CustomerInteractorTest.kt

@Mock
private lateinit var repo: CustomerRepo

private lateinit var customerUsecase: CustomerUsecase

@Before
fun setUp() {
    customerUsecase = CustomerInteractor(repo)
}
@Test
fun `should return customer data`() {
    val expectedCustomerData = mock(CustomerDetail::class.java)
    `when`(repo.fetchCustomerData()).thenReturn(Single.just(expectedCustomerData))
    customerUsecase.getCustomerData()
            .test()
            .assertComplete()
            .assertNoErrors()
            .assertValueCount(1)// Assert that this TestObserver/TestSubscriber received the specified number onNext events.
            .assertValue {
                assertEquals(expectedCustomerData, it)
                true
            }

    verify(repo).fetchCustomerDetail()
}

@Test
fun `should handle error for customer`() {
    val body = ResponseBody.create(MediaType.parse("application/json"), "Something went wrong")
    val expectedResponse = Response.error<Response>(500, body)
    `when`(repo.fetchCustomerDetail()).thenReturn(Single.error(HttpException(expectedResponse)))
    customerUsecase
            .getCustomerData()
            .test()
            .assertValueCount(0)
            .assertError {
                it is HttpException
            }

    verify(repo).fetchCustomerDatail()
}