||Implements a phantom reference, which is the weakest of the three types of
||Provides an abstract class which describes behavior common to all reference
ReferenceQueue is the container on which reference objects are
enqueued when the garbage collector detects the reachability type specified
for the referent.
||A reference that is cleared when its referent is not strongly reachable and
there is memory pressure.
||Implements a weak reference, which is the middle of the three types of