Enthuware

There is nothing in the main method that could possibly call run() method of Merger with any answer. A Merger instance is just constructed, but never used. Or am I missing something?

Actually, the Merger instance is passed to the CyclicBarrier as the barrierAction (a Runnable "which will execute the given barrier action when the barrier is tripped, performed by the last thread entering the barrier", see https://docs.oracle.com/javase/8/docs/a ... .Runnable- ).