java - Etat HTTP 500 - NullPointerException -

i'm getting above error message when trying display user table mysql db. can see, error generated displayusersservlet @ line 33 folloxing code : arraylist arr = userdb.selectusers();

java.lang.nullpointerexception         user.displayusersservlet.doget(displayusersservlet.java:33)         javax.servlet.http.httpservlet.service(httpservlet.java:621)         javax.servlet.http.httpservlet.service(httpservlet.java:728)       here classes: 

package business;

import java.io.serializable;  public class user implements serializable {     private string firstname;     private string lastname;     private string emailaddress;      public user()     {         firstname = "";         lastname = "";         emailaddress = "";     }      public user(string first, string last, string email)     {         firstname = first;         lastname = last;         emailaddress = email;     }      public void setfirstname(string f)     {         firstname = f;     }     public string getfirstname()     {          return firstname;      }      public void setlastname(string l)     {         lastname = l;     }      public string getlastname()     {          return lastname;      }      public void setemailaddress(string e)     {         emailaddress = e;     }      public string getemailaddress()     {          return emailaddress;      } } 

servlet class

package user;  import java.io.*;  import javax.servlet.*; import javax.servlet.http.*;  import java.util.arraylist;  import business.user; import data.userdb;  public class displayusersservlet extends httpservlet {       private static final long serialversionuid = 1l;      userdb userdb;      protected void doget(httpservletrequest request,             httpservletresponse response) throws servletexception, ioexception {         // todo: arraylist of user objects database          arraylist<user> arr = userdb.selectusers();          httpsession session = request.getsession();         // todo: set arraylist of user objects session attribute         session.setattribute("users", arr);          string url = "/users.jsp";         requestdispatcher dispatcher = getservletcontext()                 .getrequestdispatcher(url);         dispatcher.forward(request, response);     }      protected void dopost(httpservletrequest request,             httpservletresponse response) throws servletexception, ioexception {         doget(request, response);     } } 

dao

package data;  import java.sql.*; import java.util.arraylist;  import business.user;  public class userdb  {        // todo: add code returns arraylist of user objects             // corresponds rows in user table     public arraylist<user> selectusers() {          connectionpool pool = connectionpool.getinstance();         connection connection = pool.getconnection();         preparedstatement ps = null;         resultset rs = null;           string query = "select * user";         arraylist<user> arrusr = new arraylist<>();          try {             ps = connection.preparestatement(query);             rs = ps.executequery();             user user = null;             while (rs.next()) {                  user = resultsetrowtousers(rs);                  if (user.getfirstname()!=null)                      arrusr.add(resultsetrowtousers(rs));             }             } catch (sqlexception e) {             // todo auto-generated catch block             e.printstacktrace();         }          return arrusr;       }        public user resultsetrowtousers(resultset resultset) throws sqlexception {          //user user = new user();         return new user(resultset.getstring(1), resultset.getstring(2),                 resultset.getstring(3));      }  }     

print stacktrace (eclipse console)

jul 11, 2013 2:12:53 pm org.apache.catalina.core.standardwrappervalve invoke severe: "servlet.service()" servlet displayusersservlet has generated exception java.lang.nullpointerexception     @ user.displayusersservlet.doget(displayusersservlet.java:33)     @ javax.servlet.http.httpservlet.service(httpservlet.java:621)     @ javax.servlet.http.httpservlet.service(httpservlet.java:728)     @ org.apache.catalina.core.applicationfilterchain.internaldofilter(applicationfilterchain.java:305)     @ org.apache.catalina.core.applicationfilterchain.dofilter(applicationfilterchain.java:210)     @ org.apache.catalina.core.standardwrappervalve.invoke(standardwrappervalve.java:222)     @ org.apache.catalina.core.standardcontextvalve.invoke(standardcontextvalve.java:123)     @ org.apache.catalina.authenticator.authenticatorbase.invoke(authenticatorbase.java:472)     @ org.apache.catalina.core.standardhostvalve.invoke(standardhostvalve.java:171)     @ org.apache.catalina.valves.errorreportvalve.invoke(errorreportvalve.java:99)     @ org.apache.catalina.valves.accesslogvalve.invoke(accesslogvalve.java:947)     @ org.apache.catalina.core.standardenginevalve.invoke(standardenginevalve.java:118)     @ org.apache.catalina.connector.coyoteadapter.service(coyoteadapter.java:408)     @ org.apache.coyote.http11.abstracthttp11processor.process(abstracthttp11processor.java:1009)     @ org.apache.coyote.abstractprotocol$abstractconnectionhandler.process(abstractprotocol.java:589)     @ org.apache.tomcat.util.net.jioendpoint$socketprocessor.run(jioendpoint.java:312)     @ java.util.concurrent.threadpoolexecutor.runworker(unknown source)     @ java.util.concurrent.threadpoolexecutor$worker.run(unknown source)     @ java.lang.thread.run(unknown source) 

userdb needs initialized?