Android Academy · Lesson

Unit Testing with JUnit

Write fast, reliable unit tests with JUnit 4. Use @Test, @Before, @After, assertions, test LiveData with InstantTaskExecutorRule, and test coroutines with runTest.

Unit Testing with JUnit is a free Android Academy lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Android Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why Write Tests?

Tests are not a luxury — they are your safety net:

Catch bugs before users do
Refactor with confidence (if tests pass, nothing broke)
Document intended behavior
Prevent regressions when adding features

Well-tested code also tends to be better-designed code — if it's hard to test, the design is probably too tightly coupled.

Types of Android Tests

Two categories:

Unit tests (test/ folder) — run on JVM, no device needed. Fast. Test plain Kotlin/Java logic.
Instrumented tests (androidTest/ folder) — run on a device or emulator. Slower. Test Android-specific code (Room DB, UI).

Prefer unit tests for business logic. Use instrumented only when you must interact with Android APIs.

JUnit 4 Setup

JUnit 4 is included by default in Android projects. Add these to app/build.gradle:

// app/build.gradle:
dependencies {
    testImplementation 'junit:junit:4.13.2'
    testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-test:1.7.3'
    testImplementation 'androidx.arch.core:core-testing:2.2.0'  // for LiveData
}

Your First JUnit Test

Create a class in src/test/java/ and annotate test methods with @Test:

import org.junit.Test
import org.junit.Assert.*

class CalculatorTest {

    private val calc = Calculator()

    @Test
    fun `addition returns correct sum`() {
        val result = calc.add(2, 3)
        assertEquals(5, result)
    }

    @Test
    fun `division by zero throws exception`() {
        assertThrows(ArithmeticException::class.java) {
            calc.divide(10, 0)
        }
    }
}

@Before and @After

Use lifecycle annotations to set up and tear down state around each test:

class UserRepositoryTest {

    private lateinit var repo: UserRepository
    private lateinit var fakeDao: FakeUserDao

    @Before
    fun setUp() {
        fakeDao = FakeUserDao()          // fresh instance before each test
        repo = UserRepository(fakeDao)
    }

    @After
    fun tearDown() {
        fakeDao.clear()                  // clean up after each test
    }

    @Test
    fun `getUsers returns all users`() {
        fakeDao.insert(User(1, "Alice"))
        fakeDao.insert(User(2, "Bob"))
        assertEquals(2, repo.getUsers().size)
    }
}

Common Assertions

JUnit provides assertion methods in Assert.*:

// Equality:
assertEquals(expected, actual)
assertNotEquals(unexpected, actual)

// Null:
assertNull(value)
assertNotNull(value)

// Boolean:
assertTrue(condition)
assertFalse(condition)

// Same object reference:
assertSame(expected, actual)

// Custom message on failure:
assertEquals("User ID should be 1", 1, user.id)

// Exception:
assertThrows(IllegalArgumentException::class.java) {
    User(id = -1, name = "")
}

Testing a ViewModel

To test LiveData, add the InstantTaskExecutorRule so LiveData posts synchronously:

class UserViewModelTest {

    @get:Rule
    val instantTaskRule = InstantTaskExecutorRule()

    private val fakeRepo = FakeUserRepository()
    private lateinit var viewModel: UserViewModel

    @Before
    fun setUp() {
        viewModel = UserViewModel(fakeRepo)
    }

    @Test
    fun `users are loaded on init`() {
        fakeRepo.usersToReturn = listOf(User(1, "Alice"), User(2, "Bob"))
        viewModel.loadUsers()

        val result = viewModel.users.value
        assertNotNull(result)
        assertEquals(2, result?.size)
    }
}

Testing Coroutines

Use TestCoroutineDispatcher / StandardTestDispatcher to control coroutine execution in tests:

class SyncWorkerTest {

    @get:Rule
    val instantTaskRule = InstantTaskExecutorRule()

    private val testDispatcher = StandardTestDispatcher()

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

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

    @Test
    fun `sync completes successfully`() = runTest {
        val repo = FakeRepository()
        val worker = SyncUseCase(repo, testDispatcher)
        val result = worker.run()
        assertTrue(result.isSuccess)
    }
}

Fake vs Mock

Two strategies for replacing real dependencies in tests:

Fake — a working implementation designed for testing (e.g., an in-memory list instead of a real database). Easy to write, readable, no library needed.
Mock — an auto-generated stub that records calls and lets you verify interactions (Mockito, MockK). More powerful, but can become brittle if overused.

Prefer Fakes for simple cases; use Mocks when you need to verify method calls.

Writing a Fake

A Fake implements the same interface as the real class but with in-memory storage:

interface UserDao {
    fun getAll(): List<User>
    fun insert(user: User)
}

class FakeUserDao : UserDao {
    private val storage = mutableListOf<User>()

    override fun getAll(): List<User> = storage.toList()
    override fun insert(user: User) { storage.add(user) }

    fun clear() { storage.clear() }
}

Test Naming Convention

Good test names describe what is being tested and the expected outcome. Use backtick function names in Kotlin for readable descriptions:

`getUsers returns empty list when no users exist`
`login throws exception when password is blank`
`add returns correct sum for negative numbers`

Pattern: `subject does X when Y`

Quick Check

What is the purpose of @Before in a JUnit test class?

Recap: Unit Testing with JUnit

Reliable apps are tested apps:

Unit tests go in src/test/, run on JVM — fast
Annotate with @Test, use assertEquals, assertTrue, assertThrows
@Before / @After for setup and teardown
InstantTaskExecutorRule for testing LiveData synchronously
runTest + StandardTestDispatcher for coroutine testing
Prefer Fakes over Mocks for simple substitutions

Next: Mocking with Mockito for verifying interactions.

Frequently asked questions

Is the “Unit Testing with JUnit” lesson free?

Yes — the full text of “Unit Testing with JUnit” is free to read here on the web, and the Android Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Android Academy course, upgrade to CoddyKit PRO.

What will I learn in “Unit Testing with JUnit”?

Write fast, reliable unit tests with JUnit 4. Use @Test, @Before, @After, assertions, test LiveData with InstantTaskExecutorRule, and test coroutines with runTest. You practise Android Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Android Academy?

No prior experience is required. Android Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Unit Testing with JUnit” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Android Academy lesson?

Yes. Every Android Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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