RoomBaseConverters.kt

import android.arch.persistence.room.TypeConverter
import com.google.gson.Gson
import com.google.gson.reflect.TypeToken
import java.lang.reflect.Type

open class BaseConverter<T>(private val type: Class<T>) {

    private val gson = Gson()

    @TypeConverter
    fun fromString(value: String?): T? = value?.let { gson.fromJson(value, type) }

    @TypeConverter
    fun fromClass(value: T?): String? = value?.let { gson.toJson(value) }

}

open class BaseListConverter<T>(private val type: Type) {

    private val gson = Gson()

    @TypeConverter
    fun fromString(value: String?): List<T>? = value?.let { gson.fromJson(value, type) }

    @TypeConverter
    fun fromList(list: List<T>?): String? = list?.let { gson.toJson(list) }

}

private inline fun <reified T> type(): Type = object : TypeToken<T>() {}.type

// usage
class LocalDateConverter : BaseConverter<LocalDate>(LocalDate::class.java)
class LongListConverter : BaseListConverter<Long>(type<List<Long>>())