I see is that, you dont implement the sql interfaces is that you get a Default Implementation from the Vendor who owns the Database to which you are coding for, or the Default implementation if using ODBC Data Sources.
Simply because there is not standard as to how a Database Application provides a non SQL interface to other Systems / Languages...so sun has atleast Standardized the interface that a
java Programmer has to program to, to access the Database so that your code remains Database independent.
you could still implement the connection Class, but then that would mean you will be implementing the whole set of interfaces as all the interfaces are hirarchically linked...(that is, you get a Statement Object from a connection Object, a ResultSet Object from a Statement etc).
You should be knowing The database you are intending to write the driver inside out, because it also involves calling native methods on the Database Side.
Hope this is attempting to answer your point.