Gson List Adapter

ListAdapter.kt

import java.io.IOException
import java.util.ArrayList

import com.google.gson.Gson
import com.google.gson.TypeAdapter
import com.google.gson.reflect.TypeToken
import com.google.gson.stream.JsonReader
import com.google.gson.stream.JsonToken
import com.google.gson.stream.JsonWriter

class ListAdapter<T>(private val clazz: Class<T>, private val gson: Gson) : TypeAdapter<List<T>>() {

    @Throws(IOException::class)
    override fun read(reader: JsonReader): List<T>? = when (reader.peek()) {
        JsonToken.BEGIN_OBJECT -> {
            val item = gson.fromJson<T>(reader, clazz)
            listOf(item)
        }
        JsonToken.BEGIN_ARRAY -> {
            val list = ArrayList<T>()
            reader.beginArray()
            while (reader.hasNext()) {
                val item = gson.fromJson<T>(reader, clazz)
                list.add(item)
            }
            reader.endArray()
            list
        }
        else -> null
    }

    @Throws(IOException::class)
    override fun write(out: JsonWriter, collection: List<T>?) {
        if (collection == null) {
            out.nullValue()
            return
        }
        val elementTypeAdapter = gson.getAdapter(object : TypeToken<T>() {
        })
        out.beginArray()
        for (element in collection) {
            elementTypeAdapter.write(out, element)
        }
        out.endArray()
    }
}

ListAdapterFactory.kt

import java.lang.reflect.ParameterizedType

import com.google.gson.Gson
import com.google.gson.TypeAdapter
import com.google.gson.TypeAdapterFactory
import com.google.gson.reflect.TypeToken

class ListAdapterFactory : TypeAdapterFactory {

    @Suppress("UNCHECKED_CAST")
    override fun <T> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {
        return if (type.rawType == List::class.java) {
            try {
                val clazz: Class<T> = (type.type as ParameterizedType).actualTypeArguments[0] as Class<T>
                ListAdapter(clazz, gson) as TypeAdapter<T>
            } catch (e: Exception) {
                null
            }
        } else {
            null
        }
    }
}

Main.kt

//to register the adapter 
val gson = GsonBuilder().registerTypeAdapterFactory(ListAdapterFactory()).create()