System for saving objects

Processing Coding Questions
3

This is interesting, but I wrote this code:

void Save()
{  
  switch(mode) {
  case 1:
    for (int i = 0; i<cir.size(); i++)
    {
      save.saves(cir.get(i));
    }
    break;
  case 2:
    if (pendul)
    {
      for (int i = 0; i< pend.size(); i++)
      {
        save.saves(pend.get(i));
      }
    } else 
    {
      for (int i = 0; i< doublePend.size(); i++)
      {
        save.saves(doublePend.get(i));
      }
    }
    break;
  }
}
class SaveGame
{
  Gson gson; 
  String json;
  JSONObject js;

  SaveGame()
  {
    gson = new Gson();
  }

  void saves(Object obj)
  {
    json = gson.toJson(obj);
    js = parseJSONObject(json);
    boolean exist;
    if (js != null)
    {
      File f;
      int i = -1;
      do
      {
        i++;
        f = dataFile("saves/save" + i + ".txt");     
        exist = f.isFile();
      }
      while (exist);
      println(i, " ",f , " ",exist);//, " ",js, " ",json);
      saveJSONObject(js, "saves/save" + i + ".txt");
    }  
  }
}

But this isn’t working, and I have this errors: SEVERE: An error occured while forwarding a Controller event, please check your code at Save java.lang.reflect.InvocationTargetException Caused by: java.lang.IllegalArgumentException: class java.awt.Frame declares multiple JSON fields named state
That I don’t know how to resolve. Can you help me with that?