Explain backpressure in Kotlin Flows and its management operators
This tests your understanding of Flow's pull-based nature and performance tuning. A great answer defines suspension as the default backpressure, then details how `buffer`, `conflate`, and `collectLatest` optimize it.
Explain Kotlin's declaration-site variance with `in` and `out`
This tests your grasp of type systems and API design. Explain how `out` (covariance/producer) and `in` (contravariance/consumer) provide type safety at the declaration site, simplifying usage. A red flag is confusing variance with immutability.
What is a CoroutineDispatcher and when do you use each type?
Tests your grasp of coroutine threading. A dispatcher manages which threads a coroutine runs on. Explain `Main` for UI, `Default` for CPU-intensive work, and `IO` for blocking operations. A red flag is mixing up `IO` and `Default` or using `Unconfined`.
Hot vs. Cold Streams: `StateFlow` vs. `Flow`
Tests your grasp of stream lifecycles. A great answer defines cold `Flow` (lazy, per-collector) vs. hot `StateFlow` (shared, active), then uses a ViewModel UI state scenario to show why `StateFlow` is correct for surviving configuration changes.
Structured Concurrency in Kotlin Coroutines
Tests your understanding of coroutine lifecycles and error handling. A good answer defines the parent-child relationship, then explains how cancellation and exceptions propagate through the scope. A red flag is simply calling it a way to 'group' coroutines.
Explain Kotlin's scope functions: let, run, with, apply, also
This tests your grasp of idiomatic Kotlin and the two key differentiators: context object (`this`/`it`) and return value (object/lambda result). A great answer defines these axes, categorizes each function, and provides a clear example.
What is a `suspend` function in Kotlin?
Tests your grasp of Kotlin's core concurrency primitive. A `suspend` function can pause and resume. It must be called from another suspend function or a coroutine builder.
Difference between launch and async in Kotlin Coroutines
This tests your grasp of coroutine result handling and exception propagation. Explain that `launch` is fire-and-forget (returns `Job`), while `async` is for results (returns `Deferred`). Mention `async` defers exceptions until `await()`.
Explain `inline` and `reified` in Kotlin
Tests your grasp of JVM performance costs and type erasure. A great answer explains how `inline` avoids object allocation for lambdas, and how this enables `reified` to bypass type erasure for runtime checks. A red flag is just saying 'it's for performance'.
Compare and contrast Kotlin's `apply` and `let` scope functions
This tests your grasp of idiomatic Kotlin scope functions. `apply` uses `this` and returns the object, ideal for configuration. `let` uses `it` and returns the lambda result, useful for null-safe chaining.
When would you use a sealed class instead of an enum?
This tests your understanding of state representation. Explain that sealed classes define a hierarchy of types that can hold different data, while enums are a set of singleton constants.
Explain and implement a higher-order function in Kotlin
Tests your grasp of functions as first-class citizens. Define a higher-order function, explain it takes/returns functions, then implement `filterAndTransform` using the specified lambdas.
lateinit var vs. val by lazy in Android
Tests your grasp of Kotlin's non-nullable property initialization. Define `lateinit` (mutable, framework-init) vs. `lazy` (immutable, deferred-init). Use `lateinit` for Views in `onCreate` and `lazy` for expensive objects. Red flag: confusing their mutability.
What are the advantages of a Kotlin data class?
Tests knowledge of Kotlin's boilerplate reduction for data holders. A good answer defines the advantage (conciseness), names generated functions like `equals`, `hashCode`, and `copy`, and explains `copy` for immutable state updates.
What is a Kotlin extension function? Write one for String.
Tests your grasp of adding functionality to existing classes without inheritance. A great answer defines it as syntactic sugar over static methods, explains its use for third-party code, then implements the requested `String.hasWhitespace` function.
Purpose of Kotlin's safe call (`?.`) and Elvis (`?:`) operators?
This tests your grasp of Kotlin's null safety philosophy. A good answer defines the safe call (`?.`) for conditional access and the Elvis operator (`?:`) for default values, then combines them. A red flag is using verbose `if/else` checks for simple cases.
Explain val vs. var and null safety in Kotlin
Tests your grasp of Kotlin's core principles: immutability and compile-time null safety. Define `val` (read-only) vs. `var` (mutable), declare nullables with `?`, and identify the risk as runtime `NullPointerException`s.
Coroutine Exception Handling: launch vs. async
Coroutine builders handle exceptions differently. `launch` propagates them immediately, crashing if unhandled. `async` silently catches them, waiting for you to call `await`. Use a `CoroutineExceptionHandler` on root scopes for global logging.
Why is `LiveData<Boolean>` bad for one-time ViewModel events?
This tests your grasp of state vs. events. A `LiveData<Boolean>` is state; it re-triggers on rotation. Instead, expose events via a `StateFlow` and have the UI call an `eventConsumed()` function on the ViewModel to maintain unidirectional data flow.
How to pass data between Fragments with Jetpack Navigation?
This tests your knowledge of type-safe argument passing in Jetpack Navigation. A good answer defines the argument in XML, uses the Safe Args plugin to generate code, passes data via the Directions class, and retrieves it with the Args class.