How can I test method behaviour on not-existing interface implementation in Kotlin?

Let suppose, I have an interface Animal, it has two implementations: Dog and Cat, and we have some method for this class:

fun Animal.sound() : String {
  if (this is Dog) return "woof"
  if (this is Cat) return "meow"
  return "globglogabgalab"
}

Then, I want to test all cases of this method

How can I do that (with JUnit/Mockito), if I don't have an implementation for default behavior?

Solution

You could use kotlins Sealed classes in combination with a when statement, then you wouldn't need the last return statement, because that would statement would be unreachable.

sealed class Animal
class Dog : Animal()
class Cat : Animal()

and the sound method would look like this.

fun Animal.sound() = when (this) {
  is Cat -> "meow"
  is Dog -> "woof"
}

Alternatively, without sealed classes you could just create a TestAnimal implementation, which implements Animal in your tests, then instantiate it and run the sound() method.

class TestAnimal : Animal

@Test
fun foo() {
  TestAnimal().sound()
}

or with a anonymous implementation

@Test
fun foo() {
  val animal = object : Animal {}

  animal.sound()
}