The most common usage of weak references is for values in "lookup" Maps.
With normal (hard) value references, if the value in the map no longer has references to it elsewhere, you often don't need the lookup any more. With weakly referenced map values, once there are no other references to it, the object becomes a candidate for garbage collection
The fact that the map itself has a (the only) reference to the object does not stop it from being garbage collected because the reference is a weak reference
