Test Strategy & Pyramid

Copy code        /\
       /E2E\         ← Few: slow, fragile, but test real user flows
      /------\
     /Integr- \      ← Some: test component interactions
    /  ation   \
   /------------\
  /  Unit Tests  \   ← Many: fast, isolated, cheap to maintain
 /________________\

Copy codeclass PriceFormatterTest {
    private val formatter = PriceFormatter(currency = "USD")

    @Test
    fun `formats integer price with dollar sign`() {
        assertEquals("$5.00", formatter.format(500))  // cents
    }

    @Test
    fun `formats zero as free`() {
        assertEquals("Free", formatter.format(0))
    }

    @Test
    fun `formats negative price throws`() {
        assertThrows<IllegalArgumentException> { formatter.format(-1) }
    }
}

Copy code@RunWith(AndroidJUnit4::class)
class ArticleRepositoryTest {
    @get:Rule val instantTaskExecutorRule = InstantTaskExecutorRule()

    private lateinit var db: AppDatabase
    private lateinit var repository: ArticleRepository

    @Before
    fun setup() {
        db = Room.inMemoryDatabaseBuilder(
            ApplicationProvider.getApplicationContext(),
            AppDatabase::class.java
        ).build()
        repository = ArticleRepository(db.articleDao(), FakeApi())
    }

    @After
    fun teardown() = db.close()

    @Test
    fun `insert and retrieve article`() = runTest {
        repository.saveArticle(testArticle)
        val result = repository.getArticle(testArticle.id)
        assertEquals(testArticle, result)
    }
}

Copy code@RunWith(AndroidJUnit4::class)
class LoginFlowTest {
    @get:Rule val composeTestRule = createAndroidComposeRule<MainActivity>()

    @Test
    fun `successful login navigates to home`() {
        composeTestRule.onNodeWithTag("email_field")
            .performTextInput("user@example.com")
        composeTestRule.onNodeWithTag("password_field")
            .performTextInput("password123")
        composeTestRule.onNodeWithTag("login_button")
            .performClick()

        composeTestRule.onNodeWithText("Welcome back!").assertIsDisplayed()
    }
}