How to parse JSON objects into an enum - kotlin

Question

I have a JSON object that is something like
{
"tsp": "ABC" // can be only one of three things: "ABC", "DEF", "GHI"
"userId" : "lkajsdlk-199191-lkjdflakj"
}
Instead of writing a dataclass along the lines of
data class User(#SerializedName("tsp") val tsp: String, #SerializedName("userId") val userId: String
i'd like to have an enum that defines the three values so that my data class can be
data class User(#SerializedName("tsp") val tsp: TspEnum, #SerializedName("userId") val userId: String
I had tried writing an enum that was
enum class TspEnum(provider: String) {
AY_BEE_CEE("ABC"),
DEE_EE_EFF("DEF"),
GEE_HAYTCH_I("GHI");
}
however that did not work out
I've realized now that calling TspEnum.provider will return the value of the enum, but I'm not sure how to make GSON coöperate whilst serializing the JSON into the kotlin data class
I've read that there is an issue with Kotlin typing and GSON here: https://discuss.kotlinlang.org/t/json-enum-deserialization-breakes-kotlin-null-safety/11670
however, the way that person is serializing the hair colours to map into an enum is different enough from my tsp json object to make me scratch my head.
Any pointers on where i'm going wrong would be great, cheers!

Answer 1

You can create a deserializer for TspEnum:
class TspDeserializer : JsonDeserializer<TspEnum> {
override fun deserialize(json: JsonElement, typeOfT: Type?, context: JsonDeserializationContext?): TspEnum {
val stringValue = json.asString
for (enum in TspEnum.values()) {
if (enum.provider == stringValue) {
return enum
}
}
throw IllegalArgumentException("Unknown tsp $stringValue!")
}
}
next you have to register it:
val gson = GsonBuilder()
.registerTypeAdapter(TspEnum::class.java, TspDeserializer())
.create()
and then you can parse your user:
val user = gson.fromJson(json, User::class.java)
println(user) // prints User(tsp=AY_BEE_CEE, userId=lkajsdlk-199191-lkjdflakj)