ConcurrentModificationException in Multimap

[siva]

Just like ConcurrentHashMap is safe during iterator of keys for
HashMap. Is there any implementation for MultiMap?

We are using Multimap for keeping track of listeners on item. While
sending some event, if new subscriber is added for same item, for
which we are iterating and sending events,
ConcurrentModificationException is thrown. Is there any way to avoid
this.

static Multimap<String,String> getMap() {
Multimap<String, String> map = HashMultimap.create();
map.put("1", "1");
map.put("1", "2");
map.put("1", "3");
return map;
}

static void throwsException2() {
Multimap<String, String> map = getMap();

for (String string : map.get("1")) {
System.out.println("Get: "+string);
if (string.equals("2")) {
map.put("1", "4"); --> adding new element to same key
System.out.println("Added 3");
}
}

System.out.println(map.get("1"));
}

Exception:
Get: 3
Get: 2
Added 3
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
at java.util.HashMap$KeyIterator.next(HashMap.java:828)
at com.google.common.collect.AbstractMultimap$WrappedCollection
$WrappedIterator.next(AbstractMultimap.java:516)
at
guava.MapConcurrentAccessTest.throwsException2(MapConcurrentAccessTest.java:
37)
at guava.MapConcurrentAccessTest.main(MapConcurrentAccessTest.java:
51)

[Tim Peierls]

The CME is because you're modifying the contents of the collection while iterating over it. You can avoid it by copying the contents of the collection returned by map.get(key) and iterating over that copy.

--tim

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[Jared Levy]

Tim's suggestion is the best approach.

As another option, you could use the Multimaps.new*Multimap() methods with a Supplier that creates a CopyOnWriteArrayList or CopyOnWriteArraySet.