Hello, I have an abstract class `A` and data class `D` such that : ```abstract class A { //other...

# android

[Devanshu Kaushik]

Hello,

I have an abstract class A and data class D such that :

abstract class A {
  //other fields
  open val qty: Int?
}

data class D(
  //other fields
  override val qty: Int?
): A()

now, when I use class D for auto-serializing my JSON response using gson, I get the following error:

java.lang.IllegalArgumentException:class D declares multiple JSON fields named qty

if I’m overriding qty - won’t it mean that there is actually only one field called qty?

[Zun]

I know this isn't the answer but have you tried more modern serializing libraries? Surely they work better with Kotlin

[Devanshu Kaushik]

Haven't had an issue like this with gson so far, but if this is an actual issue with the library, I’m open to trying out the alternatives.

[ayodele]

Data classes are not meant to have supertypes, I'll advice you use sealed class or use composition (by interface)

[ephemient]

abstract val qty does not create a backing field, only an abstract accessor, so that should help.
as opposed to open val qty creates a backing field and an open accessor, then override val qty creates another backing field (with the same name) and overrides the accessor.
JVM considers these to be two distinct fields, there is no such thing as field overriding, and in fact in your case the field A.qty is not set by D.<init> so it doesn't even have the same value

[ephemient]

Moshi should be strictly better than gson, https://www.reddit.com/r/androiddev/comments/684flw/comment/dgx3gpm/

[Devanshu Kaushik]

Wow, I guess I gotta read more on open , thanks for the insight and sure will take a look at Moshi too.

[ephemient]

well it comes naturally out of the fact that open must create a non-abstract implementation, which requires a backing field, and that backing field is inaccessible so override val must create another one