Is it possible for kotlin.collections functions to return a mixture of immutable and mutable lists? I’m trying to java.collections sort a list mutated by .sortByDescending, and a very small percentage, but a substantial amount, of my users are crashing due to an UnsupportedOperationException being thrown by

AbstractList.set(int index, E element)

public E set(int index, E element) {
        throw new UnsupportedOperationException();
    }

Do you have the code you are using to create the list?

Its proprietary, but what i can give you is here: https://stackoverflow.com/questions/55802573/abstractlist-unsupportedoperationexception-when-calling-sort-on-a-list

Bear in mind that the Kotlin

List

returned from

CollectionsKt.listOf("a", "b")

is not going to be mutable but, if accessed from Java it will resolve it as a Java

List

which has no safety regarding mutability

That could be resolved by returning .toMutableList()

Indeed

Its interesting it doesnt happen on 8.0 and higher

Now, that’s interesting, yeah 🤔

Ok, well thanks for weighing in.. I appreciate it

I wonder if

java.util.Arrays$ArrayList

changed it’s implementation in the JVM and/or Dalvik/ART

The thing is, that on an emulator with 7.0 running, I’ve seen both of those list types pass through Collections.sort without throwing

Yeah, I would also try to move as much as possible to Kotlin, haha (if you can)

I agree 100%, we’re trying to but the app is big, its a delicate balance. Its really crazy though when you start migrating a huge app to Kotlin from Java with a significant number of users, you find strange things like this happen with almost every refactor

Yeah, it’s tricky. I’ve experience a couple of these WTF issues myself before in a large project that we are slowly migrating

Next I’m trying to figure out how we got here: https://github.com/openjdk-mirror/jdk7u-jdk/blob/master/src/share/classes/java/util/AbstractList.java#L132 From kotlins .map() and .sortByDescending()

Ah! it just hit me… If there is only one thing in the list there is the potential that it returns a

SingletonList

(I’m not 100% sure) which will throw this exception.

Really? Thats great info

Bam: https://try.kotlinlang.org/#/UserProjects/s3j6g8476h8047vvvsjuvletlr/rjlkkn3n6117c1aagja8dr1h67

Thank you 🙂

Hmm, bad luck? 😅

I dont think so, we have enough users so that statistically it should be happening

there you go: https://stackoverflow.com/a/55815896/458365

I’m going to verify that, to be 100% sure now that I have the knowledge to reproduce it

Thinking about this, it may be a later fix on the JDK as a list of one shouldn't be modified when sorting. Need to check the SC