Background:

I had a Groovy class which has a method that relies on an another Kotlin method imported from a dependency as the following:

class DependencyClass {
    fun remoteMethod(param1: String, param2: Int, param3: Boolean): String {
        // Method logic
        return "result"
    }

}

// Groovy class
class MyClass {

    // Google guice injection
    @Inject
    private DependencyClass depClassObj

    void myMethod(input) {

        // Call with 3 parameters
        def result = depClassObj.remoteMethod("example", 42, true)
        // Other logic...
    }
}

The tests of my Groovy class methods are written in Kotlin as the following:

    @Test
    fun `test myMethod which calls anotherMethod in DependencyClass`() {

        // Mock & bind etc..
        val depClassObjMock = mock<DependencyClass>()

        // Step 3: Define behavior for myMethod using matchers
        whenever(depClassObjMock.remoteMethod( any(), any(), any() )).thenThrow(NullPointerException)

        myClassMock.myMethod()
        
        // TestUtil class which runs the methods...
        assertThrows<NullPointerException>(flowRunner.runFlow("myClassMock.myMethod"))

}
 

Problem:

The Dependency that I was relying on got updated, as the remoteMethod() started to take 4 parameters instead of 3, where the 4th parameter is nullable as the following:

class DependencyClass {
    fun remoteMethod(param1: String, param2: Int, param3: Boolean, param4: String? = null): String {
        // Method logic
        return "result"
    }

}

Now all my tests started to fail due to an error that looks:

Invalid use of argument matchers!
4 matchers expected, 3 recorded:...

What have I tried:

First try (didn’t work)

I tried editing the tests by passing a new matcher for the new parameter as anyOrNull() or eq(Null) or isNull()` as the follwing:

whenever(depClassObjMock.myMethod( any(), any(), any(), anyOrNull() )).thenThrow(NullPointerException)

but now the whenever or when statement doesn’t match the myMethod schema causing the tests to do the actual logic of the myMethod (without mocking).

as

depClassObjMock.remoteMethod(any(), any(), any(), anyOrNull()) // takes 4 parameters
!==
depClassObj.remoteMethod("example", 42, true) // takes 3 parameters, even if the 4th is nullable it's not pointing for the whenever statement with the 4 parameters.

Second try (worked)

I had to edit the logic inside `myMethod() to make the tests work (which is not a good practice as I recall) as the following:

def result = depClassObj.remoteMethod("example", 42, true, null)

Ask:

1. Is there any way to test kotlin methods with nullable parameters without editing the actual logic (only edit the tests) ?

2. Does this issue has anything to do with the fact that we are calling a Kotlin method from a Groovy method then testing the groovy flow in Kotlin tests ?

3. How testing methods with nullable parameters usually get done in Kotlin ?

4. How Mockito usually maps the whenever / when statements to the original methods ?

Feel free guys to answer any question of the 4. You don’t have answer them all.