ConcurrentModificationException When removing element using list iterator java

Question

I have an issue removing the 1st and 2nd element of my list even by using the iterator.

I have read the following threads but can't fix my issue (those were the most relevant but I checked other material as well):

ConcurrentModificationException when trying remove element from list

Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

So my code looks like this:

List<List<String>> list = cnf.read();
List<List<String>> nlist = new ArrayList<>();
for (List<String> l : list) {
    if (l.size() <= 3) {
        nlist.add(l);
    } else {
        int size = l.size();
        while (size > 3) {
            List<String> three = l.subList(0, 2);
            three.add("Y" + (count++));
            //Iterator itr = l.iterator();
            ListIterator itr = l.listIterator();
            int v = 0;
            while (itr.hasNext()) {
                itr.next();
                if (v == 0 || v == 1) {
                    itr.remove();
                    v++;
                }
            }
            l.add(0, "Y" + (count++));
            size--;
            nlist.add(three);
        }
        nlist.add(l);
    }
}
for (List<String> l : nlist) {
    System.out.println(l.toString());
    System.out.println(l.size());
}

I get a ConcurrentModificationException at the print statement here :

System.out.println(l.toString());

I tried using iterators for my 2 for loops as well but It doesn't seem to make a difference! I am new to posting questions so let me know If I am doing it right! Thank you.

Are you familiar with interface java.util.ListIterator? According to its javadoc, it allows modification of the list while traversing it. — Abra
– Abra, Commented Apr 21, 2019 at 22:03
@Abra Yes! I am using the iterator to do itr.remove(). This should allow me to remove the items I want from the list. However, It causes an exception. My guess is that it's because it is located in another loop (nested) so it's causing issues. I don't know how to fix it. — Mamoun Debbarh
– Mamoun Debbarh, Commented Apr 21, 2019 at 22:07
@Abra So the line: Iterator itr = l.iterator(); Is not a ListIterator but a simple Iterator? I am checking it now to find a solution. It might be a lead. — Mamoun Debbarh
– Mamoun Debbarh, Commented Apr 21, 2019 at 22:12
Even by using ListIterator it = l.listIterator();. I get the same exception. As I understood from the following post: techdifferences.com/… The main difference between the 2 methods is that list iterator allows you to modify and element and traverse the list in both directions. — Mamoun Debbarh
– Mamoun Debbarh, Commented Apr 21, 2019 at 22:15

Mamoun Debbarh · Accepted Answer · 2019-04-22 14:05:00Z

0

After A long debugging, here is the solution.

The sublist function passes by reference and not by value, a sublist created by ArrayList.subList call keeps a reference to the original list and accesses its elementData array directly.

For this reason, when adding an element to the "three" list, we alter the state of the original list. this happens here:

three.add("Y" + (count++));

A way of fixing it for this specific case is to create and initialize the "three" list the following way:

                String one = l.get(0);
                String two = l.get(1);
                List<String> three = new ArrayList<>();
                three.add(one);
                three.add(two);
                three.add("Y" + (count));

This allows us to manipulate our lists without getting Concurrency Exceptions (ConcurrentModificationException). However, if you are manipulating big lists, I would suggest you use another less hardcoded method for list creation.

I will mark this thread as answered and hope it helps people.

answered Apr 22, 2019 at 14:05

Mamoun Debbarh

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2 Comments

stonar96 Over a year ago

Nevertheless, I do not understand why the ConcurrentModificationException is thrown at all. toString() should use it's own Iterator and the list isn't modified after toString() was invoked. So there is still the question where the ConcurrentModificationException comes from. It would be nice if someone could answer that.

Mamoun Debbarh Over a year ago

@stonar96 I think it's because we are iterating through 'nlist'. The iteration causes the exception because the lists it contains are altered the "wrong way". This is my best guess and I am not a pro though!

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