Android App Development Last-Minute Revision

30 quick revision notes for Android App Development

1. val is a read-only reference and var is mutable; a val list can still be mutable if its type is MutableList.
2. Kotlin separates nullable and non-null types; use ?., ?: and smart casts, and avoid !! in production code.
3. Data classes generate equals, hashCode, toString, copy and componentN only from primary constructor properties.
4. Sealed classes and interfaces give exhaustive when expressions and are ideal for modelling UI state and results.
5. apply and also return the object; let, run and with return the lambda result.
6. A suspend function does not move work off the main thread by itself; use withContext(Dispatchers.IO) for blocking calls.
7. Dispatchers.IO is for blocking I/O, Default for CPU-heavy work, and Main for UI updates.
8. viewModelScope is cancelled when the ViewModel is cleared and uses a SupervisorJob, so one failing child does not cancel others.
9. Never swallow CancellationException in a catch block; rethrow it so cancellation works correctly.
10. StateFlow always has a value and replays the latest; SharedFlow can have no value and configurable replay.
11. Use collectAsStateWithLifecycle in Compose and repeatOnLifecycle in views to collect flows safely.
12. Rotation destroys and recreates an Activity by default; ViewModel survives it but not process death.
13. SavedStateHandle and rememberSaveable survive process death; plain remember and ViewModel fields do not.
14. In fragments, observe with viewLifecycleOwner and clear view binding in onDestroyView to avoid leaks.
15. Android 13 needs runtime POST_NOTIFICATIONS permission, and Android 14 requires foreground service types.
16. Bundles and intents have a Binder limit of about 1 MB; pass IDs instead of large objects.
17. Compose recomposes only what reads changed state; composables must be fast and free of side effects.
18. Hoist state up and pass value plus event callbacks down to make composables stateless and reusable.
19. LaunchedEffect runs a coroutine tied to a key; DisposableEffect needs onDispose for cleanup.
20. Provide stable keys in LazyColumn items so state and animations follow the right item.
21. Repositories are the single source of truth; the UI observes Room flows while the network refreshes in the background.
22. Hilt @Provides builds third-party objects; @Binds maps an interface to an @Inject implementation.
23. Room migrations need a version bump and Migration SQL; test them with MigrationTestHelper.
24. DataStore replaces SharedPreferences with asynchronous, transactional, Flow-based storage that is safe to use from the main thread.
25. WorkManager is for deferrable guaranteed work with constraints; it is not for exact-time tasks.
26. Use an OkHttp Authenticator to refresh tokens on 401 and an interceptor to add auth headers.
27. ANRs come from blocking the main thread for around 5 seconds; track them in Android vitals.
28. LeakCanary finds retained Activities and Fragments; common leaks are static Context references and unremoved listeners.
29. Baseline Profiles precompile critical code paths and commonly improve startup by 20 to 30 percent.
30. The Play Store requires App Bundles, Play App Signing and a recent targetSdk; use staged rollouts for releases.

Night-before checklist

• Revise your saved questions on this page once, aloud.
• Read the quick notes and mark any you cannot explain in one line.
• Rehearse a two-minute self-introduction and two project stories.
• Keep CV, ID, laptop charger and interview link ready; sleep on time.