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"I'm almost done, I'll just need to add tests!"

Booom! You did it, that was a nuke going off in my head.

No, you shouldn't just need to add tests. The tests should have been written from the get go! You most likely won't cover all the cases. You won't know if adding the tests will break your feature, as you had none, as you refactor your untested mess in order to make your code testable.

When reading your mess of a test case and the painful mocking process you went through, I silently cry out into the void: "Why oh why!? All of this suffering could have been avoided!"

Since most of the time, your mocking pain boils down to not understanding what your "unit" in your "unit test" should be.

So let it be said:

- If you want to build a parser for an XML file, then just write a function / class whose *only* purpose is: parse the XML file, return a value object. That's it. Nothing more, nothing less.
- If you want to build a parser for an XML file, it MUST NOT: download a zip, extract that zip, merge all those files to one big file, parse that big file, talk to some other random APIs as a side-effect, and then return a value object.

Because then you suddenly have to mock away a http service and deal with zip files in your test cases.

The http util of your programming language will most likely work. Your unzip library will most likely work. So just assume it working. There are valid use cases where you want to make sure you acutally send a request and get a response, yet I am talking unit test here only.

In the scope of a class, keep the public methods to a reasonable minimum. As for each public method you shall at least create one test case. If you ever have the feeling "I want to test that private method" replace that statement in your head with: "I should extract that functionality to a new class where that method public. I then can create a unit test case a for that." That new service then becomes a dependency in your current service. Problem solved.

Also, mocking away dependencies should a simple process. If your mocking process fills half the screen, your test setup is overly complicated and your class is doing too much.

That's why I currently dig functional programming so much. When you build pure functions without side effects, unit tests are easy to write. Yet you can apply pure functions to OOP as well (to a degree). Embrace immutability.

Sidenote:

It's really not helpful that a lot of developers don't understand the difference between unit, functional acceptance, integration testing. Then they wonder why they can't test something easily, write overly complex test cases, until someone points out to them: No, in the scope of unit tests, we don't need to test our persistance layer. We just assume that it works. We should only test our businsess logic. You know: "Assuming that I get that response from the database, I expect that to happen." You don't need a test db, make a real query against that, in order to test that. (That still is a valid thing to do. Yet not in the scope of unit tests.)