I have a method with a generic type `T` bounded by `Any` and

Question

I have a method with a generic type `T` bounded by `Any` and call a method which returns a `T` but in this case `T` is a java type an enum and the method defined in java returns null I expected an exception to be thrown here but instead the value of null trickles up just fine and a kotlin method which is set to return a value `T Any` ends up happily returning null Is it expected that this won t throw somewhere Is this just a limitation of interacting with Java types from kotlin

bbaldino · Answer

I m trying some simpler cases to reproduce and it seems this isn t inherently the case TypeCastException gets thrown in my simpler case I ll have to dig some more to figure out where the safety is getting lost

streetsofboston · Answer

Without seeing your code I think what happens is that your Java method returns that enum as a so called platform type E g `MyEnum ` with the exclamation mark ` ` Values of this type can be null but can be assigned to non nullable variables At some point when you finally access the platform type value and expect it to be not null you ll get a null pointer exception

bbaldino · Answer

Yeah that s exactly what I expected that upon access I d get the exception but I didn t Something weird must be happening somewhere I m trying to reproduce with less code

bbaldino · Answer

Alright I m seeing some weird things in my real code it actually makes a difference of getting null vs an exception if I use a method reference vs a lambda in one place but I haven t repro d that in a simple example yet I did write up this <https pl kotl in vFsPfaJt7>

bbaldino · Answer

I can t simulate a platform type in playground as far as I know

bbaldino · Answer

But both bits there print null when I would ve thought at least the second one would throw What triggers treating a platform type as a Kotlin non nullable type

streetsofboston · Answer

I think the Kotlin compiler sprinkles in null checks requireNull into the byte code only in certain spots e g when providing an argument to a function parameter I would expect it to happen on the assignment of a `return` statement as well `get NewType =` but I guess it doesn t

bbaldino · Answer

Yeah I think that s what I had assumed

bbaldino · Answer

If in that snippet I change `Converter get` to ```fun get NewType = converter originalValue also if it == null println null ``` then kotlin warns me the check is redundant since it thinks `it` can t be null but it does print `null ` stuck out tongue

bbaldino · Answer

I dunno I ended up filing <https youtrack jetbrains com issue KT 40596> because it s odd to me that even explicitly declaring the type as a non nullable kotlin type not the inferred platform type doesn t throw an exception when it s null

bbaldino · Answer

It can actually make a difference if a lambda is used vs a method reference looks like

bbaldino · Answer

E g this will throw ``` val a = Converter lt String MyEnum gt blah MyEnum fromString it println a get ``` but this will print `null` ``` val c = Converter lt String MyEnum gt blah MyEnum fromString println c get ```