||Implements a phantom reference, which is the weakest of the three types of references.
||Provides an abstract class which describes behavior common to all reference objects.
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 references.