How to avoid java.util.ConcurrentModificationException when iterating through and removing elements from an ArrayList

Two options:

Create a list of values you wish to remove, adding to that list within the loop, then call originalList.removeAll(valuesToRemove) at the end
Use the remove() method on the iterator itself. Note that this means you can’t use the enhanced for loop.

As an example of the second option, removing any strings with a length greater than 5 from a list:
List list = new ArrayList();
…
for (Iterator iterator = list.iterator(); iterator.hasNext(); ) {
String value = iterator.next();
if (value.length() > 5) {
iterator.remove();
}
}

From the JavaDocs of the ArrayList

The iterators returned by this class’s iterator and listIterator
methods are fail-fast: if the list is structurally modified at any
time after the iterator is created, in any way except through the
iterator’s own remove or add methods, the iterator will throw a
ConcurrentModificationException.