I want add user on application ,but error ,why?

[putingyu]

hi :
  I want add  user on application because I don't like use telnet. but error. why?

package james;

/**
 * Title:
 * Description:
 * Copyright:    Copyright (c) 2001
 * Company:
 * @author
 * @version 1.0
 */
import org.apache.avalon.*;
import org.apache.avalon.blocks.*;
import org.apache.james.*;
import org.apache.james.transport.*;
import org.apache.james.userrepository.*;
import java.net.*;
import java.io.*;
import java.util.*;

public class addUser {
private String   username,password,restr;
private UsersRepository users;
private org.apache.james.James  mailServer;

  public addUser() {
username="pty";
password="pty";
  }
public String Add() {
mailServer=new org.apache.james.James();
if(mailServer.addUser(username,password)){
return restr="OK!";
}
else{
return restr="ERROR!";
}

}

 public static void  main(String [] args) {
    addUser adduser=new addUser();
    System.out.println(adduser.Add());
 }
}

error:
java.lang.NullPointerException

 at org.apache.james.James.addUser(James.java:464)

 at james.addUser.Add(addUser.java:44)

 at james.addUser.main(addUser.java:55)

Exception in thread "main"