async vs launch in Kotlin

Introduction

async vs launch is a fundamental concept every Kotlin developer should understand. async starts parallel work and await retrieves its result. Together they compose concurrent operations without manual thread management.

Launch is for tasks that do not return result. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• launch is for tasks that do not return result.
• async is for tasks that return result via Deferred.
• Both create concurrent coroutines.

Syntax

launch { } // Unit
async { } // Deferred<T>

async vs launch in Kotlin Example Program in Kotlin

import kotlinx.coroutines.*

fun main(args: Array<String>) = runBlocking {
    launch { println("Launch task") }
    val deferred = async { "Async result" }
    println(deferred.await())
}

Sample Output

Launch task
Async result

When to use

Use async/await when you have independent tasks that can run in parallel and you need to combine their results.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. The println("Launch task") statement writes a line to the console — this produces part of the sample output below.

3. val deferred = async { "Async result" } assigns or updates a value used later in the program.

4. The println(deferred.await()) statement writes a line to the console — this produces part of the sample output below.

5. Launch is for tasks that do not return result.

6. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• launch is for tasks that do not return result.
• async is for tasks that return result via Deferred.
• Both create concurrent coroutines.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Add the kotlinx-coroutines-core dependency when running coroutine examples outside Android or IntelliJ.
• Semicolons at the end of statements are optional in Kotlin.