try (PrintWriter out = response.getWriter()) {

            String action = request.getParameter("action");
            String username = request.getParameter("username");
            String password = request.getParameter("password");

            HttpSession s = request.getSession();
            List<Students> studentList = (List<Students>) s.getAttribute("studentList");
            if (studentList == null) {
                studentList = new ArrayList<>();
                s.setAttribute("studentList", studentList);
            }

            switch (action) {
                case "Login":
                    boolean ok = false;

                    for (Students student : studentList) {
                        if (student.getName().equals(username)) {
                            ok = true;
                            break;
                        }
                    }

                    if (ok) {
                        response.sendRedirect("partims.html");
                    } else {
                        response.sendRedirect("index.html");
                    }
                    break;
                case "Register":
                    boolean ok2 = true;

                    for (Students student : studentList) {
                        if (student.getName().equals(username)) {
                            ok2 = false;
                            break;
                        }
                    }

                    if (ok2) {
                        Students newstudent = new Students(username, password);
                        studentList.add(newstudent);
                        response.sendRedirect("partims.html");
                    } else {
                        response.sendRedirect("index.html");
                    }
                    break;
                default:
                    response.sendRedirect("index.html");
                    break;
            }
        }