Question

I have a class User that contains attributes: nickname, ipAddress, sharedFolder. The idea is to have a user with these attributes and a list of files from a shared folder.

This is my code:

import java.io.*;
import java.util.*;

public class User {

    String nickname;
    String ipAddress;
    static ArrayList<String> listOfFiles;
    File sharedFolder;
    String fileLocation;

    public User(String nickname, String ipAddress, String fileLocation) {

        this.nickname = nickname.toLowerCase();
        this.ipAddress = ipAddress;
        sharedFolder = new File(fileLocation);

        File[] files = sharedFolder.listFiles();
        listOfFiles = new ArrayList<String>();

        for (int i = 0; i < files.length; i++) {
            listOfFiles.add(i, files[i].toString().substring(fileLocation.length()));
        }
    }

    public static void showTheList() {
        for (int i = 0; i < listOfFiles.size(); i++) {
            System.out.println(listOfFiles.get(i).toString());
        }
    }

    @Override
    public String toString() {
        return nickname + " " + ipAddress;
    }

    public static void main(String[] args) {
        showTheList();
    }
}

However, when I run it I can’t get the list of files. It throws an exception:

Exception in thread “main”
java.lang.NullPointerException
at User.showTheList(User.java:35)
at User.main(User.java:52) Java Result: 1

I know it’s probably a tiny little mistake but I can’t seem to fix it 🙁
Please help.

Answer 1

listOfFiles being a static field should be initialized in a static block, not constructor.

i.e.

static{
    listOfFiles = new ArrayList<String>();
}

or you can also initialize it at the site of declaration itself.

i.e.

static ArrayList<String> listOfFiles = new ArrayList<String>();

---

In your code, you are not creating any object of the class and hence listOfFiles reference is never assigned an ArrayList instance. That’s the reason you’re getting a NullPointerException.