Caused by: java.util.ConcurrentModificationException

Question

I don't understand why this is happening. I was doing a bit of research on other questions and I found out that you can't modify an collection while using a for loop. However, I am using an Iterator, why is it not working?

int counter = 0;
    int otherCounter = 0;
    ArrayList<Character> chars = new ArrayList<Character>();
    Iterator<Character> i = chars.iterator();
    for (char s : e.getMessage().toCharArray()) {
        chars.add(s);
    }
    while (i.hasNext()) {
        char s = i.next();
        if (chars.get(otherCounter + 1) == s) {
            counter++;
        } else {
            counter = 0;
        }
        if (counter >= 2) {
            i.remove();
        }
        otherCounter++;
    }

I am getting an error on this line for some reason: char s = i.next();

Answer 1

You're adding to the collection after creating the iterator.
This throws that exception.

You need to create the iterator after you finish modifying the collection.

Answer 2

This is because an "enhanced for loop" as you are using it creates an Iterator behind the scenes.

In fact, when you write:

for (X x: whatever) {
    // do something with x
}

the "real", generated code goes something like:

Iterator<X> iterator = whatever.iterator();

while (iterator.hasNext()) {
    X x = iterator.next();
    // do something with x
}

And this is true if whatever implements Iterable<X> (which List does; in fact, any Collection does)

In your example, you create a List<Character> , create a first Iterator<Character> over it, and your for loop creates another Iterator<Character> , over the same list.

Note that you created the first before you modified your List ... Hence the result. The first Iterator , which you reuse only afterwards, will detect that in the meanwhile, its underlying list has been modified: boom.