Testing Android Apps

Relax. We'll talk through this in plain words — no textbook voice.

What you'll walk away with

Understand Testing Android Apps without the intimidation
Get comfortable running Android Studio/Compose code yourself
Apply this concept in a real project right away

Let's think about it this way for a second

A Unit Test checks an individual function/class (ViewModel logic, data transformation) without needing a UI/device — it uses the JUnit framework, runs fast, and can be run frequently in a CI pipeline. A UI Test (Compose Test), on the other hand, automates actual UI interactions (button clicks, text input) on an emulator/device — you can use `composeTestRule` to verify things like 'does the text change when I click this button.'

Let's connect this to a real scenario

To unit test the logic in `TodoViewModel.addTodo()`, you could write `@Test fun addTodo_increasesListSize() { viewModel.addTodo("Test"); assertEquals(1, viewModel.todos.size) }` — for a UI test, you could write `composeTestRule.onNodeWithText("Add").performClick(); composeTestRule.onNodeWithText("Test").assertIsDisplayed()` to verify that the todo item actually shows up on screen after clicking the button.

Unit Test = tests function/class logic without a UI/device (JUnit, fast)
UI Test (Compose Test) = automates actual UI interactions on an emulator
composeTestRule = the core testing API for writing Compose UI tests

Let's walk through it together

kotlin

// Unit test
class TodoViewModelTest {
    @Test
    fun addTodo_increasesListSize() {
        val viewModel = TodoViewModel(fakeDao)
        viewModel.addTodo("Buy milk")
        assertEquals(1, viewModel.todos.size)
    }
}

// Compose UI test
class TodoScreenTest {
    @get:Rule val composeTestRule = createComposeRule()

    @Test
    fun clickingAddButton_showsNewTodo() {
        composeTestRule.setContent { TodoScreen() }
        composeTestRule.onNodeWithText("Add").performClick()
        composeTestRule.onNodeWithText("Buy milk").assertIsDisplayed()
    }
}

You should see

$ ./gradlew test
BUILD SUCCESSFUL — 2 tests passed

5-minute try-it

Write a unit test yourself for `TodoViewModel`'s `addTodo()` function, and run it with Android Studio's 'Run Test' button.

A quick word of caution

Don't chase 100% test coverage — prioritize testing critical business logic (payments, data integrity); tests for UI details (color, spacing) tend to have a low ROI.

Easy traps

Putting off tests with 'I'll write them after I finish the code' — as features pile up, manual re-testing just keeps taking longer
Running a UI test directly against a real network/database — this makes the test flaky (passing sometimes, failing others); you should use a fake/mock dependency instead

Now try it yourself

Write a unit test yourself for `TodoViewModel`'s `addTodo()` function, and run it with Android Studio's 'Run Test' button.

You'll know it worked when: $ ./gradlew test BUILD SUCCESSFUL — 2 tests passed