Doubt about Interfaces

Hi,

I have a doubt regarding interfaces. As far as I know, methods of an interface do not have a body. The implementation of these metods have to be written by the programmer, as in the case of the MouseListener interface (i.e. when my class extends the MouseListener interface, I have to write the mouseClicked... methods)

But in the case of the ResultSet, Connection, AppletContext etc interfaces, I do not have to write the code for the methods in these interfaces. (for eg, when I say rs.next(), the resultset pointer points to the next record of the resultset.) Where is the implementation of these methods written. I have tried to look into the source code but to no avail.

Thanks,
Abu