Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

Question

We all know you can't do the following because of ConcurrentModificationException:

for (Object i : l) {
    if (condition(i)) {
        l.remove(i);
    }
}

But this apparently works sometimes, but not always. Here's some specific code:

public static void main(String[] args) {
    Collection<Integer> l = new ArrayList<>();

    for (int i = 0; i < 10; ++i) {
        l.add(4);
        l.add(5);
        l.add(6);
    }

    for (int i : l) {
        if (i == 5) {
            l.remove(i);
        }
    }

    System.out.println(l);
}

This, of course, results in:

Exception in thread "main" java.util.ConcurrentModificationException

Even though multiple threads aren't doing it. Anyway.

What's the best solution to this problem? How can I remove an item from the collection in a loop without throwing this exception?

I'm also using an arbitrary Collection here, not necessarily an ArrayList, so you can't rely on get.

Answer 1

Iterator.remove() is safe, you can use it like this:

List<String> list = new ArrayList<>();

// This is a clever way to create the iterator and call iterator.hasNext() like
// you would do in a while-loop. It would be the same as doing:
//     Iterator<String> iterator = list.iterator();
//     while (iterator.hasNext()) {
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
    String string = iterator.next();
    if (string.isEmpty()) {
        // Remove the current element from the iterator and the list.
        iterator.remove();
    }
}

Note that Iterator.remove() is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

Source: docs.oracle > The Collection Interface

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And similarly, if you have a ListIterator and want to add items, you can use ListIterator#add, for the same reason you can use Iterator#remove — it's designed to allow it.

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In your case you tried to remove from a list, but the same restriction applies if trying to put into a Map while iterating its content.