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
.
Best Answer
Iterator.remove()
is safe, you can use it like this: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 useListIterator#add
, for the same reason you can useIterator#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 aMap
while iterating its content.