Calculate the sum of elements in servlet session

[Galichka]

i have retrieved my database to Servlet, so that now it looks like this:

for (int i = 0; i < ex.getExpenses().size(); i++) {
        out.println("<tr>");
        out.println("<td > " + ex.getExpenses().get(i).getNum()+ "</td>");
        out.println("<td > " + ex.getExpenses().get(i).getPayment() + "</td>");
        out.println("<td > " + ex.getExpenses().get(i).getReceiver() + "</td>");
        out.println("<td > " + ex.getExpenses().get(i).getValue() + "</td>");
        out.println("<td><form ><input name = \"num\" type = \"hidden\" value = \""+ex.getExpenses().get(i).getNum()+"  \">");
        out.println("<input type = \"submit\" value = \"add\">");
        out.println("</form></td>");
        out.println("</tr>");


    }
    out.println("<td></td><td></td><td></td><td></td><td><input type = \"submit\" value = \"get the SUM\"></td>");
    out.println("</table>");

what else i have to do is using session tracking (and hidden field type which i have "add") write i servlet that will calculate the sum of chosen values ( like if i press add after value 22 and then after value 5555 it will show the result when clicking "get the sum button" 5577.0). it looks like adding to shopping cart and then getting the sum of what i want to buy. but i've never done this before, so i'm asking for help.
i know that i have to create an object of session (in another servlet and the servlet that stores my database will redirect it to the session servlet or smth like this) that will store all of the selected "add" buttons.

HttpSession session = request.getSession();

. but i can't go any further than this. could you tell me what are my next steps and what am i doing wrong?

[DarrylBurke]

Cross posted
java - calculate the sum of selected items in servlet using session tracking - Stack Overflow

db

[Galichka]

it's the exact same post. there is nothing there

[Tolls]

On each 'Add' call store the value of 'num' in the session.
Can you do that?
What value are you supposed to be storing in the session, though?
Is it the 'Number' or 'Value' columns?

[JosAH]

it's the exact same post. there is nothing there

There are some quite valuable tips in the replies to your cross post ...

kind regards,

Jos