NPE issue

[Joseph Smithern]

Please advise reasons for NullPointerException in a servlet dealing with request.getParameter in Tomcat 6.0.20.
The below does work where the city value does forward and show in my JSP (pageOne.jsp or pageTwo.jsp).
But it also outputs the printStackTrace() NullPointerException on this one line everytime: if(city.equals("Boston")).

public class ProServlet extends HttpServlet {
  public void doGet (HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
  {
    String city = request.getParameter("city"); 
    request.setAttribute("city", city);
    try {
        if(city.equals("Boston"))
        {
         //forward to pageOne.jsp
        }
        else
        {
          //forward to pageTwo.jsp
        }
        
    }
    catch(Exception e)
    {
           e.printStackTrace();
     }
    
 
  }
}

The NullPointerException can be eliminated if I do this:
String city = "Boston" so I assume something is wrong with the request.getParameter.

I was also able to show the city value without NullPointerException in the Servlet output page if I use the PrintWriter and comment out the conditions:

public class ProServlet extends HttpServlet {
  public void doGet (HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
  {
    String city = request.getParameter("city"); 
    request.setAttribute("city", city);
    try {
/*
        if(city.equals("Boston"))
        {
         //redirect to pageOne.jsp
        }
        else
        {
          //redirect to pageTwo.jsp
        }
*/
        

            PrintWriter out = res.getWriter();
            out.println("Show city value " + city);
            out.close();
            
    }
    catch(Exception e)
    {
           e.printStackTrace();
     }
    
  }
}

Please advise.

[Bear Bibeault]

If city is null, you'll get an NPE. If it isn't, you won't.

Catching exceptions in a servlet? Really?

[Nicola Garofalo]

As i see the code, if you tell that city is not null after you get its value from the request, is not possible that your condition arises a NullPointerException.

Try to substitute the condition with the following one, just to eliminate any doubts:

if(city!=null && city.equals("Boston")) 

if you keep getting the NPE the problem is not there

[Bear Bibeault]

Or, if it's insignificant that city might be null, the old trick of:

if ("Boston".equals(city))

could be used.

But, it's not clear yet whether city being null is significant or not.