Really?

> If you have an unit test on private members, then you can't change the implementation safely without breaking tests.

This makes absolutely no sense

You change the private implementation, you change the unit test. It's that simple

Then you keep your API the same so that external users don't break.

It seems to be from the same people that like to whine about "missing tests and lack of coverage" quite funnily. It seems they like to nitpick and idolize tests instead of shipping

Depends. If we take the example of a library, then there are some tests which should never break ( except if the major version increases). But it is entirely sensible to check if you broke something internally during development.

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.

This is true for totally isolated unit tests but false for tests that require stubbing. You can theoretically use the same tests for a map implemented with one version of a hash table or the other. But once you start testing classes with dependencies, your tests inevitably touch implementation details. For example, unit testing a hash map that is backed by a memcached server would require some form of stubbing.

The metatesting reason for this confusion is that in the example you used, the scope of unit testing is the same as the scope of integration testing. If a class has 0 dependencies, a unit test is also an integration test, because it tests all the dependencies of the class, of which there are none.

So going back to your original point, your affirmation is true for full integration tests, i.e. tests that do not stub any dependency. If you do not stub your network connection to the memcached server, the same test - albeit with a different setup - can be used for a local hash table implementation.

The method should still do the same thing, regardless if it is private or public. If you purposefully create a new method and get rid of the old one, regardless if this is done by creating and deleting or by modifying, then the unit test should be changed as well.

In short, you are testing the interface of that private method, not the implementation.

You have some tests to ensure your public methods don't change implementation.

That isn't the purpose of all tests.

A complex public API should consist of smaller private parts. When you change those smaller parts of code, you would like to know if you break something and specifically what you broke. Testing of a small, isolated chunk of code is the 'unit' in the term 'unit tests'.

Unit tests on actual units of code allow you to more quickly isolate failures.

Well yeah that's the view being taken to its logical conclusion here:

- Private methods are just an implementation detail with respect to the public methods. (You really care about whether the public methods reply correctly.)

- Public methods are just an impdet with respect to the module API. (You really care about whether the module's public surface replies correctly.)

- The module's API is JAID with respect to the user/game API. (You really just care about whether the gamer gets the right responses to controller inputs.)

- The user/game API is JAID wrt to the code/gamer interface. (You really just care about whether the gamer likes what you've written.)

- The code/gamer interface is JAID wrt the company/customer interface. (You really just care whether people still give you money.)

- The company/customer interface is JAID wrt the investor/company interface. (You really just care whether the venture throws off more money than you put in.)

Nevertheless, you'd still like to catch the failures as soon and as narrowly as possible!

To go further: define an API, write tests against that API, and then do a pass of dead-code analysis on the resulting library-plus-test-suite. Any private functions left uncalled by your public API can just be removed!