When null!=null :(

Question

hello all,
having a small issue today... i can't test for null correctly. here's some code...
<%
String thisString = null;
String thisString_sent = request.getParameter("thisString");
if (thisString_sent!=null) {
out.println(thisString);
%>
<form method="post" action="#">
<input type="button" onclick="form.submit();">
<input type="hidden" name="thisString" id="thisString" value="<%=thisString_sent%>">
</form>
when the form is submitted and thisString is null, the if statement doesn't consider the string to be null and prints "null". weird. I've also tried:
if (!thisString_sent.equals(null)) but i get a nullpointerexception... why is this happening?
signed,
newb

Answer 1

Well, because thisString_sent is never assigned a value.
This is what happens:
(1) you load the page and thisString_sent becomes null because request.getParameter("thisString") returns null.
(2) you post your form sending the hidden input 'thisString' with the value of thisString_sent, which is null
(3) the page is loaded and once again thisString_sent becomes null because request.getParameter("thisString") returns null.
(4) and so on and so on
Try making the input a textfield, remove the value="<%= thisString_sent %>" and input some string and post it.