Developing Unit tests for an Android application (JUnit)

Unit Test Development for Android Application (JUnit)

Android project without unit tests — project where scary to touch Repository or ViewModel, because unclear what breaks. JUnit 5 + Mockito + coroutines give tools to cover all business logic: fast JVM tests without emulator, isolated, reproducible.

Tool Stack

MockK preferable to Mockito for Kotlin code: correctly mocks object, companion object and suspend-functions without runBlocking hacks.

ViewModel Testing with Coroutines

Main difficulty — ViewModel works with coroutines on Dispatchers.Main, which doesn't exist in JVM tests. Solution — TestDispatcher:

@OptIn(ExperimentalCoroutinesApi::class)
class UserViewModelTest {
    private val testDispatcher = UnconfinedTestDispatcher()

    @Before
    fun setup() {
        Dispatchers.setMain(testDispatcher)
    }

    @After
    fun tearDown() {
        Dispatchers.resetMain()
    }

    @Test
    fun `loadUser emits success state`() = runTest {
        val mockRepo = mockk<UserRepository>()
        coEvery { mockRepo.getUser("1") } returns User(id = "1", name = "Test")
        val viewModel = UserViewModel(mockRepo)

        viewModel.loadUser("1")

        assertEquals(UiState.Success(User(id = "1", name = "Test")), viewModel.uiState.value)
    }
}

UnconfinedTestDispatcher executes coroutines immediately, StandardTestDispatcher — only on advanceUntilIdle(). For timing tests (debounce, delay) use advanceTimeBy(ms).

Testing Kotlin Flow via Turbine

@Test
fun `state flow emits loading then success`() = runTest {
    val mockRepo = mockk<UserRepository>()
    coEvery { mockRepo.getUser(any()) } coAnswers {
        delay(100)
        User(id = "1", name = "Test")
    }
    val viewModel = UserViewModel(mockRepo)

    viewModel.uiState.test {
        assertEquals(UiState.Loading, awaitItem())
        viewModel.loadUser("1")
        assertEquals(UiState.Success(User("1", "Test")), awaitItem())
        cancelAndIgnoreRemainingEvents()
    }
}

Turbine (app.cash.turbine) — most convenient way to check sequence of emissions from StateFlow/SharedFlow without callback mess.

Repository and UseCase

Repository test isolated from ViewModel. Mock DataSource (remote and local), check caching logic, DTO → Entity mapping, error handling:

@Test
fun `getUser returns cached data when network fails`() = runTest {
    coEvery { remoteDataSource.getUser(any()) } throws IOException("No network")
    coEvery { localDataSource.getUser("1") } returns UserEntity(id = "1", name = "Cached")

    val result = repository.getUser("1")

    assertTrue(result.isSuccess)
    assertEquals("Cached", result.getOrNull()?.name)
}

What Often Not Tested but Should Be

• Mapper classes — seems trivial, but exactly where nullable fields are lost and date-format incorrectly handled
• Extension functions — especially ones formatting strings, dates, numbers
• Pagination logic in PagingSource — PagingSource.LoadResult can be tested directly via TestPagingSource

CI Integration

./gradlew test runs all unit tests without emulator. Coverage via JaCoCo: ./gradlew jacocoTestReport. In GitHub Actions — matrix of JDK versions (17 + 21). Publish results as Test Report artifact for PR review.

Timeframe: 3–5 days depending on project size and current architecture.