Coroutine scope and structured concurrency in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested coroutine scopes

What is the output of this Kotlin code using coroutine scopes and structured concurrency?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    launch {
        delay(100)
        println("Task 1 done")
    }
    coroutineScope {
        launch {
            delay(50)
            println("Task 2 done")
        }
        println("Inside coroutineScope")
    }
    println("Outside coroutineScope")
}

A

Inside coroutineScope
Task 2 done
Task 1 done
Outside coroutineScope

B

Task 1 done
Task 2 done
Inside coroutineScope
Outside coroutineScope

C

Inside coroutineScope
Task 1 done
Task 2 done
Outside coroutineScope

D

Task 2 done
Inside coroutineScope
Task 1 done
Outside coroutineScope

💡 Hint

Remember that coroutineScope waits for all its child coroutines to complete before continuing.

The coroutineScope block prints "Inside coroutineScope" immediately, then waits for its child coroutine (Task 2) to finish. After that, the outer launch coroutine (Task 1) finishes, then "Outside coroutineScope" is printed.

❓ Predict Output

intermediate

Effect of cancelling a parent coroutine scope

What will be printed when this Kotlin code runs?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    val parentJob = launch {
        launch {
            delay(100)
            println("Child 1 finished")
        }
        launch {
            delay(200)
            println("Child 2 finished")
        }
    }
    delay(150)
    parentJob.cancel()
    delay(200)
    println("Done")
}

A Done

B

Child 1 finished
Child 2 finished
Done

C

Child 1 finished
Done

D

Child 2 finished
Done

💡 Hint

Cancelling a parent coroutine cancels all its children immediately.

Child 1 finishes before the parentJob is cancelled (after 100ms). Child 2 is cancelled before it can finish (200ms delay). Then "Done" is printed.

🔧 Debug

advanced

Identify the error in coroutine scope usage

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

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    val job = GlobalScope.launch {
        delay(100)
        println("Hello from GlobalScope")
    }
    job.join()
    println("Done")
}

A No error; code runs and prints both lines.

B The use of GlobalScope.launch breaks structured concurrency and may cause unexpected behavior.

C job.join() causes a deadlock in runBlocking.

D Missing coroutineScope block causes compilation error.

💡 Hint

GlobalScope is not tied to the lifecycle of runBlocking.

Using GlobalScope.launch launches a coroutine not bound to the runBlocking scope, breaking structured concurrency. Although this code runs, it is considered a bad practice and can cause bugs in real apps.

📝 Syntax

advanced

Correct syntax for coroutineScope usage

Which option correctly uses coroutineScope to launch two child coroutines and waits for both to finish?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    // Fill in the blank
}

A

coroutineScope {
    launch { println("Child 1") }
    launch { println("Child 2") }
}

B

launch {
    coroutineScope {
        launch { println("Child 1") }
        launch { println("Child 2") }
    }
}

C

coroutineScope {
    launch { println("Child 1") }
}
launch { println("Child 2") }

D

runBlocking {
    launch { println("Child 1") }
    launch { println("Child 2") }
}

💡 Hint

coroutineScope waits for all child coroutines inside it to complete before returning.

Option A correctly uses coroutineScope to launch two child coroutines and waits for both to finish before continuing. Option A nests coroutineScope inside launch which delays waiting. Option A launches one coroutine outside coroutineScope, so it may not wait. Option A uses runBlocking inside runBlocking which is invalid.

🚀 Application

expert

How many coroutines are active at peak in this code?

Consider this Kotlin code snippet. How many coroutines run concurrently at the peak?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    launch {
        repeat(3) { i ->
            launch {
                delay(100L * i)
                println("Inner coroutine $i done")
            }
        }
    }
    launch {
        delay(50)
        println("Another coroutine done")
    }
}

A 2

B 3

C 5

D 4

💡 Hint

Count all coroutines launched that overlap in time at the peak.

The outer launch starts 3 inner coroutines immediately. Another separate launch starts concurrently. At peak, all 4 coroutines run concurrently before delays cause some to finish.