Is it possible for kotlin.collections functions to return a mixture of immutable and mutable lists? ...

# android

[tyler]

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();
    }

[pablisco]

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

[pablisco]

And also, the code that is invoked from Java to get the list

[tyler]

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

[pablisco]

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

[tyler]

That could be resolved by returning .toMutableList()

[pablisco]

Indeed

[tyler]

Its interesting it doesnt happen on 8.0 and higher

[pablisco]

Now, that’s interesting, yeah 🤔

[tyler]

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

[pablisco]

I wonder if java.util.Arrays$ArrayList changed it’s implementation in the JVM and/or Dalvik/ART

[pablisco]

On J8 it seems to be mutable:
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Arrays.java#l3800

[tyler]

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

[tyler]

so the safest bet is to use kotlin collections to sort and have java be read only.. maybe using inference to pass a consistent list type

[tyler]

this has been an interesting exercise in kotlin/java interop 🙂

[pablisco]

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

[tyler]

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

[pablisco]

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

[tyler]

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()

[pablisco]

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.

[tyler]

Really? Thats great info

[tyler]

I dont see SingletonList in the collections.kt source

[pablisco]

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

[pablisco]

I’ll add a response on SO 😄

[tyler]

Thank you 🙂

[tyler]

Now, any reason it doesnt happen in 8.0+ ?

[pablisco]

Hmm, bad luck? 😅

[pablisco]

or good

[tyler]

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

[tyler]

Any way, you’ve been a huge help! I’ll mark your SO answer as accepted once it comes in

[pablisco]

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

[pablisco]

I’m still puzzled by why it works on 8.0 and up

[tyler]

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

[tyler]

Previously I couldnt repro, and no users on 8.0+ were crashing according to our crash reports

[tyler]

Yeah so I just verified, its not happening on 9.0. I stepped over Collections.sort on a SingletonList, whereas on 7.0 when attempting to step over, the exception is thrown

[pablisco]

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