Java concurrent modification exception is not coming while using collections remove method

Question

I have seen in many places that it says when removing an element from an ArrayList while iterating, I should be using iterator remove method instead of collections remove method to avoid concurrent modification exception.

However, below code works fine using Java 1.8 Collection remove without giving concurrent modification exception. You can see that I am not using iterator here to remove the object.

List<MyObject> list = new ArrayList<MyObject>();
list.add(new MyObject());
list.add(new MyObject());
list.add(new MyObject());

for (int i=0; i<list.size(); i++) {
        list.remove(i);     
}

The concurrent modification exception is thrown on the best effort base, your program can't always expect it to be thrown when it happens — Ferrybig, Sep 24 '17 at 13:45
Why do you think a `ConcurrentModificationException` would be thrown at all? — Mark Rotteveel, Sep 24 '17 at 14:03
If you wonder why it's not doing something you expect, check the Javadoc for all the methods you are calling. — Andy Turner, Sep 24 '17 at 14:08

score 6 · Accepted Answer · edited Sep 24 '17 at 14:04

Your example will not throw an exception. It also won't remove all elements from the list, though.

What it really does

On first iteration, it removes index 0; 2 elements remain in the list
On second iteration, it removes index 1 (last element in fact); 1 element remains
Third iteration won't happen, because by then list.size() is 1 which is less than 2

At first, I thought it would throw an IndexOutOfBoundsException, but the fact that list.size() is evaluated at the start of each iteration ensures that it won't.

Why it doesn't throw `ConcurrentModificationException`

Because you're not using an iterator.

This exception is thrown if you iterate the list using an iterator, then modify (e.g. delete from) the list, and then try to advance the iterator, like this:

Iterator<MyObject> it = list.iterator();
while (it.hasNext()) {
    list.remove(it.next());
}

As @Ferrybig noted, even in such case the iterator might not actually throw an exception - see related question.

Java concurrent modification exception is not coming while using collections remove method

1 Answers1

What it really does

Why it doesn't throw ConcurrentModificationException

Why it doesn't throw `ConcurrentModificationException`