[mrec]

I think we may be miscommunicating; a lot of these words mean different things to different people. Designing a good and minimal "interface" for a class or module, in the sense of how it exposes its functionality to the rest of the world, is absolutely important, yes.

Deciding that mentioning classes anywhere in a signature is Evil, and that everything must implement a separately-defined interface in the Java/C# sense, is IMHO not at all useful. Not because it's "expensive" - it just takes an already-designed class API and copypastes it into an interface - but it's pure busywork. More code (which some people treat as a plus), harder to understand (because now there's always extra ambiguity at a call site as to which implementation is being called), adds nothing. This type of thinking - taking a useful tool or rule of thumb and turning it into dogma - seems far more prevalent in Java-land than in any other language I've worked in; I've never really understood why.