Java's `Map.computeIfAbsent()` function documentat...

# java-to-kotlin-refactoring

[Wesley Hartford]

Java's Map.computeIfAbsent() function documentation includes:

If the mapping function returns {@code null}, no mapping is recorded.

However, when calling this function from Kotlin code, the mapping function has a signature (K) -> V, meaning that the passed in function cannot return null. Is there any way to use computeIfAbsent from kotlin passing a function which may return null?

[Klitos Kyriacou]

(mutableMapOf(1 to "one") as MutableMap<Int, String?>).computeIfAbsent(1) { null }

[Jordan Stewart]

this does seem a bit odd to me -- Kotlin allows null for compute and computeIfPresent. Why the difference? the only difference in signature is that they're BiFunction rather than Function

[Jordan Stewart]

if we define our own type

[Jordan Stewart]

we don't see the same compilation failure

[Jordan Stewart]

interesting

[Klitos Kyriacou]

It seems it was a deliberate decision relating to Java's Map:
https://discuss.kotlinlang.org/t/why-is-the-result-of-the-computeifabsent-lambda-non-nullable/19463

[Wesley Hartford]

Thanks for the replies, I ran across that discuss thread as well and though it seems like a strange decision. Not necessarily a bad one, but odd to have a case where the same function can't be used in the same ways from Kotlin as from Java. I ended up replacing the call with the compute function.

[dmcg]

I’d wondered where some of these pragmatic special cases came from, it’s good to see that they are at least visible if you know where to look