From Java 5 and onwards, we have access to some atomic thread safe structures, that are still useful with coroutines:

import java.util.concurrent.atomic.AtomicInteger

fun main(args: Array<String>) = runBlocking {
   val counter = AtomicInteger(0)

   val time = measureTimeMillis {
      repeatInParallel(1_000_000) {
         counter.incrementAndGet()
      }
   }
   println("counter = ${counter.get()}")
   println("time = $time")
}

AtomicInteger gives us many atomic operations that are thread safe. There are more thread safe structures such as other atomic primitives and concurrent collections.