How to test a private function in kotlin?

Question

I have two objects remote model and domain model

Remote model

data class InstanceRemoteModel (
    val id: String,
    val containers: List<Container>,
    val operations: List<Operation>
)

data class ContainerRemoteModel (
    val id: String,
    val capacity: Long
)

data class OperationRemoteModel (
    val id: String,
    val weight: Long,
    val containerId: String
)

Domain Model

data class Instance (
    val id: String,
    val containers: Map<String, Container>,
    val operations: Map<String, Operation>
)

data class Container (
    val id: String,
    val capacity: Long
    val total_weight: Long
)

data class Operation (
    val id: String,
    val weight: Long,
    val container: Container
)

And then I have a mapper that map objects from remote model to domain model. Here I will concentrate on mapping the containers. Note that a Container object in the domain model have different properties than ContainerRemoteModel. The same thing is done with the operations.

object Mapper {
  fun toDomainModel(remoteInstance: InstanceRemoteModel): Instance {
    val containers : Map<String, Container> = mapContainers(remoteInstance)
    val operations : Map<String, Operation> = mapOperations(remoteInstance)
    return Instance(remoteInstance.id,containers,operations)
  }



  private fun  mapContainers(remoteInstance: InstanceRemoteModel) : Map<String, Container> {
    //do some computations
    val containers : List<Container> = remoteInstance.containers.map { cont -> Container(...)}
    return containers.associateBy({ it.id }, { it }
  }

  private fun mapOperations(remoteInstance: InstanceRemoteModel): Map<String, Operation> {
    ...
  }
}

My question is : How to properly test the mapper? Note that mapOperations and mapContainers are private functions.

I added a comment below the answer. Please clarify where you are stuck. Thanks — matt freake, Dec 03 '20 at 22:08

Naor Tedgi · Accepted Answer · 2020-12-15T07:11:06.203

in unit testing, you need to test only the public methods! if a private function isn't covered by testing the class public functions then that function is an island and no one using it and the compiler will alert about it.

been says that if you still want to test this function

you can use reflection to access any field you want

by adding this dependency

<dependency>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-reflect</artifactId>
    <version>1.4.20</version>
    <scope>runtime</scope>
</dependency>

then you can test it

class MapperTest : TestCase(){

    @Test
    fun `test private method`(){
        val input = InstanceRemoteModel("test", emptyList(), emptyList())
        val returnValue = Mapper.invokePrivateFunction("mapContainers",input) as Map<String, Container>
        assert(returnValue.isEmpty())
    }

    inline fun <reified T> T.invokePrivateFunction(name: String, vararg args: Any?): Any? =
        T::class
            .declaredMemberFunctions
            .firstOrNull { it.name == name }
            ?.apply { isAccessible = true }
            ?.call(this, *args)
}

I am ok with the fact that I should not test private function. However the function `fun toDomainModel` is NOT private. So I am asking how to test it ? — LivBanana, Dec 02 '20 at 19:25
It's not clear what you are asking Giant8. You would test it the same as any function, by calling it with a variety of different inputs and checking that it behaves as wanted. Maybe if you write an example of you want to test it in the question? — matt freake, Dec 03 '20 at 07:51

How to test a private function in kotlin?

1 Answers1