queue 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 queue

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

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

      *         this queue

      * @throws NullPointerException if the specified array is null

      */

     @SuppressWarnings("unchecked")

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

         // try to use sent-in array

         int k = 0;

         Node<E> p;

         for (p = first(); p != null && k < a.length; p = succ(p)) {

             E item = p.item;

             if (item != null)

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

         }

         if (p == null) {

             if (k < a.length)

                 a[k] = null;

             return a;

         }

         // If won't fit, use ArrayList version

         ArrayList<E> al = new ArrayList<E>();

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

             E item = q.item;

             if (item != null)

                 al.add(item);

         }

         return al.toArray(a);

     }

     /**

      * Returns an iterator over the elements in this queue 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 queue in proper sequence

      */

     public Iterator<E> iterator() {

         return new Itr();

     }

     private class Itr implements Iterator<E> {

         /**

          * Next node to return item for.

          */

         private Node<E> nextNode;

         /**

          * nextItem holds on to item fields because once we claim

          * that an element exists in hasNext(), we must return it in

          * the following next() call even if it was in the process of

          * being removed when hasNext() was called.

          */

         private E nextItem;

         /**

          * Node of the last returned item, to support remove.

          */

         private Node<E> lastRet;

         Itr() {

             advance();

         }

         /**

          * Moves to next valid node and returns item to return for

          * next(), or null if no such.

          */

         private E advance() {

             lastRet = nextNode;

             E x = nextItem;

             Node<E> pred, p;

             if (nextNode == null) {

                 p = first();

                 pred = null;

             } else {

                 pred = nextNode;

                 p = succ(nextNode);

             }

             for (;;) {

                 if (p == null) {

                     nextNode = null;

                     nextItem = null;

                     return x;

                 }

                 E item = p.item;

                 if (item != null) {

                     nextNode = p;

                     nextItem = item;

                     return x;

                 } else {

                     // skip over nulls

                     Node<E> next = succ(p);

                     if (pred != null && next != null)

                         pred.casNext(p, next);

                     p = next;

                 }

             }

         }

         public boolean hasNext() {

             return nextNode != null;

         }

         public E next() {

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

             return advance();

         }

         public void remove() {

             Node<E> l = lastRet;

             if (l == null) throw new IllegalStateException();

             // rely on a future traversal to relink.

             l.item = null;

             lastRet = null;

         }

     }

     /**

      * Saves the state to a stream (that is, serializes it).

      *

      * @serialData All of the elements (each an {@code E}) in

      * the proper order, followed by a null

      * @param s the stream

      */

     private void writeObject(java.io.ObjectOutputStream s)

         throws java.io.IOException {

         // Write out any hidden stuff

         s.defaultWriteObject();

         // Write out all elements in the proper order.

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

             Object item = p.item;

             if (item != null)

                 s.writeObject(item);

         }

         // Use trailing null as sentinel

         s.writeObject(null);

     }

     /**

      * Reconstitutes the instance from a stream (that is, deserializes it).

      * @param s the stream

      */

     private void readObject(java.io.ObjectInputStream s)

         throws java.io.IOException, ClassNotFoundException {

         s.defaultReadObject();

         // Read in elements until trailing null sentinel found

         Node<E> h = null, t = null;

         Object item;

         while ((item = s.readObject()) != null) {

             @SuppressWarnings("unchecked")

             Node<E> newNode = new Node<E>((E) item);

             if (h == null)

                 h = t = newNode;

             else {

                 t.lazySetNext(newNode);

                 t = newNode;

             }

         }

         if (h == null)

             h = t = new Node<E>(null);

         head = h;

         tail = t;

     }

     /**

      * Throws NullPointerException if argument is null.

      *

      * @param v the element

      */

     private static void checkNotNull(Object v) {

         if (v == null)

             throw new NullPointerException();

     }

     private boolean casTail(Node<E> cmp, Node<E> val) {

         return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);

     }

     private boolean casHead(Node<E> cmp, Node<E> val) {

         return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);

     }

     // Unsafe mechanics

     private static final sun.misc.Unsafe UNSAFE;

     private static final long headOffset;

     private static final long tailOffset;

     static {

         try {

             UNSAFE = sun.misc.Unsafe.getUnsafe();

             Class k = ConcurrentLinkedQueue.class;

             headOffset = UNSAFE.objectFieldOffset

                 (k.getDeclaredField("head"));

             tailOffset = UNSAFE.objectFieldOffset

                 (k.getDeclaredField("tail"));

         } catch (Exception e) {

             throw new Error(e);

         }

     }

 }