PhantomReference<T> Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. 
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. 
SoftReference<T> Soft reference objects, which are cleared at the discretion of the garbage collector in response to memory demand. 
WeakReference<T> Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed.