Null problem solved !

here in this slide it shows that how i over come the problem.
that instead of using
java.sql.Date dobb= null;

i initialise dobb as string first:
string dobb= request.getParameter("debuet");
so first the value of debuet to dobb and then convert that into string.

and then this string used in date table......
So after this its saving date into to database successfully.