async/await
- Kotlin course
- Intermediate
- Lesson 86/116
- 5 min
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).
Practice 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()).
Starter code
import kotlinx.coroutines.*
// Start two async calls in runBlocking and sum with awaitSample solution
import kotlinx.coroutines.*
fun main() = runBlocking {
val a = async { 10 }
val b = async { 20 }
println(a.await() + b.await())
}Expected output
30
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the async/await concepts
| Term | Meaning |
|---|---|
async |
Start with result |
await |
Get result |
Deferred |
Type |
parallel |
Parallel |
2 Complete the await call
val a = async { 10 }
val result = a.____()val a = async { 10 }
val result = a.await()Explanation: await retrieves the value from a Deferred.
3 What does async return?
- Deferred Correct answer
- Int
- Unit
- Job
Explanation: async returns a Deferred<T>; await retrieves the value.