why does operation.map(launch thread).foreach(join()) work in kotlin?

Question

I've been trying to find an explanation on why this works in kotlin:

(1..100).map {
    launch {
        System.out.println("Hello from on ${Thread.currentThread().name}")
        delay(100)
    }
}.forEach { it.join() }

In java this would:

1. start thread 1
2. join thread 1 - and block here, never starting more than 1 thread.

In kotlin this process on multiple threads in parallel.

Why does this work?

Answer 1

The map call terminates before forEach, thus every single launch will be called before the first join (inside forEach). These collection operations don’t work like Java‘s Streams. Read this answer for further information.

In Kotlin it’s like this:

• launch coroutine 1-100
• join coroutine 1-100