Suppose we have the following interface: interface Validator { fun validate(query: String): Boolean } and suppose it’s being used as a collaborator in a few places: class FindItems( val validator: Validator, val repository: Repository ) { fun search(keyword: String) { if (validator.validate(keyword)) { repository.findItemsContaining(keyword) } } } class DeleteItems( val validator: Validator, val repository: Repository ) { fun deleteMatches(value: String) { if (validator.validate(value)) { repository.deleteItemsMatching(value) } } } Of course, we would have some tests for these functionalities, which might look something like this respectively:

⚠️ Disclaimer Any kinds of diagrams have nothing to do with TDD at all. We find the diagrams handy when we want to visualize, explain, teach, or learn something. That’s all. It’s a drawing on a sheet of paper that we throw afterward. We don’t keep the diagrams around; we don’t maintain them. That’d be a massive waste of time. Sequence Diagram Example What’s this Sequence Diagram thingy? As mentioned in the disclaimer, the sequence diagram is a tool we can use to visualize an approximation of where we want to go when working on the unit in question.

A quick overview in our industry I’m in the IT industry, particularly in Android for roughly 10 years now, and one thing I can tell with great confidence is that this industry moves incredibly fast. Looking back all these years, it’s been an extraordinary evolution of the OS itself, as well as the community around it. However, sometimes it feels like our industry lacks a lot of good practices and discipline in the development process, a lack of professionalism.