My style is similar about tiny interfaces: My usual style in Java is an interface with a single method and a nested POJO (struct) called Result. Then, I have a single implementation in production, and another implementation for testing (mocking in 2010s forward). Some of my longer lived projects might have 100s of these after a few years.

Please enjoy this silly, but illustrative example!

public interface HerdingCatsService {

    /*public static*/ final class Result {
        ...
    }

    Result herdThemCats(ABunchOfCats soMuchFun)
    throws Exception;
}