Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed.
Abstract base class for reference objects.
Reference queues, to which registered reference objects are appended by the garbage collector after the appropriate reachability changes are detected.
Soft reference objects, which are cleared at the discretion of the garbage collector in response to memory demand.
Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed.