| PhantomReference<T> | 
              Implements a phantom reference, which is the weakest of the three types of
 references.  | 
          
        
              | Reference<T> | 
              Provides an abstract class which describes behavior common to all reference
 objects.  | 
          
        
              | ReferenceQueue<T> | 
              The ReferenceQueue is the container on which reference objects are
 enqueued when the garbage collector detects the reachability type specified
 for the referent.  | 
          
        
              | SoftReference<T> | 
              Implements a soft reference, which is the least-weak of the three types of
 references.  | 
          
        
              | WeakReference<T> | 
              Implements a weak reference, which is the middle of the three types of
 references.  |