Hi, I'm trying to extend a data class, apparently this was enabled in Kotlin 1.1. Is there a working example of how to do this?

I found this comment on stack overflow, but couldnt figure out how to do this https://stackoverflow.com/questions/26444145/extend-data-class-in-kotlin#comment78087033_26467380

This is what I am trying in the REPL

data class A(val foo: String?)

data class B(val bar: String?) : A(foo)

error: function 'component1' generated for the data class conflicts with member of supertype 'A'
data class B(val bar: String?) : A(foo)
^
error: this type is final, so it cannot be inherited from
data class B(val bar: String?) : A(foo)
                                 ^
error: unresolved reference: foo

Data classes are final and cannot be open

What was introduced in 1.1 is the possibility for data classes to extend other 'open' classes

Ah thanks for the clarification guys. I was reading the release notes optimistically 🙂

What you could do is define a "generic" base class

and have those 2 data classes inherit from the base

@bamdmux Thx for the suggestion . this works

open class C

data class A(val foo: String?) : C()

data class B(val bar: String?) : C()

but I'm curious if I can do something like this?

open class C(val foo: String?)

data class A(val foo: String?) : C()

data class B(val bar: String?) : A()

where B has both foo & bar properties

No, because of what @gildor said before, you can't inherit from another data class

Ok cheers.

I don't like keeping Kotlin code in src/kotlin directory, I'm putting it into src/java, so that I do not have same package split over different directories - where part of the package is written in Java and part in Kotlin. Are there any drawbacks to this approach?

the main drawback is that you still have Java code 🧌

No, you can pick the location yourself. If you're not going to split java and kotlin anyways I'd just ditch the java directory.

Maybe you need to add it to your sourcesets though.

Well I'm not rewriting 1+ million lines of code to get clean kt-only project, so that point is moot.

java stays where it is 🙂 I'm more interested in where do I place Kotlin code, is anything harmed by placing Kotlin files in src/java?

It might surprise people, that's all. Indeed if you're porting a project to Kotlin it would suck to separate the code into two folders. I'm not sure if it works out of the box, maybe you need to mess with the sourceSets: https://kotlinlang.org/docs/reference/using-gradle.html#targeting-the-jvm

It does work, I've been using it like this from the beginning, it's just this question occurred to me now 🙂

I think it’s the best and recommended approach to use the same sourceset for kotlin and java. I use

src/main/kotlin

dir only when I have a pure kotlin project

Agree, I've started migrating project to Kotlin and keeping everything in same folder is working pretty good so far

The only thing that I'm not sure about it is the interaction with

internal

never tried befoe

silly me, it should be the same

yep only possible effects can be for the compilation/assembly process, language-wise should be no difference.

I'm looking for a way to discover the type of an identifier, including (for example) whether it is nullable. I've gotten this far:

val s = "Hello"
  println(s::class)

but that doesn't tell me everything. Pointers appreciated.

ctrl-shift-P

will show you the expression’s type

put the cursor on

s

, hit the combo and it’ll show a tool tip with

String