private void edit(HttpServletRequest request, HttpServletResponse response)
      throws IOException, ServletException {
    String linkIdParam = request.getParameter(PARAM_LINK_ID);
    String title = request.getParameter(PARAM_TITLE);
    String image = request.getParameter(PARAM_IMAGE);
    String url = request.getParameter(PARAM_URL);
    if (linkIdParam == null || !Utils.isNumeric(linkIdParam)) {
      response.sendError(400);
      return;
    }

    try {
      long linkId = Long.valueOf(linkIdParam);
      Link link = linkDao.getLinkById(linkId);
      if (title != null) {
        link.setTitle(title);
      }
      if (image != null) {
        link.setImage(image);
      }
      if (url != null) {
        link.setUrl(url);
      }
      linkDao.updateLink(link);
      response.sendRedirect(LINK_PAGE_URL);
    } catch (NumberFormatException ex) {
      logger.log(Level.SEVERE, ex.getMessage(), ex);
      response.sendError(400, ex.getMessage());
    } catch (DataAccessException ex) {
      logger.log(Level.SEVERE, ex.getMessage(), ex);
      response.sendError(500, ex.getMessage());
    }
  }
  private void add(HttpServletRequest request, HttpServletResponse response)
      throws IOException, ServletException {
    String title = request.getParameter(PARAM_TITLE);
    String image = request.getParameter(PARAM_IMAGE);
    String url = request.getParameter(PARAM_URL);
    if (title == null || url == null) {
      response.sendError(400);
      return;
    }

    try {
      if (image == null) {
        linkDao.createNewLink(title, url);
      } else {
        linkDao.createNewLink(title, image, url);
      }
      response.sendRedirect(LINK_PAGE_URL);
    } catch (DataAccessException ex) {
      logger.log(Level.SEVERE, ex.getMessage(), ex);
      response.sendError(500, ex.getMessage());
    }
  }
  private void delete(HttpServletRequest request, HttpServletResponse response)
      throws IOException, ServletException {
    String linkIdParam = request.getParameter(PARAM_LINK_ID);
    if (linkIdParam == null || !Utils.isNumeric(linkIdParam)) {
      response.sendError(400);
      return;
    }

    try {
      long linkId = Long.valueOf(linkIdParam);
      linkDao.deleteLink(linkId);
      response.sendRedirect(LINK_PAGE_URL);
    } catch (NumberFormatException ex) {
      logger.log(Level.SEVERE, ex.getMessage(), ex);
      response.sendError(400, ex.getMessage());
    } catch (DataAccessException ex) {
      logger.log(Level.SEVERE, ex.getMessage(), ex);
      response.sendError(500, ex.getMessage());
    }
  }