So I've been thinking about different kinds of contracts your software can have, hard contracts and soft contracts. If I look at the javadoc for List, it tells me what's expected of me if I implement a List.
Great, I implement a List, BsList - and pass it off to you. Technically I did everything Java cared about, there's no compile errors, I implement List and provided all the proper methods. But there's nothing that verifies if I followed the intent of the List interface. The only thing you could do if you wanted to know if I did a good job would be write some unit tests. This is a soft contract. I implemented some interface and you assumed because I did that, my code follows the expected behavior of List.add(int index, Object element) Inserts the specified element at the specified position in this list (optional operation).
Now imagine another world. Where Sun not only released the List interface with some Javadoc, but also provided developers with a set of tests. How much more confident would you be in BsList if you could run a Sun approved unit test suite?
Most interfaces talk not only about a syntactic contract (you implemented the interface), but have an expectation about how the implementation should behave. The problem is, that's only captured in javadoc. I think interfaces need harder contracts bundled with them, they need unit tests. And seriously, you can't tell me you wouldn't love implementing an interface that had a set of tests with it. The List interface is big, 23 methods, how do you know you got it right? And why should those tests ever be coded up more than once?
Who knows, maybe it would be a bad idea for Sun to do something like that, but I know I'll be providing unit tests with my interfaces.