I have a method with a generic type `T` bounded by...

# announcements

[bbaldino]

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]

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]

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]

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]

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]

(I can't simulate a platform type in playground, as far as I know?)

[bbaldino]

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]

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]

Yeah, I think that's what I had assumed

[bbaldino]

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!... 😛

[bbaldino]

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]

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

[bbaldino]

E.g. this will throw:

val a = Converter<String, MyEnum>("blah") { MyEnum.fromString(it) }
    println(a.get())

but this will print `null`:

val c = Converter<String, MyEnum>("blah", MyEnum::fromString)
    println(c.get())