# Java ConcurrentModificationException
**URL:** https://boards.straightdope.com/t/java-concurrentmodificationexception/528841
**Category:** Factual Questions
**Created:** [February 13, 2010, 8:08am UTC](https://boards.straightdope.com/t/java-concurrentmodificationexception/528841 "2010-02-13T08:08:49Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Lobot](https://boards.straightdope.com/u/Lobot)
#### Post date: [February 13, 2010, 8:08am UTC](https://boards.straightdope.com/t/java-concurrentmodificationexception/528841/1 "2010-02-13T08:08:49Z")
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.
```auto
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:
```auto
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.
---
### Author:  [@ticker](https://boards.straightdope.com/u/ticker)
#### Post date: [February 13, 2010, 9:43am UTC](https://boards.straightdope.com/t/java-concurrentmodificationexception/528841/2 "2010-02-13T09:43:18Z")
> [@](#):
>
> 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.
---
### Author:  [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)
#### Post date: [February 13, 2010, 9:51am UTC](https://boards.straightdope.com/t/java-concurrentmodificationexception/528841/3 "2010-02-13T09:51:06Z")
My guess? It’s in the line “…”.
**Indistinguishable** ’s law of debugging: it’s always in the part you were sure was irrelevant
---
### Author:  [@Lobot](https://boards.straightdope.com/u/Lobot)
#### Post date: [February 13, 2010, 9:53am UTC](https://boards.straightdope.com/t/java-concurrentmodificationexception/528841/4 "2010-02-13T09:53:17Z")
Bingo. I was conditionally removing elements. Once I used itr.remove(), it fixed itself.
Thanks!
---
### Author:  [@Lobot](https://boards.straightdope.com/u/Lobot)
#### Post date: [February 13, 2010, 9:54am UTC](https://boards.straightdope.com/t/java-concurrentmodificationexception/528841/5 "2010-02-13T09:54:36Z")
> [@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…