Trying to Unit Test an Object

With the logic of TDD in mind and trying to understand how to write unit test’s, I am having trouble with Kotlin object. The test pass but I am unsure if this is actually the right test. I am trying to make sure that the Logger.i() method is called and that it saves into the database. But at the moment I am stuck at just its called part.

My Object

object Logger {
    fun i(tag: String, msg: String, tr: Throwable? = null): Int  {
        insertIntoLogDatabase(createLogModel("i", tag, msg, tr))
        return if (BuildConfig.DEBUG) Log.i(tag, msg, tr) else 0

    private fun insertIntoLogDatabase(log: LogModel) {
        //Insert into Log DB

    private fun createLogModel(type: String, tag: String, msg: String, tr: Throwable?) = LogModel(0, type, tag, msg, if (tr != null) tr.message + "\n" + tr?.stackTrace.contentToString() else null)

    fun setLogRepo(logRepo: LogRepository) {
        this.logRepo = logRepo

with this, I know that I have to call Logger.setLogRepo(logRemp) to give the Logger access to the repo (and this works)

Where I am stuck is I am trying to unit test the Log.i method call

I have this

lateinit var log: Logger

fun setUp() {


fun `log i failed`() {

    // When
    log.i("Test", "Test1")

    // Then
    verify(log, times(1)).i("Test", "Test1")

I mean this works but is it correct (my gut tells me that something is wrong that I am not actually testing the Logger.i() method

please advise.


First, as far as I understand Mockito, you should use @Mock annotation if you’re mocking an interface. In your case you’re spying on an object, so @Spy would be the annotation to use.

Second, calling a method and then verifying it was called does not make much sense - unless method Logger.i(String, String) throws an exception your test will always pass.

Uusually you would call method Logger.i(String, String) and then verify if one of the methods that should be called down the line is actually called and if so, whether its called with correct arguments. In your case that would be methods: Logger.insertIntoLogDatabase(LogModel), Logger.createLogModel(...) and Log.i(...).

Also, since method Log.i(...) is called inside a conditional expression, you should have two tests - one where BuildConfig.DEBUG is true and the other where it’s false - and verify if said method is called or not called as expected. This way your tests will cover all possible execution paths, which is a desired outcome in TDD.

Here’s an article on spying in Mockito for further reading: Mockito - Using Spies

And another one on using Mockito with Kotlin: Kotlin with Mockito

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Yes, do not mock the Logger. You want to use the real Logger. So your “when” should be Logger.i("Test", "Test1").

Create a mock log repository and verify that it was called with the correct value. Using the Mockito-Kotlin library, that would look like:

verify(logRepo).upsert(check {
    assertEquals("i", it.type)
    assertEquals("Test", it.tag)
    assertEquals("Test1", it.msg)
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Thank you both for replying. Yes, first I was completely unaware of the Spy vs Mock (so thank you for the very helpful resources.

My solution was actually exactly as you had indicated

lateinit var logRepo: LogRepository

fun log i failed() {

// When
log.i("Test", "Test1")

// Then
verify(log, times(1)).upsert(any())


However to make the above work cause I tried using LogModel() inside the upsert but kept getting an error that things did not match (due to using a createDate that would always be different) I needed to add the following code from this site:

private fun <T> any(): T {
return uninitialized()
private fun <T> uninitialized(): T = null as T

Just curious, Because, the BuildConfig.DEBUG is static final, how would I go about creating a test that would verify what happens when it’s set to false?

I mean I know I can create this test

assertThat(“Returned class is not an Int”, Log.i(“Test”, “Test”), isA(

which verifies that an int is returned (which in either case is true). But for future knowledge and others who may be wonder also (let’s say I have a static final somewhere that the end result is different than the conditional expression equivalent, how would one go about

First, sorry for the long reply time - I was on vacation.

Second - unless the value of the constant is inlined during compilation, you can change it via reflection at the beginning of the test method.

While I don’y have an example on how to do it in a test (or in Kotlin), here’s how I used Java reflection to change options list (private static final if I recall correctly) in a H2 console in Spring Boot:

1 Like