testing.md

Testing

Press uses fakes for writing tests and avoids mocks because they’re terrible. Assertions are written using AssertK. Tests that involve storage are run on the JVM so that an in-memory SQL database can be used (example).

On mocks vs fakes

Mocks are terrible because they strongly couple tests with their implementation. When a test is written to check if certain functions on a mocked object were called, the test will be need to be updated every time those functions are refactored. Here’s an example:

class Adder {
  fun add(a: Int, b: Int): Int = a + b
}

class Calculator(val adder: Adder) {
  fun multiply(number: Int, factor: Int): Int {
    var sum = 0
    repeat(factor) {
      sum = adder.add(sum, number)
    }
    return sum
  }
}

@Test `test multiplication`() {
  val adder = mock(Adder::class)
  val calculator = Calculator(adder)
  calculator.multiply(2, 2)

  verify(adder).add(0, 2)
  verify(adder).add(2, 4)
}

Let’s make this worse. What happens if Adder#add() is changed to return a Single<Int> because we now want to offload additions to a server?

The test will continue to pass, but the implementation won’t work anymore. Mocks become worse when we start passing around a “promise” of result instead of the actual result.

If you’re still unconvinced, Artur Dryomov’s blog post is a good read this subject: Superior Testing: Make Fakes not Mocks