`listOf` returns a read-only list on kotlin side, ...

# getting-started

[Loney Chou]

listOf returns a read-only list on kotlin side, but on JVM it's actually a java.util.Arrays.ArrayList which is still mutable. Why doesn't kotlin team create a real immutable list type?

[Sam]

It’s in development 👍K

[Loney Chou]

I mean, on JVM listOf() and listOf(1) both returns immutable list, while listOf(1, 2) returns a read-only but mutable list. This is a bit unsafe.

[Sam]

There is an open issue for that I think: https://youtrack.jetbrains.com/issue/KT-9959

[Loney Chou]

Seems like this problem doesn't attract much attention.

[Vampire]

The main point probably is, that someone with evil energy can almost always somehow circumvent the immutability if he really wants to.
The read-onlyness of List is more a hint to the user to not modify the list but only use it read-only or create a new mutable list for mutating.
This way you save the extra effort to make things immutable and defensively copy things continuously.

[Klitos Kyriacou]

I think the "danger" here is that you might pass a List to a third-party Java library method that takes a List and modifies it because you didn't realise that you should be passing a mutable list.

Example:

fun main() {
    val list = listOf(1, 2, 3, 4, 5, 6)
    java.util.Collections.reverse(list)
    println(list)
}

The above may issue an IDE warning (if you're set things right), but it compiles and runs "just fine".

[Sam]

Right, or even worse,

java.util.Collections.replaceAll(listOf("foo"), "foo", "bar")

which compiles fine (without even a warning) but fails at runtime 😱.
Methods like reverse, shuffle etc. get lucky because they just happen to be a no-op for lists of size 0 and 1, which are the ones Kotlin makes immutable.

[Klitos Kyriacou]

I'd rather have it fail with an exception at runtime than it being modified unexpectedly. When I see val list = listOf(1, 2, 3, 4, 5, 6) I like to be able to rely on list being unchanged.

[Sam]

Yeah, after I wrote “even worse” I started to change my mind, I think your example is more insidious

[Loney Chou]

Same preference, listOf should be somehow unmodifieable, just like List.of from java.