1) Does server need functionality of Data class like (create, delete operation...).... My server provides only "book" and "strictSearch" methods (as needed to GUI client)....When I search this forum, I found that some people creates DBRemote inteface, that extend Remote, DB... (maybe it need for fat clients??) I'm little bit confused.....Is this ok to implement server with only business methods......
2) Can i use this interface both for local and network mode:
1/ You can use a fat client or a thin client. With the 1st one you will expose methods from the given interface to the client; with the other one you will only have 2 methods (find and book). Both approaches are acceptable. A nice discussion about this topic can be found here.
2/ an interface implementing another interface Are you sure you passed the scjp certification? But I just used the BusinessService for both local and remote implementations, other people used your approach, with either one you can pass this certification.
Here, you can find a proposal of a thin client. Although I implemented a thick client, if it was today, I would definitely implement a thin client, because I think it eases the solution. If your lock() method does not return any value, then implementing a thick client is a little bit harder.