SimpleScalaGsonExample.scala

import java.lang.reflect.Type
import java.util

import com.google.gson._

/**
  * Cannot get scala list to work nicely so resorted to Java List for the time being
  */
object SimpleExample extends App {

  lazy val gson: Gson = new GsonBuilder()
    .registerTypeHierarchyAdapter(classOf[Seq[Any]], new ListSerializer)
    .setPrettyPrinting()
    .create()

  class ListSerializer extends JsonSerializer[Seq[Any]] {//with JsonDeserializer[Seq[Any]] {
    override def serialize(src: Seq[Any], typeOfSrc: Type, context: JsonSerializationContext): JsonElement = {
      import scala.collection.JavaConverters._
      context.serialize(src.toList.asJava)
    }

//    override def deserialize(json: JsonElement, typeOfT: Type, context: JsonDeserializationContext): Seq[Any] = {
//      if (json.isJsonArray) {
//        val array: JsonArray = json.getAsJsonArray
//        (for (i <- 0 until array.size()) yield context.deserialize(array.get(i).getAsJsonObject, typeOfT)).toList
//      } else {
//        context.deserialize(json.getAsJsonObject, typeOfT)
//      }
//    }
  }

  case class User(name: String, age: Int, friends: util.List[User])

  val friends : util.List[User] = new util.ArrayList
  friends.add(User("Dan", 5, new util.ArrayList))
  friends.add(User("Janis", 7, new util.ArrayList))

  val user: User = User("Jane", 12, friends)
//  val user: User = User("Jane", 12, List(User("Dan", 5, Nil), ArrayList("Janis", 7, Nil)))

  println(gson.toJson(user))

  println(gson.fromJson(
    """
      |{
      | "name": "john",
      | "age": 55,
      | "friends": [
      |   {
      |     "name": "Gina",
      |     "age": 89,
      |     "friends": []
      |   }
      | ]
      |}
    """.stripMargin, classOf[User]))
}