Kotlin list.map() how to avoid ConcurrentModificationException

Question

I have problem with ConcurrentModificationException. Here's my part of the code :

    var deltaSum = 0
    arrDeltaBrainWaves.map {
         value -> deltaSum += value
   }

To be clear - I know why this error appears :) The problem is - I have no idea what's the solution ? Do I really need to create new temp list and put values there ? It just doesnt make sense :) Any better options, please ?

EDIT:

I changed the code to below :

var deltaSum = 0
                                with(arrDeltaBrainWaves.iterator()) {
                                    forEach {
                                        deltaSum += it
                                    }
                                }
                                avgDelta = deltaSum / arrDeltaBrainWaves.size

But problem still exists.

score 2 · Answer 1 · answered Oct 01 '21 at 21:37

2

you need to use the Iterators

example:

val myCollection = mutableListOf(1,2,3,4)
val iterator = myCollection.iterator()
while(iterator.hasNext()){
    val item = iterator.next()
    //do something
}

this will avoid ConcurrentModificationExceptions

answered Oct 01 '21 at 21:37

Reza Abedi

419
4
16

1

Unfortunatelly problem still exists. Please see edited post. – Bartos Oct 02 '21 at 09:53
1

@Bartos The main reason of happening this exception is you may have some code that modifies on this array. Just test two item and let me update about it. first, add `Volatile` to your array and run your code inside of `Synchronized` Blocks. seccond, could you change array to hashmap? if yes this should solve this exception. – Reza Abedi Oct 03 '21 at 00:30

score 0 · Answer 2 · answered Oct 01 '21 at 21:43

0

The ConcurrentModificationException can be avoided by using iterators. Here is kotlin way to do so:

with(arrDeltaBrainWaves.iterator()) {
 forEach {
    
 }
}

answered Oct 01 '21 at 21:43

akhil nair

1,371
1
11
19

2

Unfortunatelly problem still exists. Please see edited post. – Bartos Oct 02 '21 at 09:53
I tried the code but I don't see any exception. Are you sure the exception is caused due in this code snippet. Can you add more of your code please? – akhil nair Oct 02 '21 at 20:46
not fixing the error – Astha Garg Oct 28 '22 at 07:33
@AsthaGarg Can you show your code? May be create a new question and share the link. – akhil nair Oct 28 '22 at 12:50
noting different in my code same as this question, your answer does not solve the problem – Astha Garg Oct 28 '22 at 13:12

score 0 · Answer 3 · answered Oct 31 '22 at 07:39

I solved the ConcurrentModificationException by using MutableListIterator in Kotlin

OLD CODE :

registeredListeners.forEach { it.notifyObservers() }

(In my case notifyObservers() is removing object from the registeredListeners list which is the cause of the exception)

NEW CODE :

val iterator = registeredListeners.toMutableList().listIterator()
while (iterator.hasNext()) {
    iterator.next().notifyObservers()
}

Hope it helps :)

Kotlin list.map() how to avoid ConcurrentModificationException

3 Answers3