Java – the special behavior of ArrayList remove () – why?

When we delete - 1 and empty the ArrayList, it will throw concurrentmodificationexception. When we delete 0 from the same empty ArrayList, it will throw NoSuchElementException

Please find the following code:

public class Test {
    public static void main(String[] argv) {

        ArrayList<Integer> list = new ArrayList<Integer>();
        Iterator<Integer> it = list.iterator();
        try {
            list.remove(-1);
        } catch (indexoutofboundsexception e) {

        }
        try {
            it.next();// Throwing ConcurrentModificationException
        } catch (ConcurrentModificationException e) {
            System.err.println("ConcurrentModificationException 1");
        } catch (NoSuchElementException e) {
            System.err.println("NoSuchElementException 1 ");
        }

        list = new ArrayList<Integer>();
        it = list.iterator();
        try {
            list.remove(0);
        } catch (indexoutofboundsexception e) {
        }
        try {
            it.next();// Throwing NoSuchElementException
        } catch (NoSuchElementException e) {
            System.err.println("NoSuchElementException 2");
        } catch (ConcurrentModificationException e) {
            System.err.println("ConcurrentModificationException 2 ");
        }

    }
}

From my understanding, NoSuchElementException is good, but why throw concurrentmodificationexception?

Solution

If you check the code of ArrayList First perform a range check and then add a modification count

rangeCheck(index);
modCount++;

In the range check method, the range check applies only to positive numbers

if (index >= size)
     throw new indexoutofboundsexception(outOfBoundsMsg(index));

Therefore, remove (0) does not add the mod count, but remove (- 1) does Modcount causes the iterator to throw a concurrentmodificationexception