Reference objects support a limited degree of interaction with the garbage
collector. A program may use a reference object to maintain a reference to
some other object in such a way that the latter object may still be reclaimed
by the collector. A program may also arrange to be notified some time after
the collector has determined that the reachability of a given object has
changed. Reference objects are therefore useful for building simple caches as
well as caches that are flushed when memory runs low, for implementing mappings
that do not prevent their keys (or values) from being reclaimed, and for
scheduling pre-mortem cleanup actions in a more flexible way than is possible
with the Java finalization mechanism.
API Specification
(javadoc)