ConcurrentModificationException with incorrect line number in stack trace


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

Caused by: java.util.ConcurrentModificationException:
   at java.util.HashMap$HashIterator.nextNode (
   at java.util.HashMap$ (
   at MyPageAdapter.refreshBoundViewHolders (

I don’t have any 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 and this 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) {

    override fun onViewRecycled(holder: MyViewHolder?) {
        synchronized(boundViewHolders) {

    fun refreshBoundViewHolders() {
        synchronized(boundViewHolders) {
            boundViewHolders.forEach {

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!


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.

1 Like

That is a great observation, thanks for the insight!