Re: NullPointer being thrown despite check

From:

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

Newsgroups:

comp.lang.java.help

Date:

Thu, 4 May 2006 20:40:30 -0400

Message-ID:

<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)

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
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. 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 the list.

--
Rhino