Why am I unable to add lines from a text file to an ArrayList in Java?

Question

I have the following code below. If I comment out the stringList.add(row); I can get it to print the lines of the document (5 lines of text with spaces). If I don't comment it out, I get one line printed and the null. I have looked up everything I could online but I am not understanding why this is happening!

package examples.files;

import java.io.File;
import java.util.ArrayList;
import java.util.Scanner;

/**
 * @author 
 *
 */
public class ReadFileUsingScanner {
    static ArrayList<String> stringList = null;
    /**
     * @param args
     * @throws Exception 
     */
    public static void main(String[] args) {
        Scanner scanner = null;

        try {
            scanner = new Scanner(new File("customers.txt"));

            while (scanner.hasNext()) {
                String row = scanner.nextLine();
                System.out.println(row);
                stringList.add(row);
            }
        } catch(Exception e) {
            System.out.println(e.getMessage());
        } finally {
            if (scanner != null) {
                try {
                    scanner.close();
                } catch (Exception e) {}
            }
        }
    }

}

You don't have to solve the code but I want to understand the fundamentals as to why it doesn't work.

By the way, you should add the stacktrace to your questions. It isn't "printing the null", it is throwing an exception — OneCricketeer, Oct 18 '16 at 01:25
Thanks, I understand now. I will add that - which is probably why I wasn't catching such a simple mistake. — synthesize.me, Oct 18 '16 at 01:31

score 4 · Accepted Answer · answered Oct 18 '16 at 01:23

4

Your arrayList is not initialized. Change to

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

answered Oct 18 '16 at 01:23

Md Johirul Islam

5,042
4
23
56

1

Oh my god, thanks. That's embarrassing! – synthesize.me Oct 18 '16 at 01:26

score 0 · Answer 2 · answered Oct 18 '16 at 01:31

0

The bug is NPE(NullPointException).

And the next time you get a bug,you may not need to ask a question on StackOverFlow.Try Debugger Function of the IDE line by line.You will get the answer.

answered Oct 18 '16 at 01:31

Dai Kaixian

1,045
2
14
24

Thanks, I appreciate the help. – synthesize.me Oct 18 '16 at 02:15

Why am I unable to add lines from a text file to an ArrayList in Java?

2 Answers2