hello 👋 JSON fields can have 3 states ->

undefined

(field is omitted),

null

(field=null) and also can have a

value

(field=value). Within JVM we either have value or no-value (i.e.

null

) so we are trying to represent those 3 states using Kotlin sealed classes - something along the lines

sealed class OptionallyDefined<T> {
    object Undefined : OptionallyDefined<Nothing>()
    data class Defined<T>(val data: T) : OptionallyDefined<T>()
}

Unfortunately cannot get the deserialization to work. I tried using custom serializer (i.e.

@JsonDeserialize(using = NullAwareDeserializer::class)

) but cannot get it to work correctly. Any pointers would be greatly appreciated!

this appears to work, using `@JsonCreator`:

sealed class OptionallyDefined<T> {
    class Undefined<T> : OptionallyDefined<T>()
    data class Defined<T>(val data: T?) : OptionallyDefined<T>()
    companion object {
        @JvmStatic
        @JsonCreator
        fun <T> getOptDefined(input: Map<String, T>): OptionallyDefined<T> {
            return if (input.containsKey("data")) {
                OptionallyDefined.Defined(input["data"])
            } else {
                OptionallyDefined.Undefined()
            }
        }
    }
}

fun main() {
    val mapper = ObjectMapper()
    val x = OptionallyDefined.Defined<Int?>(1)
    val y: OptionallyDefined<Int?> = OptionallyDefined.Undefined()
    val z = OptionallyDefined.Defined<Int?>(null)
    println(mapper.writeValueAsString(x))
    println(mapper.writeValueAsString(y))
    println(mapper.writeValueAsString(z))
    println(mapper.readValue<OptionallyDefined<Int?>>("{\"data\":1}"))
    println(mapper.readValue<OptionallyDefined<Int?>>("{}"))
    println(mapper.readValue<OptionallyDefined<Int?>>("{\"data\":null}"))
}

👍