What are coroutines?

Learn the coroutine and async programming concept.

What are coroutines?

A coroutine is a lightweight thread that can suspend its work and resume later. A single thread can host thousands of coroutines with few resources; each coroutine takes a few hundred bytes.

Coroutines work with suspend functions: while waiting, they do not block the thread and let other work run. This replaces callback hell with clean code. The kotlinx.coroutines library provides tools like runBlocking, launch and async. In this lesson you will see the basic flow.

suspend and structured concurrency

A suspend function can pause and resume its work. While paused the thread is not blocked; other coroutines can run. delay is the simplest suspend function: it waits for a given time.

Structured concurrency ensures every coroutine has a scope. Child coroutines complete before the scope ends. This prevents resource leaks. runBlocking bridges the blocking world to the suspending world and waits for all coroutines inside.

Exercise Run your first coroutine

Add import kotlinx.coroutines.*. Write fun main() = runBlocking { println("Hello coroutine") }. runBlocking opens a coroutine scope and blocks the main thread.

Show example solution

import kotlinx.coroutines.*

fun main() = runBlocking {
    println("Hello coroutine")
}

Test yourself

What is a coroutine?
- A lightweight thread
- A heavy thread
- A class type
- A data structure
Show answer

A lightweight thread — A coroutine is a lightweight thread that can be suspended.

Complete the delay import

import kotlinx.coroutines.____

suspend fun greet() { delay(100) }

Show answer

import kotlinx.coroutines.delay

suspend fun greet() { delay(100) }

Match the coroutine concepts

coroutine

Lightweight task
suspend

Can pause
delay

Wait
lightweight

Few resources

Next lesson: launch

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