int nThreads = Runtime.getRuntime().availableProcessors(); // Number of threads
Worker[] workers = new Worker[nThreads]; // Those threads, Worker is just a thread class that can run incoming tasks
...
Worker getWorker(String dispatchId) { // Get a thread for this Task
    return workers[(dispatchId.hashCode() & Integer.MAX_VALUE) % nThreads];
}

String dispatchId = 'SomePrefix' + counter.next()

ConcurrentMap<String, CompletableFuture<Void>> dispatchQueues = ...

public CompletableFuture<Void> dispatch(String queueName, Runnable task) {
  return dispatchQueues.compute(queueName, (k, queue) -> {
    return (queue == null)
        ? CompletableFuture.runAsync(task)
        : queue.thenRunAsync(task);
  });
}