Getting a NullPointerException when creating a method that adds to an ArrayList

Question

This is my code:

private ArrayList<Integer> gradeList;  

public void addGrades(Scanner reader) {
        int grade = reader.nextInt();
        if (grade != -1) {
            this.gradeList.add(grade); // --> NullPointerException
        }
    }

I can't figure out why I am getting a NullPointerException, because I have made methods like this often without this problem. I thought changing it to this:

 public void addGrades(Scanner reader) {
        int grade = Integer.parseInt(reader.nextLine());
        while (true) {
            if (grade == -1) {
                break;
            } else {
                this.gradeList.add(grade); // --> NullPointerException
            }
        }
    }

would help, but it did not fix the problem. In both cases, the problem is on the line this.gradeList.add(grade). I also tried declaring the ArrayList to be null within the method before the loop, but realized that was redundant. So I'm not sure what I'm doing wrong.

Has gradeList been initialized?

Mena
– Mena

2014-03-24 13:18:51 +00:00
Commented Mar 24, 2014 at 13:18 — Mena
– Mena, Commented Mar 24, 2014 at 13:18
where do you initiate private ArrayList gradeList;???

nano_nano
– nano_nano

2014-03-24 13:19:32 +00:00
Commented Mar 24, 2014 at 13:19 — nano_nano
– nano_nano, Commented Mar 24, 2014 at 13:19

DaveH · Accepted Answer · 2014-03-24 13:34:31Z

5

You are never instanitating the arraylist. Try

private ArrayList<Integer> gradeList = new ArrayList<Integer>();

edited Mar 24, 2014 at 13:34

answered Mar 24, 2014 at 13:19

DaveH

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4 Comments

Balder Over a year ago

This is very likely the reason, but private ArrayList<Integer> gradeList; was added to the OP's code by an edit from RKC...

DaveH Over a year ago

OK - I'll update my answer to reflect the changes to the OP's question

user3430421 Over a year ago

I'm an idiot. I initialized the ArrayList in my constructor it fixed it. But I am getting a different error now, an "OutOfMemoryError" that still points to the same line. Is something wrong with my loop or scanner?

DaveH Over a year ago

Your while(true) loop looks suspicious to me. You are reading the input from the scanner and then adding it forever in to the arraylist

forgivenson · Accepted Answer · 2014-03-24 13:25:31Z

2

gradeList must be null. Make sure you initialize it before using it.

It can be initialized like this:

private ArrayList<Integer> gradeList = new ArrayList<Integer>();

edited Mar 24, 2014 at 13:25

answered Mar 24, 2014 at 13:19

forgivenson

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Comments

ethanfar · Accepted Answer · 2014-03-24 13:20:57Z

1

It seems like you're not initializing gradeList.

Either initialize it inline:

private ArrayList<Integer> gradeList = new ArrayList<Integer>();

Or in a constructor:

public MyClass() {
    gradeList = new ArrayList<Integer>();
    // ...
}

answered Mar 24, 2014 at 13:20

ethanfar

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Comments

Mena · Accepted Answer · 2014-03-24 13:21:09Z

1

Make sure your ArrayList is initialized.

For instance with an instance field gradeList as ArrayList<Integer>, use:

ArrayList<Integer> gradeList = new ArrayList<Integer>();

... before referencing it.

answered Mar 24, 2014 at 13:21

Mena

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Comments

jmail · Accepted Answer · 2014-03-24 14:03:38Z

0

you should must add the ArrayList is initialized. and

you should try this following as:

private List<Integer> gradeList = new ArrayList<Integer>();

answered Mar 24, 2014 at 14:03

jmail

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