fun main() {
    Topics.sub("ABC")
    Topics.sub("ZXC")
    while (true) {
        val take = Topics.topics.take()
        println(take)
    }
}

object Topics {

    val topics = LinkedBlockingQueue<String>()

    fun sub(name: String) {
        sim {
            topics.put("FROM $name $it")
        }
    }

    // Imagine this is actually a method from a Java lib. Takes callback and does its thing.
    private fun sim(cb: (String) -> Unit): () -> Unit  {
        Executors.newSingleThreadScheduledExecutor().scheduleAtFixedRate({
            cb(Random.nextInt().toString())
        }, 0, 500, TimeUnit.MILLISECONDS)
        return { println("Cancel token invoked.")}
    }
}