problem with form submission

A servlet passes parameters to a jsp. this jsp is using javascript. there is a button in this jsp "Evaluate" which when clicked i want to update the grandtotal.but when i click the button, gives me nullpointer exception because values passed from servlet are null. I dont know when i click Evaluate, why it gets the parameters from servlet again. i am not even submitting the form again.
my jsp:
<html>
<head>
<title> Check Out</title>
<script language="JavaScript">
function mode(field)
{
var val = field.value;
document.write(val);
}
</script>
</head>
<body>
<%
// code for getting values from Servlet
%>
<form name="chkout">
<table>
<tr>
<input type="radio" name="Overnight" value="overnight"> Overnight<br>
<input type="radio" name="Overnight" value="snail"> 3-5 days<br>
<input type="submit" name="evaluate" value="Evaluate" onClick="mode(Overnight)">
</td>
</tr>
</table>
</form>
</body>
</html>
if sb. could tell me what i am doing wrong and if could suggest me a right way to do it...
thanks a lot in advance.

I think the first step would be to examine the generated HTML to see if the values are really there. Do you have a prototype in plain HTML that works correctly?
Bill

------------------
author of:

servletClass
|
|
myjsp
|
|
chkoutServlet
//servlet.........
class servletClass{
.....
.....
public void forward(HttpServletRequest req,HttpServletResponse res,String url)
throws ServletException, IOException{
res.setContentType("text/html");
RequestDispatcher rd = getServletContext()
.getRequestDispatcher (url);
rd.forward(req, res);
}
public void service(HttpServletRequest req, HttpServletResponse res)throws IOException,ServletException{

//....
String overnight="Overnight";
String snail="3-5 Days";
req.setAttribute("Overnight" , overnight);
req.setAttribute("snail" , snail);
this.forward(req,res,"my.jsp");
}
}

my.jsp
-------------------
<html>
<head>
<title> Check Out</title>
<script language="JavaScript">
function mode(field)
{
var val = field.value;
document.write(val);
}
</script>
</head>
<body>
<%
String overnight=(String)request.getAttribute("Overnight");
String snail=(String)requset.getAttribute("snail");
%>
<form name="chkout" action="../chkoutServlet">
<table>
<tr>
<input type="radio" name="Overnight" value="<%=overnight%>"> Overnight<br>
<input type="radio" name="Snail" value="<%=snail%>"> 3-5 days<br>
<input type="submit" name="evaluate" value="Evaluate" onClick="mode(Overnight)">
</td>
</tr>
</table>
</form>
</body>
</html>

---------------------
class chkoutServlet{
.....
....
public void service(HttpServletRequest req, HttpServletResponse res)throws IOException,ServletException{
......
String overnight=request.getParameter("Overnight");
String snail=request.getParameter("Snail");
......
}

}
If I have done wrongly...I am very sorry and correct me

dayanand, Bill
thanks for responding..
i found out that when i click "Evaluate" button, form is being submitted which i dont want. So I made Evaluate a simple button which calls a javascript.

<input type="radio" name="Overnight" value="overnight"> Overnight<br>
<input type="radio" name="Overnight" value="snail"> 3-5 days<br>
<button type="button" name="evaluate" onClick="mode()">Evaluate</button>

now when i click Evaluate button, i want to know which radio field was selected, thats why I kept the name of both radio same.

now problem is :
document.myform.Overnight.value; does not seem to return anything.......
my javascript function is :
function mode()
{
var test = document.myform.Overnight.value;
if( test == "overnight")
{
alert('overnight');
}
else if( test == "snail")
{
alert('snail');
}

}
i donot get the alerts when any of the radio is selected.. please let me know the reason..
thanks to you all.

For radio buttons and checkboxes, you want to check the "checked" state.
if(yourControlName.checked)
doWhatEver....

Bosun

bosun
how do i retrieve the value of the checkbox.
I have following 2 checkboxes:
<input type="radio" name="Overnight" value="overnight"> Overnight<br>
<input type="radio" name="Overnight" value="snail"> 3-5 days<br>
document.myform.Overnight.checked, will let me know that checkbox is selected but wouldn't tell me which checkbox ?? how can i detemine which checkbox is selected ?
thanks