code: public void write() {
          logger.info("writing " + users.size() + " users in " + userfilename);
          try {
              Backup.backup(userfilename);
          } catch (IOException e) {
              logger.error("Problem backing up old copy of user file", e);
          }
          try (FileOutputStream fstream = new FileOutputStream(userfilename);
                  BufferedOutputStream bstream = new BufferedOutputStream(
                          fstream)) {
              writeTheLines(bstream);
          } catch (IOException e) {
              logger.error("Problem writing user file", e);
          }
      }
docs: Write the users file.