Free Coroutines & Flow Interview Questions

Structured concurrency, cancellation, and hot vs cold Flow.

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What does the suspend keyword actually change about a function, and does it make the function run off the main thread?
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In structured concurrency, what guarantee does a coroutineScope block give you that GlobalScope.launch does not?
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What is the difference between launch and async, and when is using async without ever calling await a mistake?
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Why does wrapping an async call site in try/catch sometimes fail to catch the exception?
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What is the core behavioral difference between coroutineScope and supervisorScope?
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How do Job and SupervisorJob differ in the direction that cancellation and failure propagate?
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A CoroutineExceptionHandler you installed does not fire for an exception thrown inside an async. Why?
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Why is putting a CoroutineExceptionHandler in the context of a child launch, rather than the scope, ineffective?
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What is the practical difference between withContext(Dispatchers.IO) and launching an async on the IO dispatcher and awaiting it immediately?
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What actually distinguishes Dispatchers.IO from Dispatchers.Default, and can work migrate between them cheaply?
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What does limitedParallelism on a dispatcher give you, and how does it differ from a fixed-size thread pool?
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Why does Dispatchers.Unconfined behave surprisingly, and where is it legitimately used?
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Why does a tight computational loop ignore cancellation, and what are the ways to make it cooperative?
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What is the difference between checking isActive, calling ensureActive, and calling yield inside a loop?
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Why can't you simply run suspending cleanup code in a finally block after cancellation, and what fixes it?
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What is a cold Flow, and what specifically makes it cold?
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Why does StateFlow drop values, and how do its conflation and equality check affect what collectors see?
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You update a StateFlow with a value equal to the current one and collectors do not react. Why, and how do you force an emission?
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What do replay, extraBufferCapacity, and onBufferOverflow control in a MutableSharedFlow, and how do they interact?
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Why does SharingStarted.WhileSubscribed(5000) exist, and what problem does the 5000 specifically solve?
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How do SharingStarted.Eagerly, Lazily, and WhileSubscribed differ in when the upstream runs?
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What is the difference between stateIn and shareIn, and when must you use shareIn?
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What exactly does flowOn change, and why does it only affect the upstream part of the chain?
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Why is calling withContext to change dispatcher inside a flow builder's emit wrong, and what is context preservation?
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What is the difference between buffer, conflate, and collectLatest for handling a slow collector?
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How does flatMapLatest behave differently from flatMapMerge and flatMapConcat?
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Why is callbackFlow required for bridging callback-based APIs, and what happens if you forget awaitClose?
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What does suspendCancellableCoroutine give you over suspendCoroutine, and why does it matter?
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Why is runBlocking dangerous in production Android code, and where is it acceptable?
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Why does GlobalScope.launch leak and misbehave, beyond just being discouraged?
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What lifecycle does viewModelScope have, and what dispatcher does it default to?
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Why is repeatOnLifecycle preferred over launchWhenStarted for collecting flows in the UI?
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When collecting a flow with repeatOnLifecycle(Lifecycle.State.STARTED), what happens to a StateFlow's upstream on each stop and start?
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What is exception transparency in Flow, and why is wrapping emit in try/catch a violation?
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How does the catch operator differ from a try/catch around collect, and what can catch not handle?
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What is the semantic difference between combine and zip on two flows?
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In combine, why might you miss the very first combined emission, and how does it handle initial values?
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Why is runTest preferred over runBlocking for testing suspend code, and what does it automate?
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What is the difference between StandardTestDispatcher and UnconfinedTestDispatcher in tests?
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What does advanceUntilIdle do, and why might a test not observe an emission without it?
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Why should you inject dispatchers rather than reference Dispatchers.IO directly, especially for tests?
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When should you use Mutex.withLock instead of a synchronized block in coroutine code?
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Mutex in kotlinx.coroutines is not reentrant. What problem does that cause and how do you avoid it?
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When would you choose a Channel over a SharedFlow, given both can broadcast events?
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Why can a SharedFlow with zero replay silently drop events for one-time UI actions, and what is a robust alternative?
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Why does cancelling a parent coroutine not immediately stop a child doing blocking JVM work, and how do you make it interruptible?
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Why must you never catch CancellationException and swallow it, and what is the correct handling?
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How do onEach plus launchIn differ from a plain collect, and when does the distinction matter?
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What is the difference between a SupervisorJob installed in a scope and using supervisorScope, in terms of lifetime?
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Why can awaiting an async inside supervisorScope still throw even though a child failure does not cancel the scope, and how do you handle it correctly?
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