Why does the following code using Iterator next() and remove() throw ConcurrentModificationException?

Question

I tested the next() and remove() methods of the Iterator interface. I got the following exception:

Exception in thread "main" java.util.ConcurrentModificationException

Here is my code:

import java.util.*;

public class ListTest {
    public static void main(String[] args) {
        Collection<Integer> list = new ArrayList<Integer>();
        Iterator<Integer> iterator = list.iterator();

        Collections.addAll(list, 1, 2, 3, 4, 5);
        if (iterator.hasNext()) {
            iterator.next();
            iterator.remove();
        }

        System.out.println(list);
    }
}

It's probably better to close as duplicate as the answers here explain absolutely nothing about the exception. — Denys Séguret, Aug 05 '16 at 08:30
@DenysSéguret you are right, I added more details to my answer to explain a little bit more — Nicolas Filotto, Aug 05 '16 at 08:38
@DenysSéguret this is ungrateful side of SO, if you take too much time to answer you will get less votes sometimes it is better to provide a quick and dirty answer first and then improve it — Nicolas Filotto, Aug 05 '16 at 08:46
Or not answer at all and vote to close as duplicate, since they answer it already very well... I don't see the point in duplicating content. — Tunaki, Aug 05 '16 at 08:50

Eran · Accepted Answer · 2017-09-18T14:24:39.693

There is no problem with your usage of Iterator's next() and remove().

Your ConcurrentModificationException is caused by adding elements to the List after creating the Iterator.

You should add elements to the List before the Iterator is created.

Change:

Iterator<Integer> iterator = list.iterator();
Collections.addAll(list, 1, 2, 3, 4, 5);

to:

Collections.addAll(list, 1, 2, 3, 4, 5);
Iterator<Integer> iterator = list.iterator();

and your loop will work fine.

Nicolas Filotto · Answer 2 · 2016-08-05T08:37:23.277

You get this exception because you modify the sate of your List by adding elements after having created your iterator so when you call next() on your iterator, internally it checks if the underlying List has been modified and if so it raises a ConcurrentModificationException which is the case here.

Try to inverse the order as next:

Collections.addAll(list, 1, 2, 3, 4, 5);
Iterator<Integer> iterator = list.iterator();

Why does the following code using Iterator next() and remove() throw ConcurrentModificationException?

2 Answers2