java - Trying to link two ArrayList objects together -

i've tried searching, haven't found me yet. here goes:

i'm trying connect card customer via customerid, can't seem getting work (nullpointerexceptions). objects put in arraylists.

i'm pretty sure problem in tostring() of liftcard().

import java.io.serializable;  public class liftcard implements serializable { private int cardnumber, cardtype, cardprice, cardpassing; int cardbalance; private user user; liftcard next;  public liftcard(int ct, int cn) {     cardnumber = cn;     cardtype = ct;     next = null;  }// end of konstruktqr  public int getcardnumber() {     return cardnumber; }  public int getcardtype() {     return cardtype; }  public user getuser(){     return user; }  public void setuser(user u){     user = u; }  public string tostring() {     return getuser().getcustomerid() + "\t" + cardnumber + "\t" + cardtype;  //pretty sure problem here! } 

this user-class want link card-class

import java.io.serializable;  public class user implements serializable { private string surename, firstname, phone, adress, birth; private int customerid; public liftcard liftcard; user next;  public user( int cid, string fn, string sn, string p, string a, string b) {     firstname = fn;     surename = sn;     phone = p;     adress = a;     birth = b;     customerid = cid;     liftcard = null;     next = null; }  public string getsurename() {     return surename; }  public string getfirstname() {     return firstname; }  public string getphone() {     return phone; }  public string getadress(){     return adress; }  public string getbirth() {     return birth; }  public int getcustomerid() {     return customerid; }  public liftcard getliftcard(){     return liftcard; }  public void setliftcard(liftcard liftc){     liftcard = liftc; }  public string tostring() {     return customerid + "\t" + surename + "\t" + firstname + "\t" + phone             + "\t" + adress + "\t" + birth;  }  public boolean equals(user u) {     return (u.getsurename().equals(surename) && u.getfirstname().equals(             firstname)); } } 

here i'm trying bind them together:

public void regliftcard() {     int cardtype = integer.parseint(cardtypefield.gettext());     int cardnumber = integer.parseint(cardnumberfield.gettext());     int customerid = integer.parseint(findcustomerfield.gettext());      if (cardtype != 1 && cardtype != 2 && cardtype != 3) {         joptionpane.showmessagedialog(this, "kortet må være 1,2 eller 3!");         return;     }     try {         string firstname = firstnamefield.gettext();         string surename = surenamefield.gettext();         string phone = phonefield.gettext();         string adress = adressfield.gettext();         string birth = birthfield.gettext();          user u = usera.findbyid(customerid);         liftcard l = new liftcard(cardnumber, cardtype);         if (u != null) {             if (u.getliftcard() != null) {                 joptionpane.showmessagedialog(this,                         "brukeren har allerede kort!");             } else                 carda.regliftcard(l);             joptionpane.showmessagedialog(this, "1kort registrert!");             return;         } else             u = new user(customerid++, firstname, surename, phone, adress,                     birth);         joptionpane.showmessagedialog(this, "bruker registrert!");         cardtypefield.settext("");     } catch (numberformatexception e) {         joptionpane.showmessagedialog(this, "feil nummerformat!");     }  } 

here first of 2 archive-classes. first 1 cards.

import java.util.*; import java.io.*;  import javax.swing.joptionpane;  public class cardarchive implements serializable { arraylist<liftcard> clist = new arraylist<liftcard>();  // setter inn nytt liftcard-objekt bakerst lista public void regliftcard(liftcard c) {     clist.add(c); }  public string tostring() {     string cards = "";     iterator<liftcard> iterator = clist.iterator();      while (iterator.hasnext()) {         cards += iterator.next().tostring() + "\n";     }     return cards; }  public liftcard findbycardnumber(int id) {     (liftcard c : clist) {         if (c.getcardnumber() == id) {             return c;         }     }     return null; // or empty card }   } 

and second user-archive:

import java.util.*; import java.io.*; import javax.swing.joptionpane;  public class userarchive implements serializable { arraylist<user> list = new arraylist<user>(); private static int idcounter = 1000;  // setter inn nytt user-objekt bakerst lista public void regcustomer(user u) {     list.add(u); }  // sorterer user-objektene alfabetisk på surename og first name public void sorter() {     collections.sort(list, new usercomp()); }  public user findbyid(int id) {     (user u : list) {         if (u.getcustomerid() == id) {             return u;         }     }     return null; // or empty user }  public string tostring() {     sorter();     string users = "";     iterator<user> iterator = list.iterator();      while (iterator.hasnext()) {         users += iterator.next().tostring() + "\n";     }     return users; }  } 

as can see, looking @ of code, i'm pretty new @ java. trying none less. , in advance!

edit:

used combination of both maloney , bmorris591 answers! code changed follows:

user u = usera.findbyid(customerid);         liftcard l = new liftcard(cardnumber, cardtype);         if (u != null) {             if (u.getliftcard() != null) {                 joptionpane.showmessagedialog(this,                         "brukeren har allerede kort!");             } else                 l.setuser(u);                 carda.regliftcard(l); 

and:

public string tostring() {     if (getuser() != null){         return getuser().getcustomerid() + "\t" + getuser().getsurename() + "\t" + cardnumber + "\t" + cardtype;      } else {        return null;     } } 

thanks guys!

from code looks using user field liftcard in tostring method:

public string tostring() {     return getuser().getcustomerid() //etc 

but when create liftcard not see setting user anywhere before registering it:

liftcard l = new liftcard(cardnumber, cardtype);         if (u != null) {             if (u.getliftcard() != null) {                 joptionpane.showmessagedialog(this,                         "brukeren har allerede kort!");             } else                 carda.regliftcard(l); //etc 

you need @ point call l.setuser(u) otherwise user field in liftcard null.

liftcard l = new liftcard(cardnumber, cardtype);         if (u != null) {             if (u.getliftcard() != null) {                 joptionpane.showmessagedialog(this,                         "brukeren har allerede kort!");             } else                 l.setuser(u); // <-- here set `user` on `liftcard`.                 carda.regliftcard(l); //etc