| Class | Description | 
|---|---|
| Reference<T> | 
 Abstract base class for reference objects. 
 | 
| ReferenceQueue<T> | 
 Reference queues, to which registered reference objects are appended by the garbage collector
 after the appropriate reachability changes are detected. 
 | 
| WeakReference<T> | 
 Weak reference objects, which do not prevent their referents from being made finalizable,
 finalized, and then reclaimed. 
 |