部分json文件如下：

{ "statuses": [ { "created_at": "Fri Dec 02 17:05:40 +0800 2016", "id": 4048283825629844, "mid": "4048283825629844", "idstr": "4048283825629844", "text": "前卫 //@主治医湿:造型有点害怕 //@冷知君:原谅我不懂时尚 //@城南邮局:最后是刘梓晨吧 //@话提:走远了……", "source_allowclick": 0, "source_type": 1, "source": "iPhone 7 Plus", "favorited": false, "truncated": false, "in_reply_to_status_id": "", "in_reply_to_user_id": "", "in_reply_to_screen_name": "", "pic_urls": [], "geo": null, "user": { "id": 5680719858, "idstr": "5680719858", "class": 1, "screen_name": "综艺博主",

我是使用gson来解析的，代码如下：

public class Main { public static void main(String[] args) throws IOException { Gson gson = new Gson(); jsonstring abc = gson.fromJson(json,jsonstring.class); List statuses = abc.getStatuses(); for (statuses st : statuses){ System.out.println(st.getUser().getScreen_name()); } } } } //json最外层的内容 class jsonstring{ private List statuses; private String hasvisible; public List getStatuses() { return statuses; } public String getHasvisible() { return hasvisible; } } //包含statuses中的key class statuses{ private String created_at; private String text; private user user; public com.lizhao.user getUser() { return user; } public String getCreated_at() { return created_at; } public String getText() { return text; } } //是statuses里边user这个key的内容 class user{ private String id; private String screen_name; private String description; public String getDescription() { return description; } public String getId() { return id; } public String getScreen_name() { return screen_name; } }

存在一种情况是，user这个key不存在。

我应该怎么判断user这个key是否存在呢？