code: @Override
      public void changePassword(String username, String password)
              throws AuthenticationException {
          String user = file_store.getProperty(username);
          if (user == null) {
              throw new AuthenticationException("User '" + username
                      + "' not found.");
          }
          // Encrypt the new password
          String ePwd = encryptPassword(password);
          file_store.put(username, ePwd);
          try {
              saveUsers();
          } catch (IOException e) {
              throw new AuthenticationException("Error changing password: ", e);
          }
      }
docs: Change a user's password.
  
  @param username The user changing their password.
  @param password The new password for the user.
  @throws AuthenticationException if there was an error changing the
  password.