NPE when trying to remove an element from ArrayList

Question

I have an arrayList of Objects, and each object has a list of string members.

I'm trying to iterate through the list and remove any member of the list that has a specific element (status) that is null.

the objects elements are set to null in their constructor, and remain null if they are not filled in. I can't understand why I'm getting an NPE. I've debugged it and i can step through the loop and see each element and their members, but once i hit an element who's member "status" is null, I'm getting an NPE.

Any help appreciated.

    public ArrayList<FlightObject> processResults(ArrayList<FlightObject> list) {
    for (int i = 0; i< list.size(); i++) {
        if (list.get(i).status.toString() == null) {
            list.remove(i);

        }
    }
    return list;
}

is `status` is `null`, then `status.toString()` will throw NPE! — Héctor, Aug 25 '14 at 11:59
This cannot work `if (list.get(i).status.toString() == null)`. change `if (list.get(i).status.toString() == null)` to `if (list.get(i).status == null)` — Jens, Aug 25 '14 at 12:00

score 4 · Accepted Answer · answered Aug 25 '14 at 12:05

I guess that the NPE issue can be resolved with a simple null check like if (list.get(i).status == null) but you will encounter another issue once you correct that right. Changing the list that you actually traverse over is a bad idea cause you are going to probably miss some cases as you will skip over some of the elements ("you change the indexes"). To do that correctly you'll need and iterator like this:

public ArrayList<FlightObject> processResults(ArrayList<FlightObject> list) {
    for (Iterator<FlightObject> iter = list.iterator(); iter.hasNext();) {
        FlightObject fo = iter.next();   
        if (fo.status == null) {
            iter.remove();            
        }
    }
    return list;
}

Someone at least saw the bug! +1 – Seelenvirtuose Aug 25 '14 at 12:06 — Seelenvirtuose, Aug 25 '14 at 12:06
Thank you. That's what I needed. – user3363286 Aug 25 '14 at 12:10 — user3363286, Aug 25 '14 at 12:10

score 3 · Answer 2 · answered Aug 25 '14 at 11:59

3

When list.get(i).status is null, invoking any method on it will throw a NullPointerException.

The check should be simply if (list.get(i).status == null) {

answered Aug 25 '14 at 11:59

M A

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How about `list.get(i)` becoming null ? – Suresh Atta Aug 25 '14 at 12:00
I just assumed all elements are not null. – M A Aug 25 '14 at 12:05

score 0 · Answer 3 · answered Aug 25 '14 at 12:03

        public ArrayList<FlightObject> processResults(ArrayList<FlightObject> list) {
        for (int i = 0; i< list.size(); i++) {
            if (list.get(i)!=null && list.get(i).status!= null){
                list.remove(i);    
            }
          }
        }
        return list;
    }

3 Answers3