I guess this is only tangentially related to each other Mock objects and interfaces. I stopped using interfaces in this manner. My rule if you only have one implementation of an interface you don't need an interface. Wait to create the interface when you get your second implementation, and we have great tools that make that easy.

Now for people who do lots of mock objects this rule doesn't help them out much, but my other rule is I don't overly separate my system for testing purposes. Sure there are times when you need a mock object for java mail or external services you need to mock out. But, doing it for every service you have is really a lot of work for questionable gain. This is predicated on practical experience rather than architecture theory. The reason you separate your system for testing is to find more bugs. I found that I wasn't finding anymore bugs by separating things than I was by testing it integrated. And, in fact I found more bugs in the integration of services than I would if I only tested them in separate form. Therefore, I stopped doing the extra work to separate them because the payoff was really too small to bother. It's been much more productive to think like this.