Putting into HashMap at 0 index is causing ConcurrentModificationException

I have a simple HashMap algorithm:

HashMap<Integer, Integer> map = new HashMap<>();
map.put(1, 1);
map.put(2, 2);
map.put(3, 3);

Iterator<Integer> it = map.keySet().iterator();
while(it.hasNext()) {
    Integer key = it.next();
    if (key.equals(2)) {
        map.put(1, 2);
    }
}

and this is working fine. But when I will modify the condition body to:

if (key.equals(2)) {
    map.put(0, 2); // changed index '1' to '0'
}

it’s always throwing java.util.ConcurrentModificationException. The same is happening for key values lesser than 0.

What am I missing?

Edit

Seems that if I will remove the third Map element:

map.put(1, 1);
map.put(2, 2);
// map.put(3, 3);

it’s working fine

>Solution :

You are changing the size of the map during iteration which causes the exception. Keep in mind that it is not put operation that throws the exception but an attempt to get next element via iterator via Iterator#next