Android Tech Lead Interview Questions &amp; Answers

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@Provides is used in a module function that constructs and returns an object, needed for third-party classes like Retrofit or Room that you cannot annotate with @Inject. @Binds is used on an abstract function in an abstract module to tell Hilt which implementation to use for an interface, when the implementation already has an @Inject constructor. @Binds generates less code and is slightly more efficient.

How do you test Retrofit networking code?

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Use OkHttp MockWebServer to run a local server, enqueue responses with specific status codes, bodies and delays, and point Retrofit's base URL to it. Then call the API or repository and assert parsing, headers, error mapping and retry behaviour. Store sample JSON in test resources. For ViewModel tests, replace the repository with a fake instead of using MockWebServer, which keeps them fast.

Write a Room database class with a migration that adds a column.

Room, DataStore and WorkManagerMedium

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@Database(entities = [NoteEntity::class], version = 2, exportSchema = true) abstract class AppDatabase : RoomDatabase() { abstract fun noteDao(): NoteDao } val MIGRATION_1_2 = object : Migration(1, 2) { override fun migrate(db: SupportSQLiteDatabase) { db.execSQL("ALTER TABLE notes ADD COLUMN pinned INTEGER NOT NULL DEFAULT 0") } } val db = Room.databaseBuilder(context, AppDatabase::class.java, "app.db") .addMigrations(MIGRATION_1_2) .build()

Your app has 60 Gradle modules and builds take 15 minutes. How would you speed them up?

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Profile with Gradle build scans to find slow tasks. Enable the configuration cache and build cache, including a remote cache in CI. Use KSP instead of kapt, avoid unnecessary api dependencies so changes do not recompile many modules, use convention plugins, and keep non-transitive R classes. Check heavy annotation processors and give Gradle enough memory. Developers can build only the module they work on with a sample app.

How does certificate pinning work and what are its risks?

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Certificate pinning makes the app trust only specific public key hashes for your domain, using OkHttp CertificatePinner or the Network Security Config pin-set. It protects against attackers with a rogue or user-installed CA intercepting traffic. The risk is that if the server certificate changes and the pin does not match, all users lose connectivity until they update. Always pin a backup key and set an expiration for config-based pins.

Write an OkHttp interceptor that adds a bearer token to every request.

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class AuthInterceptor @Inject constructor( private val tokenStore: TokenStore ) : Interceptor { override fun intercept(chain: Interceptor.Chain): Response { val token = tokenStore.currentAccessToken() val request = chain.request().newBuilder().apply { if (token != null) header("Authorization", "Bearer $token") }.build() return chain.proceed(request) } } val client = OkHttpClient.Builder() .addInterceptor(authInterceptor) .build()

A payment SDK integration fails only in release builds. What would you check?

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First check R8 or ProGuard: release builds shrink and obfuscate code, and the SDK may need keep rules for classes used by reflection or JSON parsing. Check the SDK documentation for required rules and add them to proguard-rules.pro. Also verify the release signing key SHA is registered with the provider, the correct production keys and base URLs are used, and network security config allows the domains.

How would you implement real-time order tracking updates in an app?

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Options are polling, WebSockets, Server-Sent Events, or push notifications. For a live tracking screen, a WebSocket via OkHttp exposed as a callbackFlow works well, started when the screen is visible and closed when it leaves. Add reconnection with backoff, heartbeat pings, and fall back to polling on failure. When the app is in background, rely on FCM pushes for status changes instead of keeping the socket open.

Compare kotlinx.serialization, Moshi and Gson for JSON parsing.

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kotlinx.serialization is Kotlin-first, uses a compiler plugin with no reflection, respects default values and nullability, and works in Kotlin Multiplatform. Moshi supports Kotlin through codegen and is also reliable. Gson uses reflection, ignores Kotlin nullability and default values, and can put null into non-null fields, causing crashes later; it is in maintenance mode. For new Android projects, kotlinx.serialization is the common choice.

How would you roll out a risky new feature to millions of users safely?

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Ship it behind a remote feature flag defaulted off, release the build with a staged rollout and monitor crash-free and ANR rates. Then enable the flag for internal users, then 1, 5, 25 and 100 percent, watching business metrics and errors at each step. Keep a kill switch ready, define rollback criteria beforehand, and keep the old path until the new one is stable.

What are Baseline Profiles?

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A Baseline Profile is a list of classes and methods used in critical journeys like startup and scrolling, shipped with the app. The Android Runtime uses it to compile that code ahead of time on install, avoiding slow interpretation and JIT on first runs. You generate it with the Baseline Profile Gradle plugin and Macrobenchmark tests. Compose and many Jetpack libraries ship their own profiles, but an app-specific one gives larger gains.

What is SavedStateHandle and when do you use it?

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SavedStateHandle is a key-value map available to a ViewModel that is saved into the instance state Bundle, so it survives process death as well as configuration changes. Navigation arguments are also placed in it automatically. Use it for small, essential UI state like a search query, selected filter or the ID of the item being edited. Do not put large lists in it; reload those from the repository.

The API team wants to switch to GraphQL. What changes on the Android side?

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You would use a client like Apollo Kotlin, which generates type-safe models from the schema and queries at build time, and supports normalised caching and coroutines. Benefits are fetching exactly the fields a screen needs and fewer round trips. Costs include learning the tooling, more complex caching and error handling since errors can come with partial data. I would pilot it on one feature before migrating everything.

How do you build an adaptive layout for phones, foldables and tablets in Compose?

Jetpack ComposeHard

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Use WindowSizeClass from the Material 3 adaptive library to choose layouts by width class, not device type. On compact width show a bottom navigation bar; on medium use a navigation rail; on expanded use a list-detail layout with ListDetailPaneScaffold or NavigationSuiteScaffold. Avoid locking orientation, since Android 16 ignores orientation restrictions on large screens. Test with resizable emulators and foldable postures.

Describe a time you improved app performance with measurable results.

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Situation: our home screen had many frozen frames on budget phones. Task: improve scroll smoothness. Action: I captured a system trace, found image decoding on the main thread and unstable list parameters causing recomposition, fixed image sizing with Coil, added keys and immutable models, and created a Baseline Profile. Result: frozen frames dropped by about 60 percent and startup improved by 25 percent in Macrobenchmark.

Users on slow 2G and 3G networks in small towns report the app keeps failing. How do you improve it?

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Set reasonable connect and read timeouts, retry idempotent requests with exponential backoff, and cache responses so screens show something offline. Reduce payload size with gzip, pagination and fields filtering, and load images in smaller sizes with placeholders. Show clear offline and retry states instead of generic errors. Test with the emulator network throttling and real devices on poor connections, and track failure rates by network type.

Network calls keep running after the user leaves a screen and results arrive late. How do you fix this?

Coroutines and FlowMedium

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The work is likely launched in GlobalScope, a custom scope never cancelled, or an Application-level scope. Move it to viewModelScope so it is cancelled when the ViewModel is cleared, and make sure the calls are cancellable, which Retrofit suspend functions are. If work must outlive the screen, like uploading a file, move it to WorkManager or an application scope deliberately, and do not update UI from it.

Explain the three phases of a Compose frame.

Jetpack ComposeHard

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Each frame goes through composition, which decides what UI to show by running composables; layout, which measures and places each element; and drawing, which renders pixels. State read in a phase only invalidates that phase and later ones. Reading scroll state inside Modifier.offset { } or drawBehind defers the read to layout or draw, skipping recomposition entirely, which is a key optimisation for animations and scrolling effects.

Your banking app must pass a security audit. What would you check on the Android side?

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Check secure storage with Keystore and no secrets in code, TLS only with pinning where justified, no sensitive data in logs, FLAG_SECURE on sensitive screens, root and tamper checks using Play Integrity, exported components reviewed, WebView with JavaScript bridges restricted, R8 obfuscation, session timeout, and biometric authentication with BiometricPrompt and CryptoObject. Use the OWASP MASVS as the checklist and fix findings by severity.

Write a use case that combines cart items and a coupon into a final total.

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class GetCartTotalUseCase @Inject constructor( private val cartRepo: CartRepository, private val couponRepo: CouponRepository ) { operator fun invoke(): Flow<Double> = combine(cartRepo.items, couponRepo.activeCoupon) { items, coupon -> val subtotal = items.sumOf { it.price * it.qty } val discount = coupon?.let { minOf(subtotal * it.percent / 100, it.maxOff) } ?: 0.0 subtotal - discount } }

The app's cold start takes 4 seconds. How do you improve it?

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Measure first with Macrobenchmark StartupTimingMetric on a release build. Then reduce work in Application.onCreate by deferring SDK initialisation, using App Startup or lazy injection, and moving disk I/O off the main thread. Simplify the first screen, avoid blocking on network before showing UI, use the SplashScreen API properly, and add a Baseline Profile, which often improves startup by about 20 to 30 percent.

Describe a situation where you had to push back on an unrealistic deadline.

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Situation: product wanted offline mode in one week. Task: give a realistic plan. Action: I broke the work into tasks with estimates, showed that sync conflicts and migrations alone needed two weeks, and proposed a smaller first release with read-only offline caching in one week. Result: we shipped the smaller version on time and the full offline sync three weeks later without quality issues.

How do you design an Android client to support API versioning and forced updates?

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Send the app version in a header on every request, have the backend support older versions for a defined period, and use versioned endpoints for breaking changes. Add a remote config value for minimum supported version and use the Play In-App Updates API with immediate flow for critical updates and flexible flow otherwise. Show a clear message rather than failing silently when the app is too old.

How do you measure app quality after release?

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Track crash-free users and ANR rate in Play vitals and Crashlytics, keeping them below Play's bad behaviour thresholds of 1.09 percent user-perceived crash rate and 0.47 percent ANR rate. Monitor startup time, slow and frozen frames, network error rates and key funnel metrics like login success. Read Play reviews and ratings by version. Set alerts and review these numbers in every release meeting.

Where should API keys go in an Android app?

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Anything shipped in the APK can be extracted, so treat client-side keys as public. Keep them out of version control using local.properties or CI secrets injected into BuildConfig, restrict them in the provider console by package name and SHA-1, and keep truly secret keys on your backend, which proxies the calls. Use Play Integrity or App Check to reduce abuse from modified apps.

Users report battery drain after the latest update. How do you investigate?

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Check Android vitals for excessive wake locks and wakeups, then use Battery Historian or the Energy Profiler to look at wake locks, alarms, location requests, jobs and network activity. Common causes are frequent location updates, a foreground service left running, polling instead of push, and WorkManager jobs scheduled too often. Fix by batching work, using lower location priority, stopping services promptly and adding proper constraints.

What is the difference between MVVM and MVI?

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Both separate UI from logic. MVI (Model-View-Intent) is stricter: the UI sends intents or actions, a reducer produces a single new immutable state, and side effects are handled separately. It gives one state object per screen and an easy-to-replay history. MVVM is more flexible, sometimes exposing several streams. In practice many Compose apps use MVVM with a single UiState data class, which is close to MVI.

How do you structure a large Android app into modules?

App Architecture, MVVM and HiltHard

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Common practice is feature modules (feature:cart, feature:profile), core modules (core:network, core:database, core:designsystem, core:model) and an app module that wires them together. Feature modules depend on core modules but not on each other; navigation is connected through the app module or route contracts. Benefits are faster incremental builds, clear ownership and enforced boundaries. Gradle convention plugins keep module build files consistent.

Product wants a feature flag to turn a new checkout on or off without releasing an update. How do you design it?

App Architecture, MVVM and HiltHard

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Use a remote config service with defaults bundled in the app, wrapped in a FeatureFlags interface injected via Hilt. Fetch and cache values at startup with a sensible timeout, and read the flag at navigation decision points rather than deep inside UI code. Keep both checkout paths tested, log which variant users see for analytics, and plan to remove the old path and the flag after rollout.

A ViewModel has grown to 1500 lines handling many responsibilities. How would you refactor it?

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First add tests around current behaviour so refactoring is safe. Then extract business logic into use cases or domain classes, move data mapping into repositories or mappers, and split UI state into smaller, focused data classes. If the screen truly has separate areas, consider separate state holders or child ViewModels per section. Refactor in small pull requests so reviewers can follow and regressions are easy to locate.

How do you mentor junior Android developers?

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I pair on their first tasks, explain the why behind patterns rather than only giving fixes, and review pull requests with specific, kind comments plus links to docs. I give them small features end to end so they see the full flow from API to UI, and I ask them to present one learning in our weekly tech sync. I check in regularly so they feel safe asking questions early.

How do you find and fix memory leaks in an Android app?

Testing, Performance and Play StoreMedium

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Add LeakCanary to debug builds; it detects retained Activities, Fragments and ViewModels and shows the reference chain. Common leaks are static or singleton references to Context or Views, listeners not unregistered, inner classes holding Activity references, and Fragment view bindings not cleared in onDestroyView. Fix by using application context, lifecycle-aware components and weak references only where needed. Confirm with Memory Profiler heap dumps.

The APK size grew to 120 MB and installs from small towns are dropping. How do you reduce it?

Testing, Performance and Play StoreMedium

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Analyse the build with APK Analyzer. Publish as an App Bundle so users get only their density, language and ABI. Enable R8 and resource shrinking, convert images to WebP or vectors, remove unused libraries and duplicate dependencies, and move large optional features or assets to Play Feature Delivery or Play Asset Delivery. Avoid bundling all native libraries for every ABI. Track size in CI.

What is a use case in the domain layer and when is it worth adding?

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A use case is a class with one job, like GetDiscountedCartTotalUseCase, typically exposing operator fun invoke. It holds business logic that combines repositories or is reused by several ViewModels. It keeps ViewModels thin and logic testable. For simple apps where the ViewModel only calls one repository method, use cases add boilerplate without benefit, so the domain layer is optional in the official guidance.

Tell me about a time you worked closely with a backend team to solve a problem.

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Situation: our feed took four seconds to load because the app made six API calls. Task: reduce load time. Action: I measured each call with network profiling, shared a timeline with the backend team, and we agreed on one aggregated endpoint with only the fields the screen used, plus pagination. I updated the client and added caching. Result: load time dropped to under one second and both teams reused the approach.

Write a Hilt module that provides Retrofit and a repository binding.

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@Module @InstallIn(SingletonComponent::class) object NetworkModule { @Provides @Singleton fun provideRetrofit(): Retrofit = Retrofit.Builder() .baseUrl("https://api.example.com/") .addConverterFactory(Json.asConverterFactory("application/json".toMediaType())) .build() @Provides @Singleton fun provideApi(retrofit: Retrofit): ShopApi = retrofit.create(ShopApi::class.java) } @Module @InstallIn(SingletonComponent::class) abstract class RepoModule { @Binds abstract fun bindRepo(impl: ProductRepositoryImpl): ProductRepository }

Your app shows duplicate orders when users tap Pay twice on a slow network. How do you fix it?

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Disable the button and show progress once tapped, but do not rely only on the UI. Generate an idempotency key on the client for each checkout attempt and send it with the request so the server returns the same result for retries. Ensure the payment call is not retried automatically by OkHttp for non-idempotent POSTs, and let the user check order status if the response is lost.

Write a HiltViewModel that exposes a single UiState using StateFlow.

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data class ProductUiState( val loading: Boolean = true, val items: List<Product> = emptyList(), val error: String? = null ) @HiltViewModel class ProductViewModel @Inject constructor( private val repo: ProductRepository ) : ViewModel() { private val _state = MutableStateFlow(ProductUiState()) val state: StateFlow<ProductUiState> = _state.asStateFlow() init { refresh() } fun refresh() = viewModelScope.launch { _state.update { it.copy(loading = true, error = null) } runCatching { repo.fetchProducts() } .onSuccess { list -> _state.update { it.copy(loading = false, items = list) } } .onFailure { _state.update { it.copy(loading = false, error = "Try again") } } } }

QA says a bug happens only on one manufacturer's phones. How do you debug it?

Testing, Performance and Play StoreMedium

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Get exact device model, OS and skin versions, and steps. Use a remote device lab or Firebase Test Lab to reproduce if you lack the device. Collect logcat and bug reports, and compare behaviour around known differences like battery optimisation, autostart restrictions, custom permission dialogs or WebView versions. Add targeted logging or a non-fatal Crashlytics event, then apply a fix guarded by checks only if necessary.

Two screens show different values for the same cart count. How do you fix the architecture?