# ConcurrentModificationException with incorrect line number in stack trace

**URL:** <https://discuss.kotlinlang.org/t/concurrentmodificationexception-with-incorrect-line-number-in-stack-trace/11792>\
**Category:** Android\
**Created:** [March 1, 2019, 6:30pm UTC](https://discuss.kotlinlang.org/t/concurrentmodificationexception-with-incorrect-line-number-in-stack-trace/11792 "2019-03-01T18:30:02Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![zsombor](https://avatars.discourse-cdn.com/v4/letter/z/4bbf92/32.png) [@zsombor](https://discuss.kotlinlang.org/u/zsombor)\
**Post date:** [March 1, 2019, 6:30pm UTC](https://discuss.kotlinlang.org/t/concurrentmodificationexception-with-incorrect-line-number-in-stack-trace/11792/1 "2019-03-01T18:30:02Z")

</div>

I am receiving intermittent crash reports with a stack trace that looks like this:

```
Caused by: java.util.ConcurrentModificationException:
   at java.util.HashMap$HashIterator.nextNode (HashMap.java:1441)
   at java.util.HashMap$KeyIterator.next (HashMap.java:1465)
   at MyPageAdapter.refreshBoundViewHolders (MyPageAdapter.java:XXX)

```

I don’t have any `MyPageAdapter.java` file in my project. There is a `MyPageAdapter.kt` file with XXX - 5 lines of Kotlin code in it, i.e. the stack trace points past the end of the file by 5 lines.

I read [this](https://discuss.kotlinlang.org/t/stacktrace-points-to-line-after-end-of-file/4129) and [this](https://discuss.kotlinlang.org/t/inline-functions-and-line-numbers/9736) so I understand that incorrect line numbers are expected when we are dealing with inline code. I am not sure if the `.java` in the stack trace for a Kotlin file is expected or significant. I am not particularly concerned about either of these discrepancies as I am pretty sure I can pinpoint where the crash is happening.

This is what the relevant code approximately looks like:

```
class MyPageAdapter(): RecyclerView.Adapter<MyViewHolder>() {
    private val boundViewHolders = HashSet<MyViewHolder>()

    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        synchronized(boundViewHolders) {
            boundViewHolders.add(holder)
        }
    }

    override fun onViewRecycled(holder: MyViewHolder?) {
        synchronized(boundViewHolders) {
            boundViewHolders.remove(holder)
        }
        super.onViewRecycled(holder)
    }

    fun refreshBoundViewHolders() {
        synchronized(boundViewHolders) {
            boundViewHolders.forEach {
                it.refresh()
            }
        }
    }
}

```

The stack trace references the `refreshBoundViewHolders` function, so I assume that is where the exception happens even though the line number doesn’t match. I am including the other parts to show that the `boundViewHolders` `HashSet` is protected by `synchronized` every time it’s accessed. These are literally all the appearances of `boundViewHolders` in the code. Am I wrong to think that this protection should be enough to avoid a `ConcurrentModificationException`?

In addition, I am doubtful whether these functions are even called from anything but the main thread. I do have background threads in this app so it is not entirely out of the question, but I don’t see how it can happen.

Is it possible that the construction or deconstruction of `boundViewHolders` conflicts with a call to `refreshBoundViewHolders` and that is causing the exception? If so, how can I protect against this?

I do find it interesting that the first of those threads is about a `ConcurrentModificationException` in a `forEach` loop, which is exactly what seems to be happening to me. Is it possible that there is a problem with `forEach` that is causing this?

Any help or ideas would be appreciated!

---

<div class="post-metadata">

**Author:** ![pdvrieze](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/pdvrieze/32/1882_2.png) [@pdvrieze](https://discuss.kotlinlang.org/u/pdvrieze)\
**Post date:** [March 4, 2019, 9:01pm UTC](https://discuss.kotlinlang.org/t/concurrentmodificationexception-with-incorrect-line-number-in-stack-trace/11792/2 "2019-03-04T21:01:56Z")

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First of all, you should remove all of the synchronization. All these methods should only be invoked on the UI/Main thread so synchronization this way just gives the illusion of doing anything (except slowing things down). I suppose that the MyViewHolder.refresh() function is a function you defined. I strongly suspect that it indirectly calls onBindViewHolder, and thus not only causes an incorrect list, it also creates a modification while you’re iterating (under water `.foreach` uses iteration) over the list of viewholders.

---

<div class="post-metadata">

**Author:** ![zsombor](https://avatars.discourse-cdn.com/v4/letter/z/4bbf92/32.png) [@zsombor](https://discuss.kotlinlang.org/u/zsombor)\
**Post date:** [March 4, 2019, 11:24pm UTC](https://discuss.kotlinlang.org/t/concurrentmodificationexception-with-incorrect-line-number-in-stack-trace/11792/3 "2019-03-04T23:24:55Z")

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That is a great observation, thanks for the insight!