Channel
- Kotlin course
- Advanced
- Lesson 9/83
- 5 min
Learn inter-coroutine communication with Channel.
What is a Channel?
A Channel is a concurrent queue for passing data between coroutines. One coroutine puts a value with send, another reads it with receive.
It is created with Channel<T>(). send and receive are suspending functions; send waits if the buffer is full, receive waits if empty. This balances producer and consumer speeds. When a channel is closed (.close()), the consumer reads the last value and learns it is done.
send and receive
The producer coroutine puts a value with channel.send(value). The consumer reads one with channel.receive(). Both are suspending; they wait automatically when needed.
When the channel is closed, receive can no longer find a value and throws ClosedReceiveChannelException. Using for (value in channel) instead ends the loop cleanly when the channel closes. This reduces blocking risk.
Practice Send and receive with a Channel
Define val channel = Channel<Int>(). In a launch, call channel.send(10) and channel.send(20), then channel.close(). In the main flow, loop for (value in channel) and print each value with println("Got: $value").
Starter code
// Channel send and receive
// write here
fun main() = runBlocking {
}Sample solution
fun main() = runBlocking {
val channel = Channel<Int>()
launch {
channel.send(10)
channel.send(20)
channel.close()
}
for (value in channel) {
println("Got: $value")
}
}Expected output
Got: 10 Got: 20
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does a Channel provide?
- Data between coroutines Correct answer
- Dispatcher choice
- UI thread handling
- Memory cache
Explanation: A Channel passes data between coroutines.
2 Complete the Channel type parameter
val channel = Channel<____>()val channel = Channel<Int>()Explanation: A Channel takes a type parameter.
3 Match Channel parts
| Term | Meaning |
|---|---|
Channel<T> |
Queue |
send |
Puts a value |
receive |
Takes a value |
close |
Closes |