/*

 * Copyright 1997-2003 Sun Microsystems, Inc.  All Rights Reserved.

 * 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.  Sun designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.com if you need additional information or

 * have any questions.

 */

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);

    }

}