How to decode json data with Reified, Generics, Interface and Kotlin? - json

Question

How to decode json data with Reified, Generics, Interface and Kotlin?
I created a project where i put the code and the instructions to run:
https://github.com/paulocoutinhox/kotlin-gson-sample
But basically the code is:
import com.google.gson.Gson
import com.google.gson.reflect.TypeToken
interface Serializer {
fun <T> decodeValue(data: String): T?
}
class JsonSerializer : Serializer {
override fun <T> decodeValue(data: String): T? {
try {
val type = object : TypeToken<T>() {}.type
val gson = Gson()
return gson.fromJson<T>(data, type)
} catch (e: Exception) {
println("Error when parse: ${e.message}")
}
return null
}
}
class Request<T>(val r: T)
inline fun <reified T> callSerializer(json: String): T? {
val serializer = JsonSerializer()
val decoded = serializer.decodeValue<Request<T>>(json)
return decoded?.r
}
fun main() {
val finalValue = callSerializer<Request<String>>("{\"r\": \"test\"}")
println("Decoded data is: $finalValue")
}
The Request class has an inner value called r with generic type.
Im trying convert the json data above to the Request class and bind r from json to string type.
But im getting the error:
> Task :run FAILED
Exception in thread "main" java.lang.ClassCastException: class com.google.gson.internal.LinkedTreeMap cannot be cast to class Request (com.google.gson.internal.LinkedTreeMap and Request are in unnamed module of loader 'app')
at MainKt.main(Main.kt:36)
at MainKt.main(Main.kt)
The gson library think that it is a LinkedTreeMap instead of the Request class.
How to solve this?
Thanks.

Answer 1

import com.google.gson.Gson
import com.google.gson.reflect.TypeToken
interface Serializer {
fun <T> decodeValue(data: String): Request<T>?
}
class JsonSerializer : Serializer {
override fun <T> decodeValue(data: String): Request<T>? {
try {
val type = object : TypeToken<Request<T>>() {}.type
val gson = Gson()
return gson.fromJson<Request<T>>(data, type)
} catch (e: Exception) {
println("Error when parse: ${e.message}")
}
return null
}
}
class Request<T>(val r: T)
inline fun <reified T> callSerializer(json: String): T? {
val serializer = JsonSerializer()
val decoded = serializer.decodeValue<T>(json)
return decoded?.r
}
fun main() {
println("Decoded data is: ${callSerializer<String>("{\"r\": \"test\"}")}")
}