Flow builder and collect in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin Flow code?

Consider the following Kotlin code using flow builder and collect. What will be printed?

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 * 2)
    }
}

A

1
2
3

B Compilation error

C

2
4
6

D

3
6
9

💡 Hint

Remember that collect receives each emitted value and you can transform it inside the lambda.

The flow emits 1, 2, and 3. The collect block multiplies each value by 2 before printing, so the output is 2, 4, 6.

❓ Predict Output

intermediate

What does this Flow builder code print?

Analyze this Kotlin code snippet using flow and collect. What is the output?

Kotlin

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

fun main() = runBlocking {
    val flow = flow {
        for (i in 1..3) {
            emit(i)
        }
    }
    flow.collect { value ->
        if (value == 2) return@collect
        println(value)
    }
}

A Compilation error

B

1
2
3

C

2

D

1
3

💡 Hint

Look at the return@collect statement inside the collect lambda.

The return@collect skips printing when value is 2, so only 1 and 3 are printed.

❓ Predict Output

advanced

What is the output of this Flow with delay and collect?

Consider this Kotlin code using flow with delays. What will be printed?

Kotlin

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

fun main() = runBlocking {
    val flow = flow {
        emit(1)
        delay(100)
        emit(2)
        delay(100)
        emit(3)
    }
    flow.collect { value ->
        println(value)
    }
}

A

1
2
3

B

3
2
1

C

1
1
1

D Compilation error

💡 Hint

The delay suspends the flow emission but does not change the order.

The flow emits 1, then waits 100ms, emits 2, waits 100ms, then emits 3. The collect prints them in order.

❓ Predict Output

advanced

What error does this Flow code produce?

What error will this Kotlin code produce when run?

Kotlin

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

fun main() = runBlocking {
    val flow = flow {
        emit(1)
        throw IllegalStateException("Error in flow")
    }
    flow.collect { value ->
        println(value)
    }
}

A No output, program runs normally

B IllegalStateException: Error in flow

C NullPointerException

D Compilation error

💡 Hint

Exceptions thrown inside flow builder propagate to the collector.

The flow emits 1, then throws an IllegalStateException. The exception is propagated and terminates the flow.

🧠 Conceptual

expert

How many items are collected from this Flow?

Given this Kotlin Flow code, how many items will be collected and printed?

Kotlin

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

fun main() = runBlocking {
    val flow = flow {
        for (i in 1..5) {
            emit(i)
            if (i == 3) break
        }
    }
    var count = 0
    flow.collect {
        println(it)
        count++
    }
    println("Total collected: $count")
}

A 3 items

B 5 items

C 0 items

D Compilation error

💡 Hint

The break statement terminates the loop after emitting 3.

The for loop emits 1, then 2, then 3 and breaks. Thus, 3 items are collected and printed, and count is 3.