How would this cause a concurrentModificationError?

Question

I understand that when using foreach loops or iterators, and removing elements from a list while iterating through it, it can cause the error by confusing the iterator. however I don't seem to be able to visualize how it can confuse it. And thus Im wondering if someone could explain to me how it can confuse an iterator? Thanks

1List<Integer> l3 = new ArrayList<>();
2 l3.add(0); l3.add(2); l3.add(3); l3.add(5);
3
4 int i = 0;
5 for (int e : l3) {
6 if (e % 2 == 0) l3.remove(i);
7 
8 }else{ i++};

I expect this to remove all even numbers and remain behind odd numbers, but i am told this could give a concurrentModificationError.

Answer 1

You are removing an element from the list using

if (e % 2 == 0) l3.remove(i);

while at the same time, iterating with an iterator here

for (int e : l3) {

So the list contents and size are out of sync with the iterator causing the exception to be thrown. You should explicitly create an iterator and use that for removal.

List<Integer> l3 = new ArrayList<>();
l3.add(0); l3.add(2); l3.add(3); l3.add(5);

Iterator<Integer> it = l3.iterator();
while (it.hasNext()) {
    int v = it.next();
    if (v % 2 == 0) {
        it.remove();
    }
}
System.out.println(l3);

prints

[3, 5]

Or without using an iterator or explicit loop, use removeIf

l3.removeIf(item -> item % 2 == 0);