R> method Null safety
Provides a view of this set as a set of
If this set contains only instances of
R, all read operations
will work correctly. If any operation tries to access an element
that is not an instance of
R, the access will throw instead.
Elements added to the set (e.g., by using add or addAll)
must be instance of
R to be valid arguments to the adding function,
and they must be instances of
E as well to be accepted by
this set as well.