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.
Returns this reference object's referent. If this reference object has
been cleared, either by the program or by the garbage collector, then
this method returns null.
Returns:
The object to which this reference refers, or
null if this reference object has been cleared