Hello! I am trying to deserialize a json element t...

# serialization

[Anuta Vlad Sv]

Hello!
I am trying to deserialize a json element that ca be a single object or an empty array when no address returned.
"Address":{"firstName":"firstNameValue"} or "Address":[]
What I did this was to use JsonTransformingSerializer.transformDeserialize() and return a null JsonElement if the element is an array. And since the JsonTransformingSerializer does not currently support nullable types I created a NullableJsonTransformingSerializer.

I was wondering if there is another way to deserialize the Address, and not use the transformDeserialize().

[Anuta Vlad Sv]

```@Serializable

data class Project(

@Serializable(with = UnwrappingJsonListSerializer::class)

val address: Address?

)

@Serializable

data class Address(val firstName: String)

object UnwrappingJsonListSerializer :

NullableJsonTransformingSerializer<Address>(Address.serializer()) {

override fun nullableTransformDeserialize(element: JsonElement): JsonElement? {

return if (element is JsonArray) null

else element

}

}```

[Adam S]

I think that JsonTransformingSerializer supports decoding as null - but you have to return the JsonNull object, instead of a literal null

[Adam S]

could you try this?

objArrKxs.kt

[Anuta Vlad Sv]

Thank you @Adam S for your suggestion! Looks like is throwing an exception

Exception in thread "main" kotlinx.serialization.json.internal.JsonDecodingException: Expected class kotlinx.serialization.json.JsonObject as the serialized body of com.stockx.stockx.sell.checkout.ui.Address, but had class kotlinx.serialization.json.JsonNull

so I tried specifying the serializer as nullable but I get a compile error

[Anuta Vlad Sv]

I was wondering if there is another way to deserialize the Address, and not use the transformDeserialize()

[Adam S]

hmm, yeah so you were right, JsonTransformingSerializer isn’t suitable.

What you could do is decode address to JsonElement, and in code convert it to Address if it’s a JsonObject

@Serializable
data class Project(
  @SerialName("address")
  private val _address: JsonElement
) {
  val address: Address? get() = when (_address) {
    is JsonObject -> Json.decodeFromJsonElement(_address)
    else          -> null
  }
}

I like this way, because there’s no custom serializer - it’s just plain Kotlin code. It’s a little weird and verbose though, and the automatic toString() doesn’t look correct

You could automate it a little by creating a value class that will wrap the logic. Value classes are nice because they’re ‘transparent’ in terms of the actual content, but you can still use them to inject logic into Kotlinx Serialization

@Serializable
data class Project(
  private val address: AddressWrapper
)

@Serializable
data class Address(val firstName: String)

@JvmInline
@Serializable
value class AddressWrapper(private val value: JsonElement) {
  val address: Address? get() = when (value) {
    is JsonObject -> Json.decodeFromJsonElement(value)
    else          -> null
  }

  override fun toString() = address.toString()
}

[Anuta Vlad Sv]

This is cool! 🤩 Agree that is more transparent and it's great that no custom serializer is needed. Thanks a lot for your help, I really appreciate it!

[Adam S]

my pleasure!