Remove element from ArrayList using ListIterator in loop inside loop

Question

My requirement is to remove from ArrayList like this:

ArrayList<User> user = new ArrayList<User>();

ListIterator<User> outerIterator = null;
ListIterator<User> innerIterator = null;

User outer = null;
User inner = null;

for(outerIterator = user.listIterator(); outerIterator.hasNext();) {
    outer = outerIterator.next();
    for(innerIterator = user.listIterator(); innerIterator.hasNext();) {
        inner = innerIterator.next();
        if(SomeOperationOn(outer,inner)) {
            innerIterator.remove();
        }
    }
}

Above code is giving exception

Exception in thread "main" java.util.ConcurrentModificationException

as expected, because I am trying to remove from innerIterator while outerIterator is Iterator on same object(user).

Is there any way to remove element from ArrayList using ListIterator in loop inside loop?

Answer 1

In isolation, calling remove() on an Iterator is the proper way to avoid a ConcurrentModificationException when removing an item from a collection you're iterating. However, you have two Iterator s iterating over the same ArrayList . When you call remove() on innerIterator , outerIterator notices that the list has changed, and it throws a ConcurrentModificationException .

In this case, if SomeOperationOn indicates that the item needs to be removed, then instead of removing it right away, store inner in a temporary List of items to be removed later. After the for loop on the iterators completes, call removeAll passing this temporary list.