Deserializing 2d array in kotlinx.serialization - json

Question

I am quite new to Kotlin but and I have successfully used Kotlin serialization on many cases - works out of the box even for nested classes, mutableLists etc. However I struggle with two dimensional arrays.
My class:
import kotlinx.serialization.*
#Serializable
data class Thing(val name:String)
#Serializable
data class Array2D(val width:Int, val height:Int,
var arrayContents:Array<Array<Thing?>> = Array(1){Array(1){null} }){
init{
arrayContents = Array(width){Array(height){null} }
}
}
And when doing this:
val a = Array2D(2, 2)
a.arrayContents[0][0] = Thing("T0")
a.arrayContents[0][1] = Thing("T1")
a.arrayContents[1][0] = Thing("T2")
a.arrayContents[1][1] = Thing("T3")
val json = Json {
allowStructuredMapKeys = true
}
val jsonString = json.encodeToString(Array2D.serializer(), a)
assertEquals(
"""
{"width":2,"height":2,"arrayContents":[[{"name":"T0"},{"name":"T1"}],[{"name":"T2"},{"name":"T3"}]]}
""".trim(),
jsonString
) // encoding is OK
val b = json.decodeFromString(deserializer = Array2D.serializer(), jsonString)
// this fails to reproduce "a" and stops at first array level
// b.arrayContents = {Thing[2][]} (array of nulls) instead of {Thing[2][2]} (array of array of Thing)
If it can encode the class to String it should decode it as well, right? Or am I missing something here? Maybe I should use custom serializer but there are not many examples that fit my case. One example is https://github.com/Kotlin/kotlinx.serialization/issues/357 but it is only one level of array.
Thanks for any help :)

Answer 1

Serialization/deserialization for arrays (including multi-dimensional) works out of the box.
Unexpected behavior is related to init section of your data class. It's executed after deserialization happens and overwrites data parsed from JSON.
It also happens when you create instance of Array2D manually:
val x = Array2D(1, 1, Array(1) { Array(1) { Thing("0") } })
println(x.arrayContents[0][0]) //will print null
You just need to replace init block with secondary constructor (default value for arrayContents is redundant, by the way), and you may declare arrayContents as val now:
#Serializable
data class Array2D(val width: Int, val height: Int, val arrayContents: Array<Array<Thing?>>) {
constructor(width: Int, height: Int) : this(width, height, Array(width) { Array(height) { null } })
}