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.

Reza Abedi · Accepted Answer · 2021-10-01 21:37:56Z

3

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 1, 2021 at 21:37

Reza Abedi

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2 Comments

Beesz Over a year ago

Unfortunatelly problem still exists. Please see edited post.

Reza Abedi Over a year ago

@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.

akhil nair · Accepted Answer · 2021-10-01 21:43:56Z

0

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

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

answered Oct 1, 2021 at 21:43

akhil nair

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5 Comments

Beesz Over a year ago

Unfortunatelly problem still exists. Please see edited post.

akhil nair Over a year ago

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?

Astha Garg Over a year ago

not fixing the error

akhil nair Over a year ago

@AsthaGarg Can you show your code? May be create a new question and share the link.

Astha Garg Over a year ago

noting different in my code same as this question, your answer does not solve the problem

Astha Garg · Accepted Answer · 2022-10-31 07:39:08Z

0

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 :)

answered Oct 31, 2022 at 7:39

Astha Garg

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