unit test coroutine nullpointerexception when mocking - unit-testing

Question

I am unit testing the following class
class LoadTrendingSearchUseCaseImp #Inject constructor(
private val searchCriteriaProvider: SearchCriteriaProvider,
private val coroutineDispatcherProvider: CoroutineDispatcherProvider
) : LoadTrendingSearchUseCase {
override suspend fun execute(): List<String> {
return withContext(coroutineDispatcherProvider.io()) {
searchCriteriaProvider.provideTrendingSearch().trendingSearches
}
}
}
interface SearchCriteriaProvider {
suspend fun provideTrendingSearch(): CatalogSearchPage
}
class SearchCriteriaProviderImp() : SearchCritieraProvider {
override suspend fun provideTrendingSearch(): CatalogSearchPage {
return withContext(coroutineDispatcherProvider.io()) {
/* long running task */
}
}
}
interface CoroutineDispatcherProvider {
fun io(): CoroutineDispatcher = Dispatchers.IO
fun default(): CoroutineDispatcher = Dispatchers.Default
fun main(): CoroutineDispatcher = Dispatchers.Main
fun immediate(): CoroutineDispatcher = Dispatchers.Main.immediate
fun unconfined(): CoroutineDispatcher = Dispatchers.Unconfined
}
class CoroutineDispatcherProviderImp #Inject constructor() : CoroutineDispatcherProvider
This is my actual test:
class LoadTrendingSearchUseCaseImpTest {
private val searchCriteriaProvider: SearchCriteriaProvider = mock()
private val coroutineDispatcherProvider = CoroutineDispatcherProviderImp()
private lateinit var loadTrendingSearchUseCaseImp: LoadTrendingSearchUseCaseImp
#Before
fun setUp() {
loadTrendingSearchUseCaseImp = LoadTrendingSearchUseCaseImp(
searchCriteriaProvider,
coroutineDispatcherProvider
)
}
#Test
fun `should provide trending searches`() {
runBlockingTest {
// Arrange
// EXCEPTION HERE whenever(searchCriteriaProvider.provideTrendingSearch().trendingSearches).thenReturn(
emptyList()
)
// Act
val actualResult = loadTrendingSearchUseCaseImp.execute()
// Assert
assertThat(actualResult).isEmpty()
}
}
}
The actual error message:
java.lang.NullPointerException
.product_search.usecase.imp.LoadTrendingSearchUseCaseImpTest$should provide trending searches$1.invokeSuspend(LoadTrendingSearchUseCaseImpTest.kt:30)

Answer 1

You tried to chain invocations when stubbing a method.
whenever(searchCriteriaProvider.provideTrendingSearch().trendingSearches)
.thenReturn(emptyList())
During stubbing, the actual methods are being called.
searchCriteriaProvider.provideTrendingSearch() returns null, as this call is not stubbed yet
subsequent call null.trendingSearches results in NPE
You need to stub each call in the chain
whenever(searchCriteriaProvider.provideTrendingSearch())
.thenReturn(catalogSearchPage)
whenever(catalogSearchPage.trendingSearches)
.thenReturn(emptyList())
Obviously, this assumes that
catalogSearchPage is also a mock
trendingSearches is a property
Alternatively, you can construct a POJO for catalogSearchPage, and return it in the first stubbing.