Flow basics in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Flow collection?

Consider this Kotlin Flow code that emits numbers and collects them. What will be printed?

Android Kotlin

import kotlinx.coroutines.flow.*
import kotlinx.coroutines.runBlocking

fun main() = runBlocking {
  val flow = flow {
    emit(1)
    emit(2)
    emit(3)
  }
  flow.collect { value ->
    println(value)
  }
}

A 1\n2\n3

B 3\n2\n1

C 1 2 3 on one line

D No output, code does not compile

💡 Hint

Remember that Flow emits values in the order they are emitted inside the flow builder.

The flow emits values 1, 2, and 3 sequentially. The collect function prints each value on its own line.

🧠 Conceptual

intermediate

What does the 'flowOn' operator do in Kotlin Flow?

Choose the correct description of the 'flowOn' operator in Kotlin Flow.

A Cancels the flow immediately

B Changes the context where the flow is collected

C Changes the context where the flow is emitted

D Buffers the flow emissions

💡 Hint

Think about where the code inside the flow builder runs.

'flowOn' changes the coroutine context where the upstream flow is executed (emission happens). Collection context remains unchanged.

❓ lifecycle

advanced

What happens if you collect a Flow multiple times?

Given a cold Flow in Kotlin, what is the behavior when you collect it multiple times?

A The flow starts emitting from the beginning each time it is collected

B The flow emits only once and shares the result with all collectors

C The flow caches the first emission and replays it to new collectors

D The flow throws an exception on the second collection

💡 Hint

Cold flows start fresh for each collector.

Cold flows start their emission anew for each collector. They do not share emissions unless converted to hot flows.

📝 Syntax

advanced

What error does this Flow code produce?

Analyze this Kotlin Flow code snippet. What error will it cause?

Android Kotlin

import kotlinx.coroutines.flow.*
import kotlinx.coroutines.runBlocking

fun main() = runBlocking {
  val flow = flow {
    emit(1)
    emit(2)
    emit(3)
  }
  flow.collect { value ->
    if (value == 2) throw Exception("Error at 2")
    println(value)
  }
}

A No error, prints 1 2 3

B Exception with message 'Error at 2' is thrown during collection

C Compilation error due to throw inside collect

D Runtime error: NullPointerException

💡 Hint

Throwing inside collect will propagate the exception.

The exception is thrown when value equals 2 during collection, stopping the flow and propagating the error.

❓ navigation

expert

How to cancel a Flow collection properly in Android?

In an Android app using Kotlin Flow, which approach correctly cancels the flow collection when the lifecycle owner is destroyed?

A Use runBlocking { flow.collect { ... } } in the main thread

B Use GlobalScope.launch { flow.collect { ... } } without cancellation

C Collect flow without any coroutine scope

D Use lifecycleScope.launch { flow.collect { ... } } and rely on lifecycleScope cancellation

💡 Hint

Think about lifecycle-aware coroutine scopes in Android.

Using lifecycleScope.launch ties the coroutine to the lifecycle owner, automatically cancelling collection on destruction.