Reference(T referent) 
{
        this(referent, null);
}

 Reference(T referent,
    ReferenceQueue<? super T> queue) 
{
        this.referent = referent;
        this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
}

 public  void clear() 
{
        this.referent = null;
}

Clears this reference object. Invoking this method will not cause this object to be enqueued.

This method is invoked only by Java code; when the garbage collector clears references it does so directly, without invoking this method.

 public boolean enqueue() 
{
        return this.queue.enqueue(this);
}

Adds this reference object to the queue with which it is registered, if any.

This method is invoked only by Java code; when the garbage collector enqueues references it does so directly, without invoking this method.

 public T get() 
{
        return this.referent;
}

Returns this reference object's referent. If this reference object has been cleared, either by the program or by the garbage collector, then this method returns null.

 public boolean isEnqueued() 
{
        /* In terms of the internal states, this predicate actually tests
           whether the instance is either Pending or Enqueued */
        synchronized (this) {
            return (this.queue != ReferenceQueue.NULL) && (this.next != null);
        }
}

Tells whether or not this reference object has been enqueued, either by the program or by the garbage collector. If this reference object was not registered with a queue when it was created, then this method will always return false.