Java如何将多个同key的json合并成一个key对应多个value的json

如将{"position":"a"},{"position":"b"},{"position":"c"}合并为{"position":["a","b","c"]}

初学者，不太懂，方便告诉一下具体的语句多么，我已经通过Gson转换得到了json的String串

public void testJsonLib()throws Exception{
String json="[{\"position\":\"a\"},{\"position\":\"b\"},{\"position\":\"c\"}]";
JSONArray jrr = JSONArray.fromObject(json);

java.util.ArrayList list=new java.util.ArrayList();
for(int i=0; i<jrr.size(); i++){
list.add( jrr.getJSONObject(i).getString("position") );
}

JSONObject obj = new JSONObject();
obj.put("position", list);
System.out.println(obj);
}

////{"position":["a","b","c"]}