Java NullPointerException in ArrayList

Question

This simple piece of code removes any Statistic in an arrayList if its properties are null. Unfortunately, I'm getting a java.lang.NullPointerException on the first line of the if Statement (if tempStat.serialName==null...). Thoughts?

public static ArrayList<Statistic> removeEmptyStats(ArrayList<Statistic> al) {
    Statistic tempStat=new Statistic();
    for (int i=0; i<al.size(); i++) {
        tempStat=al.get(i);
        if (tempStat.serialName==null || tempStat.serialPublisher==null || tempStat.serialISSN==null || tempStat.serialOnlineISSN==null) {
            al.remove(i);
        }
    }
    return al;
}

tempStat is null then

Alexis C.
– Alexis C.

2014-01-09 16:24:28 +00:00
Commented Jan 9, 2014 at 16:24 — Alexis C.
– Alexis C., Commented Jan 9, 2014 at 16:24
@ZouZou Better make that answer.

Branislav Lazic
– Branislav Lazic

2014-01-09 16:27:25 +00:00
Commented Jan 9, 2014 at 16:27 — Branislav Lazic
– Branislav Lazic, Commented Jan 9, 2014 at 16:27

M21B8 · Accepted Answer · 2014-01-09 16:26:44Z

2

It must be because tempStat itself is null. However you need to be very careful with this.

Say you have 5 elements in your list element 2 is invalid so you remove it, element 3 has now dropped back to element 2, but you're going to move on to check element 3 next, however this is now the old element 4 (if that makes any sense!)

answered Jan 9, 2014 at 16:26

M21B8

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YMomb · Accepted Answer · 2014-01-09 16:37:36Z

2

You should not remove entries of a list on which you are doing a loop.

tempstat is null, because when doing for (int i=0; i<al.size(); i++), it stores the initial value of al.size(). Then when you are doing al.get(i) after having removed some values, you can have a i which is higher than the size of the list. (BTW, once you remove a value, it also mean that you will skip one in your list).

You'd to have to use an iterator and iterator.remove().

Iterator<Statistic> iterator = al.iterator();
while(iterator.hasNext()) {
    Statistic tempStat = iterator.next();
    if (tempStat.serialName==null || tempStat.serialPublisher==null || tempStat.serialISSN==null || tempStat.serialOnlineISSN==null) {
        iterator.remove();
    }
}

edited Jan 9, 2014 at 16:37

answered Jan 9, 2014 at 16:26

YMomb

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5 Comments

rhobincu Over a year ago

I need 125 reputation... Reference for that claim? Because I don't think it's accurate.

YMomb Over a year ago

What do you mean @rhobincu?

rhobincu Over a year ago

@YMomb I mean you can very well remove the elements from an array in a for loop, you just need to make sure you decrement the loop counter after each remove. There is no language restriction to prevent you from doing that.

YMomb Over a year ago

No language restriction, just a bad practice. It is dirty to use a for loop with a counter and to update the counter inside the loop.

rhobincu Over a year ago

@YMomb There's a big difference between "cannot" and "should not".

Matteo Gatto · Accepted Answer · 2014-01-09 16:25:57Z

1

The variable tempStat is null! check the values contained in passed array al

answered Jan 9, 2014 at 16:25

Matteo Gatto

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