Suspend functions concept in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple suspend function call

What is the output of this Kotlin code using a suspend function?

Kotlin

import kotlinx.coroutines.*

suspend fun greet(): String {
    delay(100)
    return "Hello, Kotlin!"
}

fun main() = runBlocking {
    val message = greet()
    println(message)
}

A Compilation error: suspend function called from non-suspend context

B Hello, Kotlin!

C Hello, Kotlin!\nHello, Kotlin!

D Runtime exception: Unresolved reference: delay

💡 Hint

Remember that suspend functions must be called from a coroutine or another suspend function.

The suspend function greet() is called inside runBlocking, which is a coroutine builder. The delay suspends the coroutine without blocking the thread, then returns the string. The println prints the returned string.

🧠 Conceptual

intermediate

Understanding suspend function behavior

Which statement best describes what happens when a suspend function is called in Kotlin?

A It suspends the coroutine without blocking the thread, allowing other coroutines to run.

B It runs the function in a new thread automatically.

C It blocks the current thread until the function completes.

D It converts the function into a callback-based asynchronous function.

💡 Hint

Think about how suspend functions help with concurrency without blocking threads.

Suspend functions suspend the coroutine they run in, freeing the thread to do other work. They do not block the thread or create new threads automatically.

🔧 Debug

advanced

Identify the error in suspend function usage

What error will this Kotlin code produce when compiled and run?

Kotlin

suspend fun fetchData(): String {
    return "Data"
}

fun main() {
    val result = fetchData()
    println(result)
}

A Compilation error: suspend function fetchData() called from non-suspend function main()

B Compilation error: missing coroutine context

C Output: Data

D Runtime error: suspend function cannot be called directly

💡 Hint

Check where suspend functions can be called from.

Suspend functions can only be called from other suspend functions or coroutine builders. Calling fetchData() directly in main() causes a compilation error.

📝 Syntax

advanced

Correct syntax for calling a suspend function

Which option correctly calls the suspend function fetchUser() and prints its result?

Kotlin

suspend fun fetchUser(): String {
    delay(50)
    return "User123"
}

A

fun main() {
    val user = fetchUser()
    println(user)
}

B

suspend fun main() = coroutineScope {
    val user = fetchUser()
    println(user)
}

C

fun main() {
    GlobalScope.launch {
        val user = fetchUser()
        println(user)
    }
}

D

fun main() = runBlocking {
    val user = fetchUser()
    println(user)
}

💡 Hint

Remember that runBlocking is a coroutine builder that blocks the main thread until completion.

Option D uses runBlocking to call the suspend function fetchUser() properly and prints the result synchronously. Option D calls suspend function directly causing compile error. Option D launches a coroutine but main returns immediately, so println may not run before program ends. Option D is valid syntax if main is suspend, but typically main is not suspend in Kotlin JVM apps; however, the original Option D was invalid syntax because coroutineScope cannot be called like that in main without suspend. Corrected Option D marks main as suspend.

🚀 Application

expert

Predict the output of nested suspend functions with delay

What will be printed when running this Kotlin program?

Kotlin

import kotlinx.coroutines.*

suspend fun first(): String {
    delay(100)
    return "First"
}

suspend fun second(): String {
    delay(50)
    return "Second"
}

fun main() = runBlocking {
    val result = "${first()} and ${second()}"
    println(result)
}

A Compilation error: suspend functions cannot be called inside string templates

B Second and First

C First and Second

D Runtime error: IllegalStateException

💡 Hint

Suspend functions inside string templates are evaluated sequentially in runBlocking.

Inside runBlocking, the suspend functions first() and second() are called sequentially inside the string template. first() delays 100ms, then second() delays 50ms. The final string is 'First and Second'.