Exception thrown from servlet function

Question

Hi,
I have a servlet "main.java" which connects to a database and displays a table. The table contains a check box in front of each table row. It has three buttons at the bottom "add" modify" "delete". When the user clicks on some check boxes and clicks on "delete" button how can i know in the script delete.java that these are the check boxes that have been checked or selected. I tried to do in the following way
                       String[] checkedBoxes = new String[100];
                       checkedBoxes = req.getParameterValues("sel");where sel is the name of the checkboxes
but I am getting the following error:
[21/Aug/2001:10:36:54] failure ( 1118): Internal error: exception thrown from the servlet service function (uri=/servlet/delete.class):java.lang.NullPointerException, Stack: java.lang.NullPointerException
Any help would be appreciated.
Thanks

Answer 1

Does every checkbox have the same name, i.e. "sel".
In this case you're dealing with a checkbox group. Similar to a select-style dropdown or radio button group.
I know I've had problems with dropdowns in Netscape where the select parameter was not being passed at all, so I'd get null values. I got around this by having the form submission re-assign the value from the select list to a hidden variable and then getting the value from the request.getParameter("hiddenFieldName") instead of the "dropdownFieldName".
You could try to name each checkbox individually using an appended ID, such as sel1, sel2, etc. and then have these parameters picked up using a loop construct, request.getParameter("sel" + i) -- you may want to pass some value back that would also indicate how many total checkboxes you had so you could determine how many iterations of the loop to go through.
On the form side, you could probably set the checkboxes up using a similar looping mechanism so that you wouldn't have to explicitly write each checkbox name.
If this is in fact similar to the Netscape issue I was having with dropdowns, I'd be interested to hear if it'll work in IE.
I got a bit off-topic, but hope it helps.