NPE in Servlet

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.