Exception in thread "Thread-0" java.lang.NullPointerException

Question

I have a method in a client server program which is meant to create an instance of a citylist class, which is just an array list of cities...

(This method is in server)

public void listCities() {
    CityList LoadCities = new CityList();
    String CityDetails = LoadCities.cities.toString();
    try {
        dos.writeUTF("These are the cities serviced by the ssystem");
        dos.writeUTF(ServerConstants.CR_LF);
        dos.writeUTF(CityDetails);
    } catch (IOException ex) {
        Logger.getLogger(SocketHandler.class.getName()).log(Level.SEVERE, null, ex);
    }

}

The CityList Class is this:

public class CityList {

public ArrayList<City> cities;
    City melbourne = new City("Melbourne");
    City sydney = new City("Sydney");
    City darwin = new City("Darwin");
    City launceston = new City("Launceston");
    City hobart = new City("Hobart");

//default constructor
public CityList() {
    cities.add(melbourne);
    cities.add(sydney);
    cities.add(darwin);
    cities.add(launceston);
    cities.add(hobart);
}


//other constructor
public CityList(ArrayList<City> cities) {
    this.cities = cities;
}

public int size()
{
    return cities.size();
}



}

The client exists in a thread... The server calls a socket handler in its own thread class SocketHandler extends Thread class SocketHandler extends Thread

I continually get this error when I try to call that particular method (list cities)

Exception in thread "Thread-0" java.lang.NullPointerException
at hotelbroker.CityList.<init>(CityList.java:30)
at hotelbroker.SocketHandler.listCities(MultiEchoServer.java:153)
at hotelbroker.SocketHandler.run(MultiEchoServer.java:95)

I know this is cuz I'm a n00b and I need to call some kind of this instance thingummy I just have no idea...

public ArrayList<City> cities; is not initialized.

Sotirios Delimanolis
– Sotirios Delimanolis

2014-04-14 02:28:36 +00:00
Commented Apr 14, 2014 at 2:28 — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Apr 14, 2014 at 2:28

Sotirios Delimanolis · Accepted Answer · 2014-04-14 02:39:40Z

1

When you don't initialize instance fields, they get initialized to null by default. In this case, you are trying to use this instance variable

public ArrayList<City> cities;

without having first intialized it. It's therefore null.

When you try to invoke a method on a null reference, like here

cities.add(melbourne);

you get a NullPointerException.

You have to initialize it, either where it is declared

public ArrayList<City> cities = new ArrayList<>();

or before you use it

cities = new ArrayList<>();
cities.add(melbourne);

answered Apr 14, 2014 at 2:39

Sotirios Delimanolis

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Jason · Accepted Answer · 2014-04-14 02:40:43Z

0

public CityList() {
    cities = new ArrayList<City>();
    cities.add(melbourne);
    cities.add(sydney);
    cities.add(darwin);
    cities.add(launceston);
    cities.add(hobart);
}

answered Apr 14, 2014 at 2:40

Jason

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