How to modify a Collection while iterating using for-each loop without ConcurrentModificationException? [duplicate]

Question

If I modify a Collection while iterating over it using for-each loop, it gives ConcurrentModificationException. Is there any workaround?

Answer 1

Use Iterator#remove.

This is the only safe way to modify a collection during iteration. For more information, see The Collection Interface tutorial.

If you also need the ability to add elements while iterating, use a ListIterator.

Answer 2

One work around is to save your changes and add/remove them after the loop.

For example:

List<Item> toRemove = new LinkedList<Item>();

for(Item it:items){
    if(remove){
        toRemove.add(it);
    }
}
items.removeAll(toRemove);

Answer 3

A second workaround is to use a collection class whose iterators won't give the exception. For example ConcurrentLinkedQueue, ConcurrentHashMap and so on.

These avoid the need to throw exceptions by providing weaker models of consistency for the iterators. (Of course, you need to understand those models, and decide whether they are suitable for your application.)

They are typically a bit slower than non-concurrent collections, but faster than the synchronized collection wrappers if there is significant contention.

Answer 4

If you just want to remove the element from the collection, you can use Iterator instead of Iterable.

Otherwise, what you can do is not to iterate the original collection, but first make a copy of the list. For example if your collection is a list, than you can make a new ArrayList(originaList) and iterate over that. The modification should be done to the original list.

Another alternative which maybe better for your use case, is not to use for-each but the traditional for-.