RFR: JDK-8306860: Avoid unnecessary allocation in List.map() when list is empty

[ExE Boss]

On Mon, 10 Apr 2023 17:44:37 GMT, Lalit Prakash Vatsal <duke at openjdk.org> wrote:

>> Same "problem". This changes behaviour compared to what it does now. I don't have anything against that per se, but an optimization imho shouldn't really change the behaviour.
>
> I agree with @dreis2211 here. Someone might want to use/mutate the returned `List` later on.

This won’t be a change in behaviour as `ListBuffer::toList()` returns `List.nil()` when called on an empty `ListBuffer`.

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e.g.: The following evaluates to `true`:

new ListBuffer().toList() == List.nil()

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PR Review Comment: https://git.openjdk.org/jdk/pull/13407#discussion_r1162591007