code: File getNextDataDir() {
      File latest = getLatestDataDir();
      long newID;
      if (latest == null) {
        newID = 0;
      } else {
        newID = Long.parseLong(latest.getName().substring(4)) + 1;
      }
      String suffix = String.format("%015d", newID);
      return new File(this.rootDir, "data" + suffix);
    }
docs: Gets the next data directory.