Exception in thread "main" java.lang.NullPointerException

I am new to java. And I am keeping receiving this error message. I don't know why this is happening

Code:

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import java.util.*;
import javax.swing.*;
/**
 *
 * @author Administrator
 */
public class CarRegistation {
    private ArrayList CarOwner = null;
    private ArrayList CarRegInfo = null;

    public void CarRegistation(){
        CarOwner = new ArrayList();
        CarRegInfo = new ArrayList();
    }

    public void newCarRegistation(String oc, String nic, String man, String Reg){
        //try{
              //PersonsInfo p = new PersonsInfo(oc, nic);
              PersonsInfo p = new PersonsInfo(oc, nic);
              Cars c = new Cars(man, Reg);
              CarOwner.add(p);        //error comes here Null Pointer
              CarRegInfo.add(c);
        /*}catch (Exception e){
            JOptionPane.showMessageDialog(null, e.getMessage());
      }*/
    }

    public void searchCarRegistation (String n) {
        // try{
            for(int i=0; i<CarOwner.size(); i++){
                PersonsInfo p = (PersonsInfo)CarOwner.get(i);
                Cars c = (Cars)CarRegInfo.get(i);
                if (n.equals(p.getName())){
//                    WindowsApplication wa = new WindowsApplication();
//                    wa.txtManufacturer.setText(c.getCarManufacturer());
//                    wa.txtOwnerOfCar.setText(p.getName());
//                    wa.txtNicNo.setText(p.getNicNo());
//                    wa.txtRegistationNo.setText(c.getRegistation());
                }
            }
        /*}catch (Exception e){
            JOptionPane.showConfirmDialog(null, e.getMessage());
      }*/
    }

    public void deleteCarRegistation(String nic){
        /*try{*/
            for(int i=0; i<CarOwner.size(); i++){
                PersonsInfo p = (PersonsInfo)CarOwner.get(i);
                //Cars c = (Cars)CarRegInfo.get(i);
                if (nic.equals(p.getNicNo())){
                    CarOwner.remove(i);
                    CarRegInfo.remove(i);
                    JOptionPane.showMessageDialog(null, "Car having NIC: " + p.getNicNo() + "has been remove from list");
                }
            }
        //}catch (Exception e){
          //  JOptionPane.showConfirmDialog(null, e.getMessage());
        //}
    }

    public int countCarRegistation(String n){
        int size = 0;
        //try{
            for(int i=0; i<CarOwner.size();i++){
                PersonsInfo p = (PersonsInfo)CarOwner.get(i);
                if(n.equals(p.getName())){
                    size = size + 1;
                }
            }
        //}
        //catch (Exception e){
        //    JOptionPane.showConfirmDialog(null, e.getMessage());
        // }
        return size;
    }

}

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Please help me out as I have to submit this code ASAP. Thanks in advance.

I am guessing it has to do with your arraylists? is pointing into a null position. post you java error stack. Its had to tell without knowing where the error occurs

Exception in thread "main" java.lang.NullPointerException
at mytesting.CarRegistation.newCarRegistation(CarRegi station.java:29)
at mytesting.Main.main(Main.java:29)

I have four class right now. One of then is PersonalInfo and Cars these are classes which I want to make collection and main class

CarOwner is null
how can a null object call method size

i cant find constructor in your class

public void CarRegistation(){
CarOwner = new ArrayList();
CarRegInfo = new ArrayList();
}

this is the constructor

just ran a couple of quick tests and this seemed to work

When declaring your arrays try :

Code:

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private ArrayList<PersonsInfo> CarOwner = new ArrayList<PersonsInfo>();

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And i know it is sometimes easy to over look stuff when passing Variables so make sure the types in the constructor for Car and personInfo accept strings!!

Not Working anyone else help please please do it

thx for everyone i have resolve my problem 'mtyoung' is very close i have constructor but its not working for me

public void CarRegistation(){}
never be a constructor

above is a void method call CarRegistation

without self defined constructor, default constructor will initialize your ArrayList

myToung is right

thx myToung, Nisha and everyone else who help to solve my problem.