Testing a private method in Kotlin

Question

I'm trying to test a private method which takes the below parameters as input:

ClassToBeTested

delete(
        someId1: String,
        aList: List<CustomObject>,
        someId2: String,
        someId3: String,
        someId4: String
) : Long? {
}

TestClass

val method = ClassToBeTested.javaClass.getDeclaredMethod("delete", String::class.java,
    List::class.java, String::class.java, String::class.java, String::class.java)

method.isAccessible = true
val testId = method.invoke(ClassToBeTested, someId1, aList, someId2, someId3, someId4)

I end up getting the below error:

java.lang.NoSuchMethodException:

ClassToBeTested$Companion.delete(java.lang.String, java.util.Arrays$ArrayList, java.lang.String, java.lang.String, java.lang.String)

When I tried changing the above method declaration as:

val method = ClassToBeTested.javaClass.getDeclaredMethod("delete", String::class.java,
    List<CustomObject>::class.java, String::class.java, String::class.java, String::class.java)

I get the below error:
Kotlin: Only classes are allowed on the left hand side of a class literal

Is there anyway we can test the private function which takes a parameter of List of Custom Objects?

Answer 1

You can use Kotlin's declaredMemberFunctions property to find it based on its name alone:

data class CustomObject(val id: String)

class ClassToBeTested {
    private fun delete(
        someId1: String,
        aList: List<CustomObject>,
        someId2: String,
        someId3: String,
        someId4: String
    ): Long? {
        return someId1.toLongOrNull()
    }
}

class SampleTest {

    @Test
    fun test1() {
        val instance = ClassToBeTested()
        val method = instance::class.declaredMemberFunctions.first { it.name == "delete" }.apply { isAccessible = true }
        val result = method.call(instance, "1", emptyList<CustomObject>(), "2", "3", "4")
        assertEquals(1L, result)
    }
}

Answer 2

Don't use reflection for this. Simply make the method internal and call it directly from your tests. You can even add an annotation like @VisibleForTesting or @TestOnly , those are available in various libraries.