String json = "[{'day1':'work','day2':26},{'day1':123,'day2':26}]";
        StringJoiner sj1 = new StringJoiner(",");
        JSONArray jsonArray = JSONArray.fromObject( json);
        for(int i=0; i<jsonArray.size(); i++){
            JSONObject obj = (JSONObject) jsonArray.get(i);
            String day1 = String.valueOf(obj.get("day1"));
            sj1.add(day1);
          }
        System.out.println(sj1.toString());
        }

结果：