java.lang.ref

Class Reference<T>

Known Direct Subclasses:
PhantomReference<T>, SoftReference<T>, WeakReference<T>

public abstract class Reference<T>
extends Object

This is the base class of all references. A reference allows refering to an object without preventing the garbage collector to collect it. The only way to get the referred object is via the get()-method. This method will return null if the object was collected.
A reference may be registered with a queue. When a referred element gets collected the reference will be put on the queue, so that you will be notified.
There are currently three types of references: soft reference, weak reference and phantom reference.
Soft references will be cleared if the garbage collector is told to free some memory and there are no unreferenced or weakly referenced objects. It is useful for caches.
Weak references will be cleared as soon as the garbage collector determines that the refered object is only weakly reachable. They are useful as keys in hashtables (see WeakHashtable) as you get notified when nobody has the key anymore. Phantom references don't prevent finalization. If an object is only phantom reachable, it will be finalized, and the reference will be enqueued, but not cleared. Since you mustn't access an finalized object, the get method of a phantom reference will never work. It is useful to keep track, when an object is finalized.
See Also:
java.util.WeakHashtable

Method Summary

void
clear()
Clears the reference, so that it doesn't refer to its object anymore.
boolean
enqueue()
Enqueue an object on a reference queue.
T
get()
Returns the object, this reference refers to.
boolean
isEnqueued()
Tells if the object is enqueued on a reference queue.

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Details

clear

public void clear()
Clears the reference, so that it doesn't refer to its object anymore. For soft and weak references this is called by the garbage collector. For phantom references you should call this when enqueuing the reference.

enqueue

public boolean enqueue()
Enqueue an object on a reference queue. This is normally executed by the garbage collector.

get

public T get()
Returns the object, this reference refers to.
Returns:
the object, this reference refers to, or null if the reference was cleared.

isEnqueued

public boolean isEnqueued()
Tells if the object is enqueued on a reference queue.
Returns:
true if it is enqueued, false otherwise.

java.lang.ref.Reference Copyright (C) 1999, 2002, 2003, 2006 Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath 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 for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.