Programming to an interface. How to decide where its needed?

Question

I understand that programming to interfaces helps with loose coupling. However, is there a guideline that explains when its most effective?

For example, I have a simple web application that collects data on employees, their training plans, expenses and computes their expense for the year etc. This is a fairly simple application and I can of course use an interface but wonder if there would be any use. I will be using it for the sake of using it.

It can always be argued that as the application grows in complexity and I pass around objects it would make more sense to pass the type (interface) than an implementation. So, should I wait for the application to become complex or use it right away? I'm wondering the best practice might turn out to be "this guy is over doing things".

Answer 1

The reason to program to an interface is to insulate higher-level classes from changes in lower-level classes. If you don't anticipate a lower-level class to change at all, then it's reasonable not to program to an interface in that case. This article (PDF) elaborates on the idea, coining the term the Dependency-Inversion Principle.

Answer 2

If you ever get yourself interested in Test Driven Development, the need for "programming to an interface" really becomes apparent. If you want a class to be tested in isolation of its dependencies, you need to pass around interfaces instead of objects.