FlowOn for changing dispatcher in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin Flow code with flowOn?

Consider the following Kotlin code using flowOn. What will be printed?

Kotlin

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

fun main() = runBlocking {
    flow {
        println("Emitting on thread: ${Thread.currentThread().name}")
        emit(1)
    }
    .flowOn(Dispatchers.Default)
    .collect {
        println("Collected $it on thread: ${Thread.currentThread().name}")
    }
}

A

Emitting on thread: DefaultDispatcher-worker-1
Collected 1 on thread: DefaultDispatcher-worker-1

B

Emitting on thread: main
Collected 1 on thread: DefaultDispatcher-worker-1

C

Emitting on thread: main
Collected 1 on thread: main

D

Emitting on thread: DefaultDispatcher-worker-1
Collected 1 on thread: main

💡 Hint

Remember that flowOn changes the dispatcher of the upstream flow emissions.

The flowOn(Dispatchers.Default) makes the emission happen on the Default dispatcher thread, but the collect happens on the original coroutine context, which is main here.

🧠 Conceptual

intermediate

Which statement about flowOn is correct?

Choose the correct statement about flowOn in Kotlin Flow.

A <code>flowOn</code> delays the flow emissions by switching dispatcher after collection.

B <code>flowOn</code> cancels the flow if the dispatcher is not specified.

C <code>flowOn</code> changes the dispatcher only for the upstream flow emissions.

D <code>flowOn</code> changes the dispatcher of the entire flow including collection.

💡 Hint

Think about which part of the flow flowOn affects.

flowOn changes the dispatcher only for the upstream flow emissions, not the downstream collection.

🔧 Debug

advanced

Why does this flowOn usage cause a runtime error?

Examine the code below. Why does it throw an exception at runtime?

Kotlin

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

fun main() = runBlocking {
    flow {
        emit(1)
    }
    .flowOn(Dispatchers.IO)
    .collect {
        delay(1000)
        println(it)
    }
}

A Because delay cannot be called inside collect without a suspend function.

B Because delay inside collect is allowed, no runtime error occurs.

C Because flowOn cannot be used with Dispatchers.IO.

D Because collect is blocking and delays the flow emission causing deadlock.

💡 Hint

Recall that collect is a suspend function and can call delay.

The code runs fine because collect is a suspend function and can call delay. There is no runtime error.

❓ Predict Output

advanced

What is the output of this flow with multiple flowOn calls?

What will this Kotlin program print?

Kotlin

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

fun main() = runBlocking {
    flow {
        println("Start emitting on: ${Thread.currentThread().name}")
        emit(1)
    }
    .flowOn(Dispatchers.IO)
    .map {
        println("Mapping $it on: ${Thread.currentThread().name}")
        it * 2
    }
    .flowOn(Dispatchers.Default)
    .collect {
        println("Collected $it on: ${Thread.currentThread().name}")
    }
}

A

Start emitting on: IO-thread
Mapping 1 on: DefaultDispatcher-worker-1
Collected 2 on: main

B

Start emitting on: DefaultDispatcher-worker-1
Mapping 1 on: DefaultDispatcher-worker-2
Collected 2 on: main

C

Start emitting on: main
Mapping 1 on: IO-thread
Collected 2 on: DefaultDispatcher-worker-1

D

Start emitting on: IO-thread
Mapping 1 on: IO-thread
Collected 2 on: main

💡 Hint

Remember each flowOn affects upstream operators only.

The first flowOn(Dispatchers.IO) moves emission to IO thread. The second flowOn(Dispatchers.Default) moves the map operator upstream to Default dispatcher. Collection happens on main.

🧠 Conceptual

expert

How does flowOn affect upstream and downstream operators in a flow chain?

Given a flow chain with multiple operators and flowOn calls, which statement is true?

A <code>flowOn</code> changes the dispatcher of all operators upstream from its position.

B <code>flowOn</code> changes the dispatcher of all operators downstream from its position.

C <code>flowOn</code> changes the dispatcher of the operator immediately before it only.

D <code>flowOn</code> changes the dispatcher of the entire flow regardless of position.

💡 Hint

Think about the direction of data flow and which operators flowOn affects.

flowOn changes the dispatcher of all upstream operators before it in the flow chain, not downstream.