How to register an InstanceCreator with Gson in Kotlin?

Question

I can use Code 1 to save MutableList<MDetail> to json string using Gson correctly, But I get the error when I try to restore MutableList<MDetail> object from json string with Code 2. I have search some resources, it seems that I need to register an InstanceCreator.

How can I write a register an InstanceCreator code with Kotlin? Thanks!

Error

Caused by: java.lang.RuntimeException: Unable to invoke no-args constructor for interface model.DeviceDef. Registering an InstanceCreator with Gson for this type may fix this problem.

Code 1

private var listofMDetail: MutableList<MDetail>?=null
mJson = Gson().toJson(listofMDetail) //Save

Code 2

var mJson: String by PreferenceTool(this, getString(R.string.SavedJsonName) , "")
var aMListDetail= Gson().fromJson<MutableList<MDetail>>(mJson)

inline fun <reified T> Gson.fromJson(json: String) = this.fromJson<T>(json, object: TypeToken<T>() {}.type)

My Class

interface DeviceDef

data class BluetoothDef(val status:Boolean=false):  DeviceDef
data class WiFiDef(val name:String, val status:Boolean=false) : DeviceDef

data class MDetail(val _id: Long, val deviceList: MutableList<DeviceDef>)
{
    inline fun <reified T> getDevice(): T {
        return deviceList.filterIsInstance(T::class.java).first()
    }
}

Added

After I use val myGson = GsonBuilder().setPrettyPrinting().registerTypeAdapterFactory(adapter).create(), I can get the correct result when I use open class DeviceDef , why?

open class DeviceDef

data class BluetoothDef(val status:Boolean=false):  DeviceDef()
data class WiFiDef(val name:String, val status:Boolean=false) : DeviceDef()

val adapter = RuntimeTypeAdapterFactory
        .of(DeviceDef::class.java)
        .registerSubtype(BluetoothDef::class.java)
        .registerSubtype(WiFiDef::class.java)


data class MDetail(val _id: Long, val deviceList: MutableList<DeviceDef>)
{
    inline fun <reified T> getDevice(): T {
        return deviceList.filterIsInstance(T::class.java).first()
    }
}

val myGson = GsonBuilder().setPrettyPrinting().registerTypeAdapterFactory(adapter).create()

Answer 1

Gson is having a hard time deserialising polymorphic objects as in your MutableList<DeviceDef>. Here's what you need to do:

1. Add the RuntimeTypeAdapterFactory.java to your project manually (does not seem to be part of gson library). See also this answer.

2. Change your code to use the factory

   Create Gson instance:

   val adapter = RuntimeTypeAdapterFactory
           .of(DeviceDef::class.java)
           .registerSubtype(BluetoothDef::class.java)
           .registerSubtype(WiFiDef::class.java)
   
   val gson = GsonBuilder().setPrettyPrinting().registerTypeAdapterFactory(adapter).create()
   

3. register each of your subtypes in the factory and it will work as intended :)