open class Myclass {

    fun asdf(myArg: String?) {
        println(myArg!!.length)
    }
}


class MyclassTest {

    @Test
    fun asdf() {

        val myClassMock: Myclass = mock {
            on {
                asdf(any()) /* ??? */
            }
        }

        myClassMock.asdf(“bla”)
    }

}