Html select change to make JSP page forward keeping the exsting field value

Question

Hello, All,
I am trying to make the change of a selection box in HTML form to reload the page and populate the second selection box based on the value in the first selection box. The content of the second selection box is from database....
i used a javascript funtion and called it by the onChange(), the onChange redirect the page to the same page. However all my previous input is lost and the second selection box is not populated based on the first selection box.
The question is how do I keep the current input and reload the page based on the change of a selection box?
Please help..
Thanks..
Here is the code
<script language=javascript type="text/javascript">
function forward(){
location.href="trdCapDetail.jsp";
</script>
<select name=vehicle onChange="forward()">
<%for(int i = 0; i < Vehicle.getAll().size(); i++){
     Vehicle vehicle = (Vehicle)(Vehicle.getAll().elementAt(i));     
     out.write("<option value=" + vehicle.getKey()+" >" + vehicle.getCode() + "</option>");
%>

Answer 1

Ok two things.
1 - you need to submit the value which was selected.
2 - you need to repopulate the select box with it.
1 - you need to submit the value which was selected.
You do this by either appending the parameter to the url, or calling form.submit().
<script language=javascript type="text/javascript">
// either set parameter on url manually
function forward(control){
var param = control.value;
location.href="trdCapDetail.jsp?vehicle=" + param;
// or submit the form:
function forward(control){
  control.form.submit();
<% pageContext.setAttribute("vehicles", Vehicle.getAll() ) ; %>
<form name="myForm" action="trdCapDetail.jsp"> 
  <select name="vehicle" onchange="forward(this)">
      <c:forEach var="vehicle" items="${vehicles}">
        <option value="<c:out value="${vehicle.key}"/>"> <c:out value="${vehicle.code}"/> </option>
      </c:forEach>
   </select>   
</form>2: populating the control
One way is to use javascript again:
<c:if test="${not empty param.vehicle}">
  <script language="javascript">
    document.myForm.vehicle.value = "<c:out value="${param.vehicle}"/>";
  </script>
  </c:if>
// or scriptlet code if you must
<%
String selectedVehicle = request.getParameter("vehicle");
if (selectedVehicle != null){  %>
  <script language="javascript">
    document.myForm.vehicle.value = "<%= selectedVehicle%>";
  </script>
<%
%>The other way to keep the current input is to generate the string "selected" inside the option tag for the selected item. In your loop, check if the current id equals the submitted id, and print "selected" if it is.
Hope this helps,
evnafets