使用Java中的JSONObject，对象的JSON字符串返回null

Question

通过JSONObject循环会产生null元素。我的jsonString是：

{"user1":[{"43":1558631338651},{"40":1558631347443}],"user2":[{"367":1558631214488},{"365":1558631222849},{"362":1558631231942}]}

@SuppressWarnings("unchecked")
    public void loadMapFromJsonString(String jsonString) {
        try {
            JSONObject json = (JSONObject) new JSONParser().parse(jsonString);
        //json converts to an object... and the for loop works, but
        //list is always null
            for (int i = 0; i < json.size(); i++) {
                List<HashMap<String, Long>> list = (List<HashMap<String, Long>>) json.get(i);
                System.out.println(list); //always null..??
            }
        }
    catch (Exception e) {
        System.out.println(e);
        //log this
        }
    }

for循环中的列表始终为空。为什么会这样？