java interop How can I overcome java ` Nullable` which is ov

Question

java interop How can I overcome java ` Nullable` which is overly pessimistic For example I want to create a shortcut for transactions execution ```fun lt T gt PlatformTransactionManager execute block gt T T = TransactionTemplate this execute gt block ``` This doesn t compile because `execute` is marked as ` Nullable` therefore the return type must be `T ` Instead would like it to be `T` that is whatever `block` returns which might also be null

Rob Elliot · Answer

Can you just throw an exception ```fun lt T gt PlatformTransactionManager execute block gt T T = TransactionTemplate this execute gt block throw IllegalStateException Expected it to be impossible for execute to return null ```

David Kubecka · Answer

As I said the block can return null I highlighted that part of my question slightly smiling face

Rob Elliot · Answer

Oh I see sorry

Riccardo Lippolis · Answer

```fun lt T gt PlatformTransactionManager execute block gt T T = TransactionTemplate this execute gt block as T``` this appears to be working and it removes the warning

David Kubecka · Answer

Yes it works but it generates a warning not removes one disappointed

Riccardo Lippolis · Answer

oh weird my IDE did not give any warning there

Riccardo Lippolis · Answer

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ephemient · Answer

if you don t want to see the warning on `as T` ``` Suppress UNCHECKED CAST ```

Riccardo Lippolis · Answer

hmm weird that I don t need that suppression did not disable that warning in IntelliJ

David Kubecka · Answer

Weird indeed Does it compile without warnings btw I also use IDEA

kqr · Answer

i feel stupid right now but if it can return null how can it be `T`

David Kubecka · Answer

Plain ` lt T gt ` doesn t say anything about its nullability in contrast to e g ` lt T Any gt `

kqr · Answer

ah thx

David Kubecka · Answer

I guess the confusion might be about the difference between `T` and `T ` Well `T` in this case means the exact return type of the block preserving nullability while `T ` might not preserve the nullability i e even if `block` returned non null then `TransactionTemplate execute` might still return null I guess that s a principal deficiency of those java annotations

Riccardo Lippolis · Answer

gt Weird indeed Does it compile without warnings btw I also use IDEA yes no compiler warnings as well I use Kotlin 1 8 10 btw

deive · Answer

If you know that what you run in execute never returns a null then why not use a ` ` ```fun lt T gt PlatformTransactionManager execute block gt T T = TransactionTemplate this execute gt block ```

Rob Elliot · Answer

It can return null if the block returns null It should have the same type as the block which may or may not be nullable

deive · Answer

As in if `TransactionTemplate execute` returns T but you are always running `block gt T` then `TransactionTemplate this execute gt block ` should be OK

Rob Elliot · Answer

i e ```val x String = tm execute not null val y String = tm execute if Random nextInt 2 == 1 not null else null ```

ephemient · Answer

basically a mismatch between Java where ` lt T gt ` is neither nullable nor non nullable and Kotlin where there is such a distinction

ephemient · Answer

Java ` Nullable lt T gt T foo ` becomes Kotlin `fun lt T gt foo T ` Java ` NonNull lt T gt T foo ` becomes Kotlin `fun lt T gt foo T amp Any` and ` lt T gt T foo ` can t be written directly in Kotlin due to platform nullability none of them result in `fun lt T gt foo T`

deive · Answer

I missed part of the original question as well that is whatever `block` returns which might also be null this means that block will have to return `T ` so what I said was moot

Rob Elliot · Answer

No the block returns `T` Whether or not `T` s concrete type is nullable depends on how you define the block

Rob Elliot · Answer

```val notNullBlock gt String = not null val x String = tm execute notNullBlock val nullableBlock gt String = if Random nextInt 2 == 1 not null else null val y String = tm execute nullableBlock ```

deive · Answer

yeah sorry would you change `PlatformTransactionManager execute` to be `fun lt T Any gt ` for a non nullable version

Rob Elliot · Answer

Yes

deive · Answer

Thanks sorry for my confusion there

Rob Elliot · Answer

I missed the important bit too right at the start of the conversation