remove elements from CopyOnWriteArrayList

Question

I am getting an exception when I try to remove elements from CopyOnWriteArrayList using an iterator. I have noticed that it is documented

Element-changing operations on iterators themselves (remove, set, and add) are not supported. These methods throw UnsupportedOperationException.

(from http://download.oracle.com/javase/6/docs/api/java/util/concurrent/CopyOnWriteArrayList.html)

Now, surprisingly i can iterate it with foreach and use the remove() function . But then I get the famous bug - when trying to remove an item from a list using a for loop - you skip the element next to the removed element. any suggestions then?

Answer 1

Since this is a CopyOnWriteArrayList it is totally safe to remove elements while iterating with forEach. No need for fancy algorithms.

list.forEach(e -> {
    if (shouldRemove(e))
        list.remove(e);
});

EDIT: Well of course that works if you want to delete elements by reference, not by position.

Answer 2

If you want to delete all use just clear(). If you want to keep elements put them in a temporary ArrayList and get them back from there.

List<Object> tKeepThese= new ArrayList<>();
for(ListIterator<Object> tIter = theCopyOnWriteArrayList; tIter.hasNext();)
{
    tObject = tIter.next();
    if(condition to keep element)
        tKeepThese.add(tObject);
}
theCopyOnWriteArrayList.clear();
theCopyOnWriteArrayList.addAll(tKeepThese);