Re: NullPointer being thrown despite check

From:

"Rhino" <no.offline.contact.please@nospam.com>

Newsgroups:

comp.lang.java.help

Date:

Fri, 5 May 2006 00:04:16 -0400

Message-ID:

<q7A6g.2128$VV2.155075@news20.bellglobal.com>

"Rhino" <no.offline.contact.please@nospam.com> wrote in message
news:p8x6g.2036$VV2.141348@news20.bellglobal.com...

"fb" <fb@noway.com> wrote in message
news:dAu6g.124611$WI1.83430@pd7tw2no...

I have a select list in an HTML file the eventually has the selected item
passed to a servlet. However, if I don't select an item in the list, it
seems to throw a NullPointerException. I was hoping to catch this with an
if/else statement (as seen below), but it doesn't seem to be working.
Other than catching the exception are there other ways to deal with this
problem? To be sure, is this what is causing the exception?

Thanks! Code snippet below:

            PW.println("<p>Please fill in the following: </p>");
            PW.println("<form ACTION=\"ViewProductInfoSV\"
METHOD=\"POST\">");
            PW.println("<p>Category List:</p>");
            PW.println("<p><select NAME=\"CategoryList\" MULTIPLE
size=\"5\">");

            while (myRSet.next()) {
                PW.println("<option
value=\""+myRSet.getString("category_id")+"\">"+myRSet.getString("description"));
            }

            PW.println("</select></p>");

            PW.println("<p>Code: <input TYPE=\"TEXT\" NAME=\"catCode\"
size=\"20\"></p>");
            PW.println("<p>Description: <input TYPE=\"TEXT\"
NAME=\"catDesc\" size=\"20\"></p>");
            PW.println("<p><input TYPE=\"SUBMIT\"> </p>");
            PW.println("</form>");

            PW.println("<br /><br />");
            PW.println("The following information was selected:<br />");

            //The problem appears to be here
            for (int i=0; i < theCatList.length; i++) {
                if (!theCatList[i].equals("")) {
                    PW.println("From the Category List: " +
theCatList[i]+"<br />");
                }else{
                    PW.println("A selection must be made from the
category list");
                }
            }
            PW.println("Code entered: "+ theCatCode+"<br />");
            PW.println("Description Entered: "+ theCatDesc+"<br />");

            theC.close();

        }

        catch (ClassNotFoundException ce) { PW.println("DB Error: " +
ce); }
        catch (SQLException se) { PW.println("SQL Error: " + se); }

        PW.println("</body></html>");
        PW.close();

    }

}

//Stack Trace Follows:

Servlet.service() for servlet viewProdInf threw exception
java.lang.NullPointerException
at ViewProductInfo.doGet(ViewProductInfo.java:59)
at ViewProductInfo.doPost(ViewProductInfo.java:12)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:237)
at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:157)
at
org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:214)
at
org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at
org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at
org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:825)
at
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:731)
at
org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:526)
at
org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Unknown Source)

I was a bit rushed when I wrote this so please let me supply a few
corrections....

Checking for an SQLException or a ClassNotFoundException in your try block
is absolutely pointless since you don't do anything that would throw those
exceptions.

I think the best solution for your problem is to test the value chosen by
the Catalog List listbox and see if anything was chosen; if it wasn't, do
the appropriate error handling, such as displaying a clear message to the
user so that he can try again.

Simply examine the value that was actual chosen for the Catalog List; if
no value was chosen, you should find the value to be null.

Now, I don't know how much work you've done with servlets so forgive me if
I'm saying things you already know. When I write servlets that are
managing forms, I have one servlet present the form to the user; the user
complets

I meant to write 'completes'....

the form and presses submit; then a second servlet examines the data
obtained from the form and verifies that it is all valid and complete. So,
if you follow this approach, the code you have already is fine for the
first servlet except that you should delete the try block and catches that
are doing anything for you.

I didn't mean to delete the entire try/catch block; keep the code that is in
the try block but remove the try/catch structure that you put around it.

Then, your first servlet should point to the second servlet with a line
like this:

out.println("<form action=\"FormChecker\" method=\"POST\"
ENCTYPE=\"multipart/form-data\">");

This line in the form servlet says that the servlet which will evaluate
the form is called FormChecker; within FormChecker, you will write a
doPost() that grabs each of the form values via
httpRequest.getParameter(). Then, if the form field is a list, and if the
value of that variable is null, then you know that the user chose no value
form

Oops, I meant to say 'from'....

the list.

--
Rhino

--
Rhino