||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.