Getting null pointer exception while using HttpSession

Question

I am currently learning Servlets. I am getting NullPointerException when using the code below. It is returning null value whenever getAttribute() function is called in SqServlet to fetch value in k. Can anyone please tell me where I am going wrong? Thanks.

public class AddServelet extends HttpServlet {
    public void doGet(HttpServletRequest req, HttpServletResponse res) {
        try {
            int i = Integer.parseInt(req.getParameter("num1"));
            int j = Integer.parseInt(req.getParameter("num2"));
            int k = i + j;

            PrintWriter out = res.getWriter();
            HttpSession session = req.getSession();

            req.setAttribute("k", k);
            res.sendRedirect("sq");
        } catch(Exception e) {
            e.printStackTrace();
        }
    }
}

public class SqServlet extends HttpServlet {
    public void doGet(HttpServletRequest req, HttpServletResponse res) {
        try {
            HttpSession session = req.getSession(false);
            int k = (int)session.getAttribute("k");

            k = k * k;
            PrintWriter out = res.getWriter();
            out.println("Square of sum: " + k);
        } catch(Exception e) {
            e.printStackTrace();
        }
    }
}

Answer 1

You are setting the "k" in request then getting it from session

in AddServelet ,you should do : session.setAttribute("k", k);

rather than req.setAttribute("k", k);