TestCase - https://android.jlelse.eu/instrumentation-testing-with-dagger2-retrofit2-mockwebserver-android-4cca7cb7373c

BasicActivityTest.kt

@RunWith(AndroidJUnit4::class)
class BasicActivityTest {

    private lateinit var mockServer: MockWebServer

    @Rule
    @JvmField
    val rule = ActivityTestRule(DummyActivity::class.java, false, false)

    @Before
    fun setUp() {
        //  init mock server
        mockServer = MockWebServer()
        //  at port 8080
        mockServer.start(8080)
    }

    @After
    fun tearDown() {
        //  shut down
        mockServer.shutdown()
    }

    @Test
    fun happyTest() {
        mockServer.dispatcher = MockServerDispatcher.ResponseDispatcher()
        val intent = Intent(InstrumentationRegistry.getInstrumentation().targetContext, DummyActivity::class.java)
        //  passing valid MOCK url to the activity
        intent.putExtra(
            ConstantsUtil.MOCK_URL,
            mockServer.url("/todos/1").toString()
        )
        rule.launchActivity(intent)
        //  checks text in text view
        Espresso.onView(ViewMatchers.withId(R.id.dddd))
            .check(ViewAssertions.matches(ViewMatchers.withText("success!!!")))
    }

    @Test
    fun unHappyTest() {
        mockServer.dispatcher = MockServerDispatcher.ErrorDispatcher()
        val intent = Intent(InstrumentationRegistry.getInstrumentation().targetContext, DummyActivity::class.java)
        //  passing invalid MOCK url to the activity
        intent.putExtra(
            ConstantsUtil.MOCK_URL,
            mockServer.url("jack and jill URL").toString()
        )
        rule.launchActivity(intent)
        //  checks text in text view
        Espresso.onView(ViewMatchers.withId(R.id.dddd))
            .check(ViewAssertions.matches(ViewMatchers.withText("somethingWentWrong!!!")))
    }
}