I've been a game developer for 15+ years (mainly C++), never used unit tests, just good old plain asserts, sometimes ad-hoc code that creates/simulates errors or slowdowns. Pretty much QA people testing your game/tools and some form of automated tests (run this level, expect this to happen). Then I changed jobs, started writing in Java (+ GoogleWebKit and Javascript), was exposed to Unit, integration and end-to-end testing. Do I know it properly? Hell no. I'm still confused.

But this is what I seems to be getting out of it: You are given a black box, with inputs and outputs. There is also a spec (it could be in your head for all I know) that defines that for certain inputs, certain outpus are expected. This spec also tries to cover quite a lot distinct cases. Each such representative case of input and output is an unit test. (If your spec was really in your head, your unit tests kind of becomes it, or I like to think about it in this way - a Unit Spec :)).

The tricky part is when this blackbox is internally working with other blackboxes. Unit testing is all about testing the blackbox in isolation from other blackboxes. As such one needs to isolate them away. Currently what I'm using is DI (Dependency Injection) with guice/gin/dagger to achieve that.

Thanks for all comments, it seems I have to fill my gaps in what I know.