/*

  * 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.atomic;

 import sun.misc.Unsafe;

 /**

  * An object reference that may be updated atomically. See the {@link

  * java.util.concurrent.atomic} package specification for description

  * of the properties of atomic variables.

  * @since 1.5

  * @author Doug Lea

  * @param <V> The type of object referred to by this reference

  */

 public class AtomicReference<V>  implements java.io.Serializable {

     private static final long serialVersionUID = -1848883965231344442L;

     private static final Unsafe unsafe = Unsafe.getUnsafe();

     private static final long valueOffset;

     static {

       try {

         valueOffset = unsafe.objectFieldOffset

             (AtomicReference.class.getDeclaredField("value"));

       } catch (Exception ex) { throw new Error(ex); }

     }

     private volatile V value;

     /**

      * Creates a new AtomicReference with the given initial value.

      *

      * @param initialValue the initial value

      */

     public AtomicReference(V initialValue) {

         value = initialValue;

     }

     /**

      * Creates a new AtomicReference with null initial value.

      */

     public AtomicReference() {

     }

     /**

      * Gets the current value.

      *

      * @return the current value

      */

     public final V get() {

         return value;

     }

     /**

      * Sets to the given value.

      *

      * @param newValue the new value

      */

     public final void set(V newValue) {

         value = newValue;

     }

     /**

      * Eventually sets to the given value.

      *

      * @param newValue the new value

      * @since 1.6

      */

     public final void lazySet(V newValue) {

         unsafe.putOrderedObject(this, valueOffset, newValue);

     }

     /**

      * Atomically sets the value to the given updated value

      * if the current value {@code ==} the expected value.

      * @param expect the expected value

      * @param update the new value

      * @return true if successful. False return indicates that

      * the actual value was not equal to the expected value.

      */

     public final boolean compareAndSet(V expect, V update) {

         return unsafe.compareAndSwapObject(this, valueOffset, expect, update);

     }

     /**

      * Atomically sets the value to the given updated value

      * if the current value {@code ==} the expected value.

      *

      * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>

      * and does not provide ordering guarantees, so is only rarely an

      * appropriate alternative to {@code compareAndSet}.

      *

      * @param expect the expected value

      * @param update the new value

      * @return true if successful.

      */

     public final boolean weakCompareAndSet(V expect, V update) {

         return unsafe.compareAndSwapObject(this, valueOffset, expect, update);

     }

     /**

      * Atomically sets to the given value and returns the old value.

      *

      * @param newValue the new value

      * @return the previous value

      */

     public final V getAndSet(V newValue) {

         while (true) {

             V x = get();

             if (compareAndSet(x, newValue))

                 return x;

         }

     }

     /**

      * Returns the String representation of the current value.

      * @return the String representation of the current value.

      */

     public String toString() {

         return String.valueOf(get());

     }

 }