Useless interfaces

Question

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

Answer 1

If you are working with a system that allows refactoring, you should only add interfaces if either it is needed for the specification (say, an external API) or if you need multiple implementations. I consider test objects to be valid interpretations, so if you need an interface in order to get it under test, that is a fine use of interfaces.

Value objects should rarely become interfaces, service objects should frequently become interfaces.