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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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).
 *
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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 to schedule post-mortem
 * cleanup actions.
 *
 * <p>Suppose the garbage collector determines at a certain point in time that an object is <a
 * href="package-summary.html#reachability">phantom reachable</a>. At that time it will atomically
 * clear all phantom references to that object and all phantom references to any other
 * phantom-reachable objects from which that object is reachable. At the same time or at some later
 * time it will enqueue those newly-cleared phantom references that are registered with reference
 * queues.
 *
 * <p>In order to ensure that a reclaimable object remains so, the referent of a phantom reference
 * may not be retrieved: The {@code get} method of a phantom reference always returns {@code null}.
 *
 * @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}.
   *
   * @return {@code null}
   */
  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 {@code null} queue, but such a reference
   * is completely useless: Its {@code get} method will always return {@code 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 {@code null} if
   *     registration is not required
   */
  public PhantomReference(T referent, ReferenceQueue<? super T> q) {
    super(referent, q);
  }
}
