Coroutines vs threads mental model in Kotlin - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of coroutine vs thread switching

What is the output of this Kotlin code that uses a coroutine and a thread?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    launch {
        println("Start coroutine")
        delay(100)
        println("End coroutine")
    }
    Thread {
        println("Start thread")
        Thread.sleep(50)
        println("End thread")
    }.start()
    delay(200)
}

A

Start coroutine
Start thread
End thread
End coroutine

B

Start thread
Start coroutine
End coroutine
End thread

C

Start coroutine
End coroutine
Start thread
End thread

D

Start thread
End thread
Start coroutine
End coroutine

💡 Hint

Remember coroutines are lightweight and can suspend without blocking threads.

The coroutine starts and prints immediately, then suspends for 100ms. The thread starts and prints immediately, then sleeps for 50ms. So the thread finishes before the coroutine resumes.

🧠 Conceptual

intermediate

Difference in resource usage between coroutines and threads

Which statement best describes the difference in resource usage between Kotlin coroutines and threads?

A Coroutines use less memory and are more lightweight than threads because they share threads and suspend without blocking.

B Threads use less memory than coroutines because threads are managed by the OS.

C Coroutines and threads use the same amount of memory because both run concurrently.

D Threads are more lightweight because they can suspend without blocking, unlike coroutines.

💡 Hint

Think about how coroutines suspend without blocking threads.

Coroutines are lightweight because they do not require a dedicated thread and can suspend execution without blocking the underlying thread, saving memory and resources.

🔧 Debug

advanced

Identify the error in coroutine and thread usage

What error will this Kotlin code produce?

Kotlin

import kotlinx.coroutines.*

fun main() {
    GlobalScope.launch {
        println("Coroutine running")
        delay(100)
        println("Coroutine finished")
    }
    Thread.sleep(50)
    println("Main finished")
}

A The program throws a runtime exception because delay cannot be called outside runBlocking.

B The program prints "Coroutine running" then "Main finished" and exits before "Coroutine finished" prints.

C The program prints all three lines in order without error.

D The program deadlocks and never finishes.

💡 Hint

Consider the lifecycle of GlobalScope coroutines and main thread.

The main thread sleeps 50ms and then prints "Main finished" and exits. The coroutine is launched but may not complete before the program ends, so "Coroutine finished" may never print.

❓ Predict Output

advanced

Output order with multiple coroutines and threads

What is the output of this Kotlin program?

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    launch {
        println("Coroutine 1 start")
        delay(100)
        println("Coroutine 1 end")
    }
    launch {
        println("Coroutine 2 start")
        delay(50)
        println("Coroutine 2 end")
    }
    Thread {
        println("Thread start")
        Thread.sleep(75)
        println("Thread end")
    }.start()
}

A

Thread start
Coroutine 1 start
Coroutine 2 start
Thread end
Coroutine 2 end
Coroutine 1 end

B

Coroutine 2 start
Coroutine 1 start
Thread start
Coroutine 2 end
Coroutine 1 end
Thread end

C

Coroutine 1 start
Thread start
Coroutine 2 start
Coroutine 2 end
Thread end
Coroutine 1 end

D

Coroutine 1 start
Coroutine 2 start
Thread start
Coroutine 2 end
Thread end
Coroutine 1 end

💡 Hint

Coroutines start immediately but suspend at delay; thread runs independently with sleep.

Both coroutines print start immediately. Coroutine 2 delays 50ms and ends first. Thread sleeps 75ms and ends after coroutine 2. Coroutine 1 delays 100ms and ends last.

🧠 Conceptual

expert

Why coroutines are preferred over threads for concurrency in Kotlin

Which reason best explains why Kotlin coroutines are often preferred over threads for concurrency?

A Coroutines require more memory but provide better debugging tools than threads.

B Threads are faster than coroutines because they run on multiple CPU cores automatically.

C Coroutines allow writing asynchronous code sequentially and use fewer resources by suspending without blocking threads.

D Threads are easier to cancel and manage than coroutines in Kotlin.

💡 Hint

Think about how coroutines simplify asynchronous programming and resource use.

Coroutines let you write code that looks like normal sequential code but runs asynchronously. They suspend without blocking threads, so they use less memory and are easier to manage than threads.