Phantom reference objects, which are enqueued after the collector
determines that their referents may otherwise be reclaimed.  Phantom
references are most often used for scheduling pre-mortem cleanup actions in
a more flexible way than is possible with the Java finalization mechanism.

 If the garbage collector determines at a certain point in time that the
referent of a phantom reference is phantom reachable, then at that
time or at some later time it will enqueue the reference.

 In order to ensure that a reclaimable object remains so, the referent of
a phantom reference may not be retrieved: The get method of a
phantom reference always returns null.

 Unlike soft and weak references, phantom references are not
automatically cleared by the garbage collector as they are enqueued.  An
object that is reachable via phantom references will remain so until all
such references are cleared or themselves become unreachable.

Abstract base class for reference objects.  This class defines the
operations common to all reference objects.  Because reference objects are
implemented in close cooperation with the garbage collector, this class may
not be subclassed directly.

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.  Soft references are most often used
to implement memory-sensitive caches.

 Suppose that the garbage collector determines at a certain point in time
that an object is softly
reachable.  At that time it may choose to clear atomically all soft
references to that object and all soft references to any other
softly-reachable objects from which that object is reachable through a chain
of strong references.  At the same time or at some later time it will
enqueue those newly-cleared soft references that are registered with
reference queues.

 All soft references to softly-reachable objects are guaranteed to have
been cleared before the virtual machine throws an
OutOfMemoryError.  Otherwise no constraints are placed upon the
time at which a soft reference will be cleared or the order in which a set
of such references to different objects will be cleared.  Virtual machine
implementations are, however, encouraged to bias against clearing
recently-created or recently-used soft references.

 Direct instances of this class may be used to implement simple caches;
this class or derived subclasses may also be used in larger data structures
to implement more sophisticated caches.  As long as the referent of a soft
reference is strongly reachable, that is, is actually in use, the soft
reference will not be cleared.  Thus a sophisticated cache can, for example,
prevent its most recently used entries from being discarded by keeping
strong referents to those entries, leaving the remaining entries to be
discarded at the discretion of the garbage collector.

Weak reference objects, which do not prevent their referents from being
made finalizable, finalized, and then reclaimed.  Weak references are most
often used to implement canonicalizing mappings.

 Suppose that the garbage collector determines at a certain point in time
that an object is weakly
reachable.  At that time it will atomically clear all weak references to
that object and all weak references to any other weakly-reachable objects
from which that object is reachable through a chain of strong and soft
references.  At the same time it will declare all of the formerly
weakly-reachable objects to be finalizable.  At the same time or at some
later time it will enqueue those newly-cleared weak references that are
registered with reference queues.