Source for java.lang.ref.PhantomReference

   1: /* java.lang.ref.PhantomReference
   2:    Copyright (C) 1999, 2004 Free Software Foundation, Inc.
   3: 
   4: This file is part of GNU Classpath.
   5: 
   6: GNU Classpath is free software; you can redistribute it and/or modify
   7: it under the terms of the GNU General Public License as published by
   8: the Free Software Foundation; either version 2, or (at your option)
   9: any later version.
  10: 
  11: GNU Classpath is distributed in the hope that it will be useful, but
  12: WITHOUT ANY WARRANTY; without even the implied warranty of
  13: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14: General Public License for more details.
  15: 
  16: You should have received a copy of the GNU General Public License
  17: along with GNU Classpath; see the file COPYING.  If not, write to the
  18: Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  19: 02110-1301 USA.
  20: 
  21: Linking this library statically or dynamically with other modules is
  22: making a combined work based on this library.  Thus, the terms and
  23: conditions of the GNU General Public License cover the whole
  24: combination.
  25: 
  26: As a special exception, the copyright holders of this library give you
  27: permission to link this library with independent modules to produce an
  28: executable, regardless of the license terms of these independent
  29: modules, and to copy and distribute the resulting executable under
  30: terms of your choice, provided that you also meet, for each linked
  31: independent module, the terms and conditions of the license of that
  32: module.  An independent module is a module which is not derived from
  33: or based on this library.  If you modify this library, you may extend
  34: this exception to your version of the library, but you are not
  35: obligated to do so.  If you do not wish to do so, delete this
  36: exception statement from your version. */
  37: 
  38: 
  39: package java.lang.ref;
  40: 
  41: /**
  42:  * A phantom reference is useful, to get notified, when an object got
  43:  * finalized.  You can't access that object though, since it is
  44:  * finalized.  This is the reason, why <code>get()</code> always
  45:  * returns null.
  46:  *
  47:  * @author Jochen Hoenicke
  48:  */
  49: public class PhantomReference<T>
  50:   extends Reference<T>
  51: {
  52:   /**
  53:    * Creates a new phantom reference.
  54:    * @param referent the object that should be watched.
  55:    * @param q the queue that should be notified, if the referent was
  56:    * finalized.  This mustn't be <code>null</code>.
  57:    * @exception NullPointerException if q is null.
  58:    */
  59:   public PhantomReference(T referent, ReferenceQueue<? super T> q)
  60:   {
  61:     super(referent, q);
  62:   }
  63: 
  64:   /**
  65:    * Returns the object, this reference refers to.
  66:    * @return <code>null</code>, since the refered object may be
  67:    * finalized and thus not accessible.
  68:    */
  69:   public T get()
  70:   {
  71:     return null;
  72:   }
  73: }