Messages messages = new Messages(); messages.setMessage("Some message here"); //Add all the regn IDs to the regIDs List. ArrayList<String> regIDs = messages.getRegIDs() regIDs.add("regnID1"); regIDs.add("regnID2"); regIDs.add("regnID3"); regIDs.add("regnID4"); //Add as many IDs as you want. //then create a new GSON Object. Gson gson = new Gson(); String jsonString = gson.toJSON(messages);