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exception thrown from servlet function

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I have a servlet "" 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 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.
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Check boxes only come back in the request if they are selected. If you query a parameter value and it wasn't sent, you get a null value.
Consider Paul's rocket mass heater.
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