Flow operators (map, filter, transform) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of map and filter on a Flow

What is the output of this Kotlin Flow code snippet?

Kotlin

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

fun main() = runBlocking {
    val flow = (1..5).asFlow()
        .map { it * 2 }
        .filter { it % 3 == 0 }
    flow.collect { println(it) }
}

A 6

B

2
4
6
8
10

C

3
6
9
12
15

D

4
8
12
16
20

💡 Hint

Remember map transforms each item, then filter keeps only those divisible by 3.

The flow emits numbers 1 to 5. map multiplies each by 2: 2,4,6,8,10. filter keeps only those divisible by 3, which is only 6.

❓ Predict Output

intermediate

Transform operator output

What does this Kotlin Flow code print?

Kotlin

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

fun main() = runBlocking {
    val flow = (1..3).asFlow()
        .transform {
            emit(it)
            emit(it * 10)
        }
    flow.collect { println(it) }
}

A

1
2
3

B

1
2
3
10
20
30

C

10
20
30

D

1
10
2
20
3
30

💡 Hint

transform can emit multiple values per input.

For each number 1 to 3, transform emits the number itself and then the number multiplied by 10.

🧠 Conceptual

advanced

Behavior of filter with suspend predicate

Which statement about the filter operator in Kotlin Flow is true when using a suspend predicate?

A filter cannot use suspend functions as predicates.

B filter evaluates the predicate synchronously and blocks the flow.

C filter supports suspend predicates and evaluates them asynchronously for each element.

D filter only works with non-suspend predicates and throws a compile error otherwise.

💡 Hint

Check the official Kotlin Flow documentation about filter operator.

The filter operator in Kotlin Flow supports suspend predicates, allowing asynchronous evaluation for each element.

❓ Predict Output

advanced

Output of combined map and transform with delay

What is the output of this Kotlin Flow code?

Kotlin

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

fun main() = runBlocking {
    val flow = (1..2).asFlow()
        .map {
            delay(100)
            it * 2
        }
        .transform {
            emit(it)
            emit(it + 1)
        }
    flow.collect { println(it) }
}

A

2
3
4
5

B

1
2
2
3

C

2
4
3
5

D

3
4
5
6

💡 Hint

map doubles each number with delay, transform emits the number and number+1.

Numbers 1 and 2 are doubled to 2 and 4 with delay. transform emits each doubled number and that number plus one.

🔧 Debug

expert

Why does this Flow code throw an exception?

This Kotlin Flow code throws an exception at runtime. What is the cause?

Kotlin

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

fun main() = runBlocking {
    val flow = (1..3).asFlow()
        .map { if (it == 2) throw IllegalStateException("Error at 2") else it }
        .filter { it > 0 }
    flow.collect { println(it) }
}

A filter operator causes the exception because it cannot handle negative numbers.

B The exception is thrown inside map and stops the flow collection immediately.

C The flow completes normally without exceptions because filter ignores errors.

D The exception is caught and suppressed by the flow operators automatically.

💡 Hint

Check where the exception is thrown and how flows handle exceptions.

The exception is thrown inside the map operator when the value is 2, which stops the flow collection immediately.