code: public String nextId() {
          String id = "-1";
          do {
              // when we reach max int, reset to zero
              if (nextId.get() == Integer.MAX_VALUE) {
                  nextId.set(0);
              }
              id = String.format("%d", nextId.getAndIncrement());
          } while (hasClient(id));
          return id;
      }
docs: Return next client id
  
  @return Next client id