Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop
collections, iteration, java
Solution
`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
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.
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.
Problem
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`.