Channels

One way for two coroutines to communicate (or for a coroutine to the external world as with async) is throughDeferred<T>:

import kotlinx.coroutines.experimental.delay
import kotlinx.coroutines.experimental.launch
import kotlinx.coroutines.experimental.runBlocking

fun main(args: Array<String>) = runBlocking {
    val result = CompletableDeferred<String>()

   val world = launch {
      delay(500)
      result.complete("World (from another coroutine)")
   }

   val hello =launch {
      println("Hello ${result.await()}")
   }

   hello.join()
   world.join()
}

Deferreds are fine for single values, but sometimes we want to send a sequence or a stream. In that case, we can use Channel. Channel which is similar to BlockingQueue, but with suspending operations instead of blocking ones, also Channel can be close:

import kotlinx.coroutines.experimental.channels.*

fun main(args: Array<String>) = runBlocking<Unit> {
   val channel = Channel<String>()

 ...