2

class UserStats {

        private Map<String, AtomicLong> hitsPerUser = new ConcurrentHashMap<>()

        private void onUserCall(String userId) {
            if (userId == null) {
                return;
            }

            hitsPerUser.putIfAbsent(userId, new AtomicLong());
            hitsPerUser.get(userId).addAndGet(1L);
        }

    }