Ship Android features you can change without fear. This intermediate course teaches test-driven development the way it works on a real Android codebase: write a failing test, make it pass with the simplest thing that could work, then refactor while the suite stays green. You will get fluent in the toolkit that actually matters — JUnit 5, assertion libraries, parameterized tests, and MockK for test doubles — and then go where Android gets hard. You will learn to keep the Android framework out of your unit tests so they run on the JVM in milliseconds, to test a ViewModel properly, to control coroutines with runTest and an injected TestDispatcher instead of sleeping, and to assert on Flow emissions with Turbine. You will learn why the Android community increasingly prefers hand-written fakes over mocks, where Robolectric and instrumented tests genuinely belong, and how to tame a legacy Activity with characterization tests. Assumes you already write Kotlin comfortably. By the end, tests will be how you design Android code, not a chore bolted on at the end.