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How exactly should unit tests be written without mocking extensively?

By minimising side-effects in your code.

Taking your example code, if calculator for example talks to a web API, then either you create fragile tests that rely on being able to interact with that web API, or you create a mock of it. If however its a deterministic, state-free set of calculation functions, then you don't (and shouldn't) mock it. If you do, you risk your mock behaving differently to the real code, leading to bugs in your tests.

Mocks should only be needed for code that read/writes to the file system, databases, URL endpoints etc; that are dependent on the environment you are running under; or that are highly stateful and non-deterministic in nature. So if you keep those parts of the code to a minimum and hide them behind abstractions, then they are easy to mock and the rest of your code avoids the need for mocks.

For the code points that do have side effects, it's worth writing tests that mock and tests that don't. The latter though need care as they will inherently be fragile and possibly slow. So you may want to only run them say overnight on a CI server, rather than every time you save and build your code. The former tests though should be run as often as practicable.
As to whether each test is then a unit or integration test becomes academic and avoids "flame wars" over what is and isn't a unit test.

These questions are quite different in their difficulty. Let's take question 2 first.

Unit tests and integration tests are clearly separated. A unit test tests one unit (method or class) and uses other units only as much as necessary to achieve that goal. Mocking may be necessary, but it is not the point of the test. An integration test tests the interaction between different actual units. This difference is the entire reason why we need both unit and integration testing - if one did the job of the other well enough, we wouldn't, but it's turned out that it's usually more efficient to use two specialized tools rather than one generalized tool.

Now for the important question: How should you unit test? As said above, unit tests should construct auxiliary structures only as far as necessary. Often it is easier to use a mock database than your real database or even any real database. However, mocking in itself has no value. If often happens that it is in fact easier to use actual components of another layer as input for a mid-level unit test. If so, don't hesitate to use them.

Many practitioners are afraid that if unit test B reuses classes that were already tested by unit test A, then a defect in unit A causes test failures in multiple places. I consider this not a problem: a test suite has to succeed 100% in order to give you the reassurance you need, so it is not a big problem to have too many failures - after all, you do have a defect. The only critical problem would be if a defect triggered too few failures.

Therefore, don't make a religion of mocking. It is a means, not an end, so if you can get away with avoiding the extra effort, you should do so.

OK, so to answer your questions directly:

How should unit tests be written properly?

As you say, you should be mocking dependencies and testing just the unit in question.

Where exactly does the line between them and integration tests lie?

An Integration test is a unit test where your dependencies are not mocked.

Can a test that tests the Person.calculate method without mocking the
Calculator be considered a unit test?

No. You need to inject the calculator dependency into this code and you have a choice between a mocked version or a real one. If you use a mocked one its a unit test, if you use a real one its an integration test.

However, a caveat. do you really care what people think your tests should be called?

But your real question seems to be this:

a quick Google search about mocking reveals tons of articles that
claim that "mocking is a code smell" and should mostly (though not
completely) be avoided.

I think the problem here is that a lot of people use mocks to completely recreate the dependencies. For example I might mock the calculator in your example as

public class MockCalc : ICalculator

{

     public Add(int a, int b) { return 4; }

}

I would not do something like:

myMock = Mock<ICalculator>().Add((a,b) => {return a + b;})

myPerson.Calculate()

Assert.WasCalled(myMock.Add());

I would argue that, that would be "testing my mock" or "testing the implementation". I would say "Don't write Mocks! *like that".

Other people would disagree with me, we would start massive flame wars on our blogs about the Best way to Mock, which really would make no sense unless you understood the whole background of the various approaches and really don't offer a whole lot of value to someone who just wants to write good tests.

1. How should unit tests be implemented properly?

My rule of thumb is that proper unit tests:

• 
  Are coded against interfaces, not implementations. This has many benefits. For one, it ensures that your classes follow the Dependency Inversion Principle from SOLID. Also, this is what your other classes do (right?) so your tests should do the same. Also, this allows you to test multiple implementations of the same interface while reusing much of the test code (only initialization and some assertions would change).


• 
  Are self-contained. As you said, changes in any outside code cannot affect the test result. As such, unit tests can execute at build-time. This means you need mocks to remove any side effects. However, if you are following the Dependency Inversion Principle, this should be relatively easy. Good test frameworks like Spock can be used to dynamically provide mock implementations of any interface to use in your tests with minimal coding. This means that each test class only needs to exercise code from exactly one implementation class, plus the test framework (and maybe model classes ["beans"]).


• 
  Do not require a separate running application. If the test needs to "talk to something", whether a database or a web service, it's an integration test, not a unit test. I draw the line at network connections or the filesystem. A purely in-memory SQLite database, for example, is fair game in my opinion for a unit test if you really need it.

If there are utility classes from frameworks that complicate unit testing, you may even find it useful to create very simple "wrapper" interfaces and classes to facilitate mocking of those dependencies. Those wrappers would then not necessarily be subject to unit tests.

2. Where exactly does the line between them [unit tests] and integration tests lie?

I have found this distinction to be the most useful:

• 
  Unit tests simulate "user code", verifying behavior of implementation classes against the desired behavior and semantics of code-level interfaces.


• 
  Integration tests simulate the user, verifying behavior of the running application against specified use cases and/or formal APIs. For a web service, the "user" would be a client application.

There is gray area here. For example, if you can run an application in a Docker container and run the integration tests as the final stage of a build, and destroy the container afterwards, is it OK to include those tests as "unit tests"? If this is your burning debate, you're in a pretty good place.

3. Is it true that virtually every unit test needs to mock?

No. Some individual test cases will be for error conditions, like passing null as a parameter and verifying you get an exception. Lots of tests like that will not require any mocks. Also, implementations that have no side effects, for example string processing or math functions, may not require any mocks because you simply verify the output. But most classes worth having, I think, will require at least one mock somewhere in the test code. (The fewer, the better.)

The "code smell" issue you mentioned arises when you have a class that is overly complicated, that requires a long list of mock dependencies in order to write your tests. This is a clue that you need to refactor the implementation and split things up, so that each class has a smaller footprint and a clearer responsibility, and is therefore more easily testable. This will improve quality in the long run.

Only one unit test should break by a bug in the tested unit.

I don't think this is a reasonable expectation, because it works against reuse. You may have a private method, for example, that is called by multiple public methods published by your interface. A bug introduced into that one method might then cause multiple test failures. This doesn't mean you should copy the same code into each public method.

1. They should not break by any unrelated code change elsewhere in the codebase.

I'm not really sure how this rule is useful. If a change in one class/method/whatever can break the behaviour of another in production code, then the things are, in reality, collaborators, and not unrelated. If your tests break and your production code doesn't, then your tests are suspect.

2. Only one unit test should break by a bug in the tested unit, as opposed to integration tests (which may break in heaps).

I'd regard this rule with suspicion too. If you're really good enough to structure your code and write your tests such that one bug causes exactly one unit test failure, Then you're saying you've identified all the potential bugs already, even as the codebase evolves to use cases you haven't anticipated.

Where exactly does the line between them and integration tests lie?

I don't think that's an important distinction. What is a 'unit' of code anyhow?

Try to find entry points at which you can write tests that just 'make sense' in terms of the problem domain/business rules that that level of the code is dealing with. Often these tests are somewhat 'functional' in nature - put in an input, and test that the output is as expected. If the tests express a desired behaviour of the system, then they often remain quite stable even as the production code evolves and is refactored.

How exactly should unit tests be written without mocking extensively?

Don't read too much into the word 'unit', and lean towards using your real production classes in tests, without worrying too much if you're involving more than one of them in a test. If one of them is hard to use (because it takes a lot of initialisation, or it needs to hit a real database/email server etc), then let your thoughts turn to mocking/faking.