problem with MVC pattern

hi everybody

i was trying this Model View Controller pattern but got stuck.

the servlet(EmailServlet_2) that is the controller looks like this:

_________________________________________________________________
package servpack;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import business.*;
import data.*;

public class EmailServlet_2 extends HttpServlet {

    public void doGet(HttpServletRequest request,HttpServletResponse
response)
        throws ServletException, IOException{
            String firstName=request.getParameter("firstName");
            String lastName=request.getParameter("lastName");
            String eID=request.getParameter("eID");
            String[] music=request.getParameterValues("music");

            //page forward control
            if((firstName.length()==0)||(lastName.length()==0)||
(eID.length()==0)){

                RequestDispatcher
dispatcher=getServletContext().getRequestDispatcher("/
get_missing_fields.jsp");
                dispatcher.forward(request,response);
            }
            else if(music[0].length()==0){
                RequestDispatcher
dispatcher=getServletContext().getRequestDispatcher("/
select_an_option.jsp");
                dispatcher.forward(request,response);
            }

            User user=new User(firstName, lastName, eID);
         UserIO.addRecord(user,"C:/Program Files/Apache Software
Foundation/Tomcat 6.0/webapps/m_egs/WEB-INF/etc/UserEmail.txt");

         RequestDispatcher
dispatcher=getServletContext().getRequestDispatcher("/
show_email_entry_2.jsp");
         dispatcher.forward(request,response);
    }

    public void doPost(HttpServletRequest request,HttpServletResponse
response)
        throws ServletException, IOException{
            doGet(request,response);
    }
}
___________________________________________________________________

the 'music' element is a list box.

when i fill all fields i.e. firstName, lastName and eID and when i
select
at least one option in 'music' list box the 'show_email_entry_2.jsp'
is called
(as it should be) and application works correctly.
but when i leave any of the 3 fields (firstName, lastName and eID)
empty,
rather than showing 'get_missing_fields.jsp' this shows up in browser:

_________________________________________________________________________________
HTTP Status 500 -

type Exception report

message

description The server encountered an internal error () that prevented
it from fulfilling this request.

exception

java.lang.IllegalStateException: Cannot forward after response has
been committed
    servpack.EmailServlet_2_ex6.doGet(EmailServlet_2_ex6.java:33)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

note The full stack trace of the root cause is available in the Apache
Tomcat/6.0.13 logs.
Apache Tomcat/6.0.13
_________________________________________________________________________________

also when all three fields (firstName, lastName and eID) are filled
but
no option in 'music' list box is selected then server returns this:

_________________________________________________________________________
HTTP Status 500 -

type Exception report

message

description The server encountered an internal error () that prevented
it from fulfilling this request.

exception

java.lang.NullPointerException
    servpack.EmailServlet_2_ex6.doGet(EmailServlet_2_ex6.java:24)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

note The full stack trace of the root cause is available in the Apache
Tomcat/6.0.13 logs.
Apache Tomcat/6.0.1
________________________________________________________________________

please tell me where i m wrong