Why I'm not getting ConcurrentModificationException while removing element from ArrayList during iteration

Question

I am using the following code to loop through an arraylist and then removing one element from the arraylist.

Here i'm expecting ConcurrentModificationException. But didn't get that exception. especially when you are checking condition with (n-1)th element. Please help me. Below is my code.

    ArrayList<Integer> arrayList = new ArrayList<Integer>();

    for (int i = 1; i <= 10; i++) {
        arrayList.add(5 * i);
    }
    System.out.println(arrayList);

    Iterator<Integer> iterator = arrayList.iterator();
    while (iterator.hasNext()) {
        Integer temp = iterator.next();
        if (temp == 45) {
            /**
             * temp == 40 (then i'm getting *ConcurrentModificationException) why not i'm
             * getting ConcurrentModificationException if (temp == 45)
             */
            arrayList.remove(1);
        }
    }
    System.out.println(arrayList);

Thanks in Advance

It's not foolproof. The exception is thrown if modification is noticed, it's not always noticed. — Kayaman, Oct 10 '19 at 09:45
use `iterator.remove();` instead to get rid of the exception. — Another coder, Oct 10 '19 at 09:47

score 3 · Accepted Answer · answered Oct 10 '19 at 09:49

The implementation makes a best effort to detect concurrent modification, but there are cases where it fails to do so.

The Iterator implementation returned for ArrayList's Iterator checks for concurrent modification in next() and remove(), but not in hasNext(), whose logic is:

public boolean hasNext() {
    return cursor != size;
}

Since you removed an element when the Iterator's cursor was at the element before the last element, the removal causes hasNext() to return false (since size becomes equal to cursor after the removal), which ends your loop without throwing an exception.

Why I'm not getting ConcurrentModificationException while removing element from ArrayList during iteration

1 Answers1