Useless interfaces

Question

Why would you ever use an interface if you are only going to have one implementation of it?

Answer 1

One situation for an interface for only one implementation: RMI. You have an object on one application and will use it from another app via RMI; you need to have an interface. That way you don't even have to include the implementation class on the calling app, which might save the calling app from including a bunch of J2EE stuff or external libraries that the implementation might use.

Another reason: Today you only have one implementation, but tomorrow there may be something in the JDK or an external library that could lead to changing that implementation (maybe creating a new one). Your system could change and you need to add a new implementation while keeping the old one; some maintenance, etc.