async/await

Learn parallel coroutines with async and await.

async and await

async starts a coroutine that returns a result and returns a Deferred<T>. The await() call retrieves the result. Example: val a = async { 10 }; println(a.await()) }.

Multiple async calls start in parallel; each runs in its own coroutine. Even if await() is called sequentially, the total time approaches the longest one because computations run in parallel. This is the difference from launch: launch is resultless, async is resultful. Keep the await order fixed for deterministic output.

async/await parallelism

When two async calls start in parallel each runs concurrently. In val a = async { 10 }; val b = async { 20 } both computations start at once. println(a.await() + b.await()) waits for both and gives the sum.

The output is deterministic: 30. Because await collects results in a fixed order. Parallel execution shortens time but the result values are fixed. async is called only inside a coroutine scope (such as runBlocking).

Exercise Sum with async and await

Add import kotlinx.coroutines.*. Write fun main() = runBlocking; inside define val a = async { 10 } and val b = async { 20 }. Print the sum with println(a.await() + b.await()).

Show example solution

import kotlinx.coroutines.*

fun main() = runBlocking {
    val a = async { 10 }
    val b = async { 20 }
    println(a.await() + b.await())
}

Test yourself

Match the async/await concepts

async

Start with result
await

Get result
Deferred

Type
parallel

Parallel

Complete the await call

val a = async { 10 }
val result = a.____()

Show answer

val a = async { 10 }
val result = a.await()

What does async return?
- Deferred
- Int
- Unit
- Job
Show answer

Deferred — async returns a Deferred<T>; await retrieves the value.

Next lesson: Summary - Coroutine basics

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