How come the test case is still passing even if I have not provided correct mocking in my opinion - mockito

Question

I am testing this function. The main bit for me is the call to add method of a respository (partitionsOfATagTransactionRepository.add(transaction, infoToAdd,mutationCondition))
def updateOrCreateTagPartitionInfo(transaction:DistributedTransaction,currentTagPartition: Option[TagPartitions], tag: String) = {
val currentCalendar = Calendar.getInstance() //TODOM - should I use a standard Locale/Timezone (eg GMT) to keep time consistent across all instances of the server application
val currentYear = currentCalendar.get(Calendar.YEAR).toLong
val currentMonth = currentCalendar.get(Calendar.MONTH).toLong
val newTagParitionInfo = TagPartitionsInfo(currentYear.toLong, currentMonth.toLong)
val (infoToAdd,mutationCondition) = currentTagPartition match {
case Some(tagPartitionInfo) => {
//checktest-should add new tag partition info to existing partition info
(TagPartitions(tagPartitionInfo.tag, tagPartitionInfo.partitionInfo + (newTagParitionInfo)),new PutIfExists)
}
case None => {
//checktest-should add new tag partition info if existing partition doesn't exist
(TagPartitions(tag, Set(newTagParitionInfo)),new PutIfNotExists)
}
}
partitionsOfATagTransactionRepository.add(transaction, infoToAdd,mutationCondition) //calling a repositoru method which I suppose needs mocking
infoToAdd
}
I wrote this test case to test the method
"should add new tag partition info if existing partition doesn't exist" in {
val servicesTestEnv = new ServicesTestEnv(components = components)
val questionTransactionDBService = new QuestionsTransactionDatabaseService(
servicesTestEnv.mockAnswersTransactionRepository,
servicesTestEnv.mockPartitionsOfATagTransactionRepository,
servicesTestEnv.mockPracticeQuestionsTagsTransactionRepository,
servicesTestEnv.mockPracticeQuestionsTransactionRepository,
servicesTestEnv.mockSupportedTagsTransactionRepository,
servicesTestEnv.mockUserProfileAndPortfolioTransactionRepository,
servicesTestEnv.mockQuestionsCreatedByUserRepo,
servicesTestEnv.mockTransactionService,
servicesTestEnv.mockPartitionsOfATagRepository,
servicesTestEnv.mockHelperMethods
)
val currentCalendar = Calendar.getInstance() //TODOM - should I use a standard Locale/Timezone (eg GMT) to keep time consistent across all instances of the server application
val currentYear = currentCalendar.get(Calendar.YEAR).toLong
val currentMonth = currentCalendar.get(Calendar.MONTH).toLong
val newTagParitionInfo = TagPartitionsInfo(currentYear.toLong, currentMonth.toLong)
val existingTag = "someExistingTag"
val existingTagPartitions = None
val result = questionTransactionDBService.updateOrCreateTagPartitionInfo(servicesTestEnv.mockDistributedTransaction,
existingTagPartitions,existingTag) //calling the funtion under test but have not provided mock for the repository's add method. The test passes! how? Shouldn't the test throw Null Pointer exception?
val expectedResult = TagPartitions(existingTag,Set(newTagParitionInfo))
verify(servicesTestEnv.mockPartitionsOfATagTransactionRepository,times(1))
.add(servicesTestEnv.mockDistributedTransaction,expectedResult,new PutIfNotExists())
result mustBe expectedResult
result mustBe TagPartitions(existingTag,Set(newTagParitionInfo))
}
The various mocks are defined as
val mockCredentialsProvider = mock(classOf[CredentialsProvider])
val mockUserTokenTransactionRepository = mock(classOf[UserTokenTransactionRepository])
val mockUserTransactionRepository = mock(classOf[UserTransactionRepository])
val mockUserProfileAndPortfolioTransactionRepository = mock(classOf[UserProfileAndPortfolioTransactionRepository])
val mockHelperMethods = mock(classOf[HelperMethods])
val mockTransactionService = mock(classOf[TransactionService])
val mockQuestionsCreatedByUserRepo = mock(classOf[QuestionsCreatedByAUserForATagTransactionRepository])
val mockQuestionsAnsweredByUserRepo = mock(classOf[QuestionsAnsweredByAUserForATagTransactionRepository])
val mockDistributedTransaction = mock(classOf[DistributedTransaction])
val mockQuestionTransactionDBService = mock(classOf[QuestionsTransactionDatabaseService])
val mockQuestionNonTransactionDBService = mock(classOf[QuestionsNonTransactionDatabaseService])
val mockAnswersTransactionRepository = mock(classOf[AnswersTransactionRepository])
val mockPartitionsOfATagTransactionRepository = mock(classOf[PartitionsOfATagTransactionRepository])
val mockPracticeQuestionsTagsTransactionRepository = mock(classOf[PracticeQuestionsTagsTransactionRepository])
val mockPracticeQuestionsTransactionRepository = mock(classOf[PracticeQuestionsTransactionRepository])
val mockSupportedTagsTransactionRepository = mock(classOf[SupportedTagsTransactionRepository])
val mockPartitionsOfATagRepository = mock(classOf[PartitionsOfATagRepository])
The test case passes even though I have not provided any mock for partitionsOfATagTransactionRepository.add. Should I get a NullPointer exception when the add method is called.
I was expecting that I would need to write something like doNothing().when(servicesTestEnv.mockPartitionsOfATagTransactionRepository).add(ArgumentMatchers.any[DistributedTransaction],ArgumentMatchers.any[TagPartitions],ArgumentMatchers.any[MutationCondition]) or when(servicesTestEnv.mockPartitionsOfATagTransactionRepository).add(ArgumentMatchers.any[DistributedTransaction],ArgumentMatchers.any[TagPartitions],ArgumentMatchers.any[MutationCondition]).thenReturn(...) for the test case to pass.

Answer 1

Mockito team made a decision to return default value for a method if no stubbing is provided.
See: https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html#stubbing
By default, for all methods that return a value, a mock will return either null, a primitive/primitive wrapper value, or an empty collection, as appropriate. For example 0 for an int/Integer and false for a boolean/Boolean.
This decision was made consciously: if you are focusing on a different aspect of behaviour of method under test, and the default value is good enough, you don't need to specify it.
Note that other mocking frameworks have taken opposite path - they raise an exception when unstubbed call is detected (for example: EasyMock).
See EasyMock vs Mockito: design vs maintainability?