[COLLECTIONS-663] Unexpected ConcurrentModificationException when altering Collection of a MultiValuedMap

• Type: Bug
• Status: Open
• Priority: Trivial
• Resolution: Unresolved
• Assignee: Bruno P. Kinoshita
• Reporter: Christophe Schmaltz
• Created: 04/Nov/17 17:38
• Updated: 30/Apr/20 01:17

Description

Testcase:

	@Test
	public void test() {
		MultiValuedMap<Integer, Integer> multiMap = new HashSetValuedHashMap<>();
		multiMap.put(1, 10);
		multiMap.put(2, 20);
		for (Collection<Integer> innerCollection : multiMap.asMap().values()) {
			for (Iterator<Integer> iterator = innerCollection.iterator(); iterator.hasNext();) {
				Integer i = iterator.next();
				iterator.remove(); // only the innerCollection is altered
			}
			// innerCollection.add(6); // adding stuff back should also work...
		}
	}

This test unexpectedly throws a ConcurrentModificationException.
The issue is that when calling iterator.remove() the AbstractMultiValuedMap.ValuesIterator detects that the Collection is empty and calls AbstractMultiValuedMap.this.remove(key);.

It may be better if the iterator of the inner collection had a reference on the iterator if the outer map and called containerIterator.remove() instead.
Note: this solution would again present issues if the user tries to add new elements in this now empty collection (which was removed from the parent).

In the current state, it is quite unclear why an exception is thrown, without debugging the code.

Issue Links

• links to: GitHub Pull Request #110