Should I Mock the Trait or Mock the Class Using Scala Mock

Question

Say I have the following:

@ImplementedBy(classOf[DefaultFoo])
trait Foo {
  def a (s : String) : Int
}

class DefaultFoo @Inject()() extends Foo{

  override def a (s : String) = 1

}

@ImplementedBy(classOf[DefaultBaz])
trait Baz {
  def b (s : String) : Int
}

class DefaultBaz @Inject()(val f :Foo) extends Baz{

  override def a (s : String) = 1

}

If I want to test, say DefaultBaz, I am normally using ScalaMock and I would mock as follows in my test spec:

class DefaultBazSpec extends AnyWordSpec with MockFactory{

   val mockFoo = mock[Foo]
val b = new DefaultBaz(mockFoo)

   // write tests
}

But I could also do this:

val mockFoo = mock[DefaultFoo]

Which one is better? Mock the trait or the default class implementation?

Answer 1

Better to use neither. Mocking should be in general avoided (especially in Scala).

But even if you decided to proceed with mocks, mocking the class under test destroys the whole purpose of testing: the code under test has nothing in common with the production code.

Mocks are for mocking dependencies (e.g. constructor parameters or method arguments) of the class under test.