gistfile1.txt

public class BleCommandExecutor implements CommandExecutor {
    private final BleDevice                                   bleDevice;
    private final Executor                                    executor  = Executors.newSingleThreadExecutor();
    private final Map<String, Queue<CommandCallback<String>>> callbacks = new ConcurrentHashMap<>();

    public BleCommandExecutor(BleDevice bleDevice) {
        this.bleDevice = bleDevice;
    }

    @Override
    public void requestAsync(@NonNull String command, CommandCallback<String> callback) {
        Queue<CommandCallback<String>> callbacksQueue = callbacks.get(command);
        if (callbacksQueue != null) {
            callbacksQueue.add(callback);
        } else {
            callbacksQueue = new ConcurrentLinkedQueue<>();
            callbacksQueue.add(callback);
            callbacks.put(command, callbacksQueue);

            executor.execute(() -> {
                String result = bleDevice.requestSync(command);
                Queue<CommandCallback<String>> cbQueue = callbacks.remove(command);
                if (cbQueue != null) {
                    for (CommandCallback<String> cb : cbQueue) {
                        cb.onResponse(result);
                    }
                }
            });
        }
    }
}

лучше newFixedThreadPool вместо newSingleThreadExecutor, а то долго отрабатывает