public boolean updatePassword(String username, String oldPassword, String newPassword)
     throws IOException {
   if (!validate(username, oldPassword)) return false;
   props.put(PASSWORD_PROP_PFX + username, newPassword);
   return true;
 }
 public boolean validate(String username, String password) throws IOException {
   return (password.equals(props.getProperty(PASSWORD_PROP_PFX + username)));
 }