Is it not safe to insert into a list while iterating through it?

Question

Is it true that it is not safe to insert into a list while iterating through it? Thoughts? I'm not sure personally...

"A list" is not all that helpful. Some lists may be "safe" to use in such a way, others may not. "Safe" is not particularly well defined either! — Affe, Mar 27 '13 at 03:25
I think it will throw an concurrentmodification exception if you modify the list directly — Arun P Johny, Mar 27 '13 at 03:25
See this [answer](http://stackoverflow.com/a/7409771/2024761) — Rahul, Mar 27 '13 at 03:26
@Affe It's not possible without the iterator right? Apart from that — Thihara, Mar 27 '13 at 03:40

score 1 · Answer 1 · answered Mar 27 '13 at 04:19

If you are iterating through a collection using an Iterator object, then changing the underlying collection will create a ConcurrentModificationError that will crash the code. This applies even if you are using a for-each loop, because this type of loop implicitly declares an Iterator.

More information on ConcurrentModificationException.

score 1 · Accepted Answer · answered Mar 27 '13 at 04:24

As I expected it throw ConcurrentModificationException. I test it on simple example:

public class Test {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>();
        list.add(1);
        list.add(2);
        list.add(3);
        for(Iterator<Integer> it = list.iterator(); it.hasNext();it.next()){
            System.out.println(it.toString());
            list.add(4);
        }
    }
}

changen ArrayList to LinkedList give the same result. If I remember exactly only remove operation are valid

Is it not safe to insert into a list while iterating through it?

2 Answers2