A test sample on a use-case which returns an Arrow Either value

FetchNumberFactUcTest.kt

private const val DEFAULT_INTEGER_VALUE = -1
private const val DEFAULT_STRING_VALUE = "none"

@ExperimentalCoroutinesApi
class FetchNumberFactUcTest {

    private lateinit var usecase: DomainlayerContract.Presentation.UseCase<NumberFactRequest, NumberFactResponse>
    private lateinit var mockRepository: DomainlayerContract.Data.DataRepository<NumberFactResponse>

    @Before
    fun setUp() {
        mockRepository = mock()
        usecase = FetchNumberFactUc(numberDataRepository = mockRepository)
    }

    @After
    fun tearDown() {
    }

    @Test
    fun `Given null parameters, when usecase is invoked -- 'InputParamsError' is returned`() = runBlockingTest {
        // given
        val nullParams = null
        // when
        val response = usecase.run(params = nullParams)
        // then
        Assert.assertTrue(response.isLeft() && (response as? Either.Left<Failure>)?.a is Failure.InputParamsError)
    }

    // TODO: this test fails because the 'repository' instance is not accessible, i.e. cannot be mocked/stubbed
    @Test
    fun `Given right parameters, when usecase is invoked -- 'NumberFactResponse' data is returned`() = runBlockingTest {
        // given
        val rightParams = NumberFactRequest(number = DEFAULT_INTEGER_VALUE)
        whenever(mockRepository.fetchNumberFact(request = rightParams)).doReturn(getDummyNumberFactResponse().right())
        // when
        val response = usecase.run(params = rightParams)
        // then
        Assert.assertTrue(response.isRight() && (response as? Either.Right<NumberFactResponse>) != null)
    }

    private fun getDummyNumberFactResponse() = NumberFactResponse(fact = DEFAULT_STRING_VALUE)

}