Java ConcurrentModificationException

[Lobot]

OK, I’m not a Java guru, but I have no idea as to what’s going wrong here. Whatever it is, it looks like I’m just misunderstanding something.

Iterator itr = lhm.keySet().iterator();
while (itr.hasNext()) {
    int[] foo = (int[]) itr.next();
    ...
}

…where lhm is a LinkedHashMap whose keys are arrays of ints.

Once we get to the cast, the following exception occurs:

Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.java:390)
	at java.util.LinkedHashMap$KeyIterator.next(LinkedHashMap.java:401)

So what am I doing wrong?

ETA: There is only one thread running, FWIW.

[ticker]

The iterators returned by the iterator method of the collections returned by all of this class’s collection view methods are fail-fast:** if the map is structurally modified at any time after the iterator is created**, in any way except through the iterator’s own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

My guess is that within your ‘while’ loop you are somehow messing with the map.

[Indistinguishable]

My guess? It’s in the line “…”.

Indistinguishable’s law of debugging: it’s always in the part you were sure was irrelevant

[Lobot]

Bingo. I was conditionally removing elements. Once I used itr.remove(), it fixed itself.

Thanks!

[Lobot]

@Indistinguishable

My guess? It’s in the line “…”.

Indistinguishable’s law of debugging: it’s always in the part you were sure was irrelevant

Your law is worth remembering…