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   * 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
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   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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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);
      }
  
  }
  