<form action="begin.jsp">
                                <h2 align="left">
                                    <select name="combobox">
                                        <%for (Product next : (List<Product>) request.getAttribute("productList")) {
                                                out.println("<option value=" + next.getId() + ">"
                                                        + next.getDescription()
                                                        + "</option>");
                                            }%>
                                    </select>
                                    <input type="submit" value="OK">
                                </h2>
                            </form>

request.setAttribute("productList", productDAO.getProduct());
dispatcher = request.getRequestDispatcher("index.jsp");
if (dispatcher != null) {
    dispatcher.forward(request, response);
}

request.setAttribute("productList", productDAO.getProduct());
request.setAttribute("showProduct", productDAO.getProductById(Integer.parseInt(request.getParameter("combobox"))));
dispatcher = request.getRequestDispatcher("index.jsp");
if (dispatcher != null) {
    dispatcher.forward(request, response);
}

request.setAttribute("productList", productDAO.getProduct());
if(request.getParameter("combobox") != null){
request.setAttribute("showProduct", productDAO.getProductById(
      Integer.parseInt(request.getParameter("combobox"))));
}
dispatcher = request.getRequestDispatcher("index.jsp");
if (dispatcher != null) {
     dispatcher.forward(request, response);
}