Coroutine scope and structured concurrency in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to launch a coroutine inside the given scope.

Kotlin

fun main() = runBlocking {
    launch [1]
        println("Hello from coroutine")
}

Drag options to blanks, or click blank then click option'

A{}

B() -> Unit

C()

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses () instead of curly braces {} after launch

Using square brackets [] which are invalid here

The launch function takes a lambda block enclosed in curly braces {} to define the coroutine body.

2fill in blank

medium

Complete the code to create a new CoroutineScope with a Job.

Kotlin

val scope = CoroutineScope([1] + Dispatchers.Default)

Drag options to blanks, or click blank then click option'

ASupervisorJob()

BJob()

Claunch()

DrunBlocking()

Attempts:

3 left

💡 Hint

Common Mistakes

Using launch() which is a coroutine builder, not a job

Using runBlocking() which blocks the thread

CoroutineScope requires a Job instance to manage its lifecycle; Job() creates a new job.

3fill in blank

hard

Fix the error in the coroutine builder to properly wait for completion.

Kotlin

fun main() = runBlocking {
    val job = launch {
        delay(1000L)
        println("Done")
    }
    [1]
}

Drag options to blanks, or click blank then click option'

Ajob.join()

Bjob.cancel()

Cjob.start()

Djob.wait()

Attempts:

3 left

💡 Hint

Common Mistakes

Using cancel() which stops the coroutine prematurely

Using wait() which is not a coroutine function

Calling job.join() suspends until the coroutine completes, ensuring "Done" is printed before main ends.

4fill in blank

hard

Fill both blanks to create a coroutine scope and launch a child coroutine.

Kotlin

fun main() = runBlocking {
    val scope = CoroutineScope([1] + Dispatchers.IO)
    scope.launch [2]
        println("Child coroutine running")
}

Drag options to blanks, or click blank then click option'

AJob()

B{}

C()

DSupervisorJob()

Attempts:

3 left

💡 Hint

Common Mistakes

Using SupervisorJob() instead of Job() for basic scope

Using parentheses () instead of curly braces {} after launch

Job() creates the scope's job; launch requires a lambda block {} to define the coroutine body.

5fill in blank

hard

Fill all three blanks to create a map of coroutine results filtering only successful ones.

Kotlin

val results = mapOf(
    "task1" to 10,
    "task2" to 0,
    "task3" to 5
)

val filtered = results.filter { ([1], [2]) -> [3] > 0 }

Drag options to blanks, or click blank then click option'

Akey

Bvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using key instead of value in the condition

Mixing up the order of destructured variables

The filter destructures the map entries into key and value; filtering keeps entries where value > 0.

Practice

1. What is the main purpose of a CoroutineScope in Kotlin?

easy

A. To block the main thread until coroutines finish

B. To create threads manually for parallel execution

C. To group coroutines and manage their lifecycle together

D. To convert coroutines into synchronous functions

Solution

1. Step 1: Understand CoroutineScope role

   A CoroutineScope groups coroutines so they can be managed as one unit.

2. Step 2: Compare other options

   The other options describe unrelated or incorrect behaviors for CoroutineScope.

3. Final Answer:

   To group coroutines and manage their lifecycle together -> Option C

4. Quick Check:

   CoroutineScope groups coroutines [OK]

Hint: CoroutineScope manages coroutines lifecycle as a group [OK]

Common Mistakes:

• Thinking CoroutineScope creates threads
• Confusing CoroutineScope with blocking calls
• Assuming CoroutineScope converts coroutines to sync

2. Which of the following is the correct way to launch a coroutine within a CoroutineScope?

easy

A. scope.launch { /* coroutine code */ }

B. launch.scope { /* coroutine code */ }

C. CoroutineScope.launch { /* coroutine code */ }

D. scope.run { /* coroutine code */ }

Solution

1. Step 1: Recall coroutine launch syntax

   Within a CoroutineScope, you call launch as a function on the scope instance.

2. Step 2: Check options for syntax correctness

   scope.launch { /* coroutine code */ } uses scope.launch {} which is correct. Others misuse syntax or functions.

3. Final Answer:

   scope.launch { /* coroutine code */ } -> Option A

4. Quick Check:

   launch is called on CoroutineScope instance [OK]

Hint: Use scope.launch { } to start coroutine inside scope [OK]

Common Mistakes:

• Swapping order like launch.scope
• Using run instead of launch
• Calling launch as a static method

3. What will be the output of this Kotlin code snippet?

import kotlinx.coroutines.*

fun main() = runBlocking {
    val job = launch {
        delay(100)
        println("Task done")
    }
    println("Waiting")
    job.join()
    println("Finished")
}

medium

A. Task done\nWaiting\nFinished

B. Waiting\nTask done\nFinished

C. Waiting\nFinished\nTask done

D. Finished\nWaiting\nTask done

Solution

1. Step 1: Analyze coroutine launch and delay

   The coroutine launched delays 100ms before printing "Task done". Meanwhile, "Waiting" prints immediately.

2. Step 2: Understand job.join() effect

   The main coroutine waits for the launched job to finish before printing "Finished".

3. Final Answer:

   Waiting\nTask done\nFinished -> Option B

4. Quick Check:

   Output order respects delay and join [OK]

Hint: join() waits for coroutine to finish before continuing [OK]

Common Mistakes:

• Assuming launched coroutine runs instantly
• Ignoring delay effect
• Printing Finished before job completes

4. Identify the problem in this Kotlin coroutine code and choose the correct fix:

fun main() = runBlocking {
    val job = GlobalScope.launch {
        delay(1000)
        println("Done")
    }
    println("Waiting")
}

medium

A. Replace delay(1000) with Thread.sleep(1000)

B. Use CoroutineScope().launch {} instead of GlobalScope.launch

C. Remove runBlocking and use GlobalScope.launch only

D. Add job.join() after println("Waiting")

Solution

1. Step 1: Identify lifecycle issue with GlobalScope

   Using GlobalScope.launch starts coroutine that may outlive runBlocking, so "Done" may never print.

2. Step 2: Fix by waiting for job completion

   Calling job.join() ensures main coroutine waits for launched coroutine to finish before exiting.

3. Final Answer:

   Add job.join() after println("Waiting") -> Option D

4. Quick Check:

   join() waits for coroutine to complete [OK]

Hint: Always join launched jobs to wait for completion [OK]

Common Mistakes:

• Ignoring job.join() causing premature exit
• Replacing delay with blocking sleep
• Removing runBlocking causing main to exit early

5. Given this Kotlin code using structured concurrency, what will be printed?

import kotlinx.coroutines.*

fun main() = runBlocking {
    try {
        coroutineScope {
            launch {
                delay(100)
                println("Child 1 done")
            }
            launch {
                delay(50)
                throw RuntimeException("Error in Child 2")
            }
        }
    } catch (e: Exception) {
        println("Caught: " + e.message)
    }
    println("End of main")
}

hard

A. Caught: Error in Child 2\nEnd of main

B. Child 1 done\nCaught: Error in Child 2\nEnd of main

C. Caught: Error in Child 2

D. Child 1 done\nEnd of main

Solution

1. Step 1: Understand coroutineScope behavior on exceptions

   If any child coroutine throws an exception, coroutineScope cancels all children and throws the exception.

2. Step 2: Analyze timing and cancellation

   Child 2 throws after 50ms, cancelling Child 1 before it prints. The exception is caught and message printed.

3. Final Answer:

   Caught: Error in Child 2\nEnd of main -> Option A

4. Quick Check:

   Exception cancels siblings, caught prints message [OK]

Hint: Exception cancels sibling coroutines in coroutineScope [OK]

Common Mistakes:

• Assuming Child 1 prints despite cancellation
• Missing try-catch around coroutineScope
• Thinking exceptions are ignored in coroutineScope