Java in a Nutshell, 5th Edition

Name

ReferenceQueue<T>

Synopsis

This class represents a queue (or linked list) of Reference objects that have been enqueued because the garbage collector has determined that the referent objects to which they refer are no longer adequately reachable. It serves as a notification system for object-reachability changes. Use poll( ) to return the first Reference object on the queue; the method returns null if the queue is empty. Use remove( ) to return the first element on the queue, or, if the queue is empty, to wait for a Reference object to be enqueued. You can create as many ReferenceQueue objects as needed. Specify a ReferenceQueue for a Reference object by passing it to the SoftReference( ), WeakReference( ), or PhantomReference( ) constructor.

A ReferenceQueue is required to use PhantomReference objects. It is optional with SoftReference and WeakReference objects; for these classes, the get( ) method returns null if the referent object is no longer adequately reachable.

public class ReferenceQueue<T> {
// Public Constructors
     public ReferenceQueue( );  
// Public Instance Methods
     public Reference<? extends T> poll( );  
     public Reference<? extends T> remove( ) throws InterruptedException;  
     public Reference<? extends T> remove(long timeout) throws IllegalArgumentException, 
InterruptedException;  
}

Passed To

PhantomReference.PhantomReference( ), SoftReference.SoftReference( ), WeakReference.WeakReference( )