[jira] [Work logged] (COLLECTIONS-663) Unexpected ConcurrentModificationException when altering Collection of a MultiValuedMap

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/COLLECTIONS-663?focusedWorklogId=428547&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-428547 ]

ASF GitHub Bot logged work on COLLECTIONS-663:
----------------------------------------------

                Author: ASF GitHub Bot
            Created on: 29/Apr/20 13:04
            Start Date: 29/Apr/20 13:04
    Worklog Time Spent: 10m 
      Work Description: garydgregory commented on a change in pull request #110:
URL: https://github.com/apache/commons-collections/pull/110#discussion_r417296559



##########
File path: src/main/java/org/apache/commons/collections4/multimap/AbstractMultiValuedMap.java
##########
@@ -879,7 +879,8 @@ public boolean remove(final Object o) {
             public Map.Entry<K, Collection<V>> next() {
                 final Map.Entry<K, Collection<V>> entry = super.next();
                 final K key = entry.getKey();
-                return new UnmodifiableMapEntry<>(key, wrappedCollection(key));
+                final Collection<V> value = entry.getValue();
+                return new UnmodifiableMapEntry<>(key, value);

Review comment:
       You can just inline `key` and `value` IMO.




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Issue Time Tracking
-------------------

    Worklog Id:     (was: 428547)
    Time Spent: 20m  (was: 10m)

> Unexpected ConcurrentModificationException when altering Collection of a MultiValuedMap
> ---------------------------------------------------------------------------------------
>
>                 Key: COLLECTIONS-663
>                 URL: https://issues.apache.org/jira/browse/COLLECTIONS-663
>             Project: Commons Collections
>          Issue Type: Bug
>            Reporter: Christophe Schmaltz
>            Assignee: Bruno P. Kinoshita
>            Priority: Trivial
>          Time Spent: 20m
>  Remaining Estimate: 0h
>
> Testcase:
> {code}	@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...
> 		}
> 	}{code}
> 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. 



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