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

  *

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  * 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

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 package java.lang.ref;

 /**

  * Phantom reference objects, which are enqueued after the collector

  * determines that their referents may otherwise be reclaimed.  Phantom

  * references are most often used for scheduling pre-mortem cleanup actions in

  * a more flexible way than is possible with the Java finalization mechanism.

  *

  * <p> If the garbage collector determines at a certain point in time that the

  * referent of a phantom reference is <a

  * href="package-summary.html#reachability">phantom reachable</a>, then at that

  * time or at some later time it will enqueue the reference.

  *

  * <p> In order to ensure that a reclaimable object remains so, the referent of

  * a phantom reference may not be retrieved: The <code>get</code> method of a

  * phantom reference always returns <code>null</code>.

  *

  * <p> Unlike soft and weak references, phantom references are not

  * automatically cleared by the garbage collector as they are enqueued.  An

  * object that is reachable via phantom references will remain so until all

  * such references are cleared or themselves become unreachable.

  *

  * @author   Mark Reinhold

  * @since    1.2

  */

 public class PhantomReference<T> extends Reference<T> {

     /**

      * Returns this reference object's referent.  Because the referent of a

      * phantom reference is always inaccessible, this method always returns

      * <code>null</code>.

      *

      * @return  <code>null</code>

      */

     public T get() {

         return null;

     }

     /**

      * Creates a new phantom reference that refers to the given object and

      * is registered with the given queue.

      *

      * <p> It is possible to create a phantom reference with a <tt>null</tt>

      * queue, but such a reference is completely useless: Its <tt>get</tt>

      * method will always return null and, since it does not have a queue, it

      * will never be enqueued.

      *

      * @param referent the object the new phantom reference will refer to

      * @param q the queue with which the reference is to be registered,

      *          or <tt>null</tt> if registration is not required

      */

     public PhantomReference(T referent, ReferenceQueue<? super T> q) {

         super(referent, q);

     }

 }