Async coroutine builder in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin coroutine code?

Consider this Kotlin code using async coroutine builder. What will it print?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    val deferred = async {
        delay(100)
        "Hello"
    }
    println("Start")
    println(deferred.await())
    println("End")
}

A Start\nHello\nEnd

B Hello\nStart\nEnd

C Start\nEnd\nHello

D Start\nHello

💡 Hint

Remember that async starts a coroutine immediately and await() waits for its result.

The async block starts immediately but suspends for 100ms. The main coroutine prints "Start" first, then waits for the async result with await(), printing "Hello", and finally prints "End".

❓ Predict Output

intermediate

What is the output when using async with different CoroutineScopes?

What will this Kotlin program print?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    val scope = CoroutineScope(Dispatchers.Default)
    val deferred = scope.async {
        delay(50)
        42
    }
    println("Result: ${deferred.await()}")
}

A Compilation error

B Result: 42

C Result: 0

D Runtime exception

💡 Hint

async returns a Deferred that you can await() to get the result.

The async coroutine runs in the Dispatchers.Default scope, delays 50ms, then returns 42. The await() call waits for completion and prints the result.

❓ Predict Output

advanced

What happens if you call async without awaiting?

What will this Kotlin program print?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    val deferred = async {
        delay(100)
        "Done"
    }
    println("Before delay")
    delay(200)
    println("After delay")
}

A Compilation error

B Before delay\nDone\nAfter delay

C Before delay\nAfter delay

D Runtime exception

💡 Hint

If you don't call await(), the result is not printed.

The async coroutine runs concurrently but its result is not used. The program prints "Before delay", waits 200ms, then prints "After delay". The "Done" string is never printed.

❓ Predict Output

advanced

What is the output of nested async coroutines?

What will this Kotlin code print?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    val deferred = async {
        val inner = async {
            delay(50)
            10
        }
        inner.await() * 2
    }
    println(deferred.await())
}

A 20

B 10

C Compilation error

D Runtime exception

💡 Hint

Inner async returns 10 after delay, outer async multiplies by 2.

The inner async returns 10 after 50ms delay. The outer async awaits inner and multiplies by 2, so the final result is 20.

🧠 Conceptual

expert

Which option correctly describes the behavior of async coroutine builder?

Choose the statement that best describes the async coroutine builder in Kotlin.

A <code>async</code> creates a coroutine that cannot be cancelled once started.

B <code>async</code> blocks the current thread until the coroutine finishes and returns the result directly.

C <code>async</code> launches a coroutine that runs only after all other coroutines complete.

D <code>async</code> starts a coroutine and immediately returns a Deferred that represents a future result, which must be awaited to get the value.

💡 Hint

Think about how async returns a Deferred and how await() works.

async immediately starts a coroutine and returns a Deferred object. You must call await() on it to get the result. It does not block the thread and can be cancelled.