Testing private methods in Kotlin

Question

How to test private methods in Kotlin? I tried to add @VisibleForTesting(otherwise = VisibleForTesting.PRIVATE) from androidx.annotation.VisibleForTesting but it doesn’t make my function private

This is how I’m using it

@VisibleForTesting(otherwise = VisibleForTesting.PRIVATE)
fun doSomething() {}

[EDIT]

I understand that I shouldn't test private methods, but it's now always trivial. What about below case.

I have a CsvReader class

class CsvReader(private val inputStream: InputStream, private val separator: String = "\t") {
    fun read(): List<String> {
        return read(inputStream.bufferedReader())
    }
    private fun read(bufferedReader: BufferedReader): List<String> {
        val line = bufferedReader.use { it.readLine() } // `use` is like try-with-resources in Java
        return parse(line)
    }
    private fun parse(line: String): List<String> {
        return line.split(separator)
    }
}

And I wrote tests for it

class CsvReaderTest {
    private val stream = mock<InputStream>()
    private val reader = CsvReader(stream)
    private val bufferedReader = mock<BufferedReader>()
    @Test
    fun read() {
        whenever(bufferedReader.readLine()).thenReturn("Jakub\tSzwiec")
        reader.read(bufferedReader) shouldEqual listOf("Jakub", "Szwiec")
    }
    @Test
    fun readWhenEmpty() {
        whenever(bufferedReader.readLine()).thenReturn("")
        reader.read(bufferedReader) shouldEqual listOf("")
    }
    @Test
    fun throwIOExceptionWhenReadingProblems() {
        whenever(bufferedReader.readLine()).thenThrow(IOException::class.java)
        val read = { reader.read(bufferedReader) }
        read shouldThrow IOException::class
    }
}

Unfortunately, for tests I need to call private function fun read(bufferedReader: BufferedReader): List<String> because when mocking File, file.bufferedReader gives NullPointerException Unable to mock BufferedWriter class in junit

Answer 1

You can use java reflections:

To test the method:

class ClassUnderTest {
      private fun printGreetings(name: String): String {
        return "Hello, $name"
      }
}

That's enough:

  private val classUnderTest = spyk(ClassUnderTest()) 

  @Test
    fun `should return greetings`() {
      val method = classUnderTest.javaClass.getDeclaredMethod("printGreetings", String::class.java)
      method.isAccessible = true
      val parameters = arrayOfNulls<Any>(1)
      parameters[0] = "Piotr"

      assertEquals("Hello, Piotr", method.invoke(classUnderTest, *parameters) )
  }

Answer 2

like this:

fun callPrivate(objectInstance: Any, methodName: String, vararg args: Any?): Any? {
        val privateMethod: KFunction<*>? =
            objectInstance::class.functions.find { t -> return@find t.name == methodName }

        val argList = args.toMutableList()
        (argList as ArrayList).add(0, objectInstance)
        val argArr = argList.toArray()

        privateMethod?.apply {
            isAccessible = true
            return call(*argArr)
        }
            ?: throw NoSuchMethodException("Method $methodName does not exist in ${objectInstance::class.qualifiedName}")
        return null
    }

you need to pass instance of the object you want the method to be called on, method name and required arguments