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Kotlin and coroutine interview drills

High-frequency questions​

val, immutability and data races​

val prevents reassignment of the reference; it does not make the referenced object immutable or thread-safe. Prefer immutable state and single ownership, then synchronize unavoidable mutation.

Unit versus Nothing​

Unit means the function completes normally with the single Unit value. Nothing has no values: the function never completes normally because it throws or never returns. Nothing helps type inference in error(...)/throw branches.

inline, noinline, crossinline, reified​

Inlining can place higher-order function body/lambda at the call site, enabling non-local returns and reified type parameters while potentially increasing bytecode. noinline keeps a lambda as a value; crossinline forbids non-local return when invoked in another context. Explain with generated behavior, not “always faster.”

Structured concurrency​

Child work belongs to a scope/lifetime; failure/cancellation propagates by defined parent/supervisor rules and completion waits for children. GlobalScope breaks ownership. coroutineScope fails siblings together; supervisorScope isolates child failure but does not make errors disappear.

suspend fun loadDashboard(): Dashboard = coroutineScope {
    val profile = async { api.profile() }
    val orders = async { api.orders() }
    Dashboard(profile.await(), orders.await())
}

If orders fails, the scope cancels sibling work and throws; cancellation must not be converted into a generic failure.

Coding drills​

  1. Implement a cancellable retry with exponential backoff, jitter, max attempts and retryable-error predicate.
  2. Merge search input with debounce/latest-result semantics and lifecycle ownership.
  3. Protect token refresh so concurrent 401s cause one refresh and all callers resume/fail correctly.
  4. Model an immutable reducer and test concurrent events.

Follow-up traps​

  • Dispatchers.IO does not make shared mutation safe.
  • suspend does not mean background thread.
  • Flow exception operators act by position/direction; cancellation is special.
  • StateFlow conflates by equality and requires initial state; it is not an event queue.
  • Channels deliver values under channel semantics, not durable process-death events.

Self-check​

Explain dispatcher, context, job, scope, cancellation, exception propagation, hot/cold flow, stateIn/shareIn, buffering/conflation and lifecycle collection with one end-to-end example.