JSON.parseObject ：是将Json字符串转化为相应的对象；JSON.toJSONString ：则是将对象转化为Json字符串。
JSON.toJSON(user2) ：把Java对象 转 JSON对象
JSONObject jsonObject1 = (JSONObject) JSON.toJSON(user2);
// {“sex”:“男”,“name”:“秦疆2号”,“age”:3}

public class fastJson {



    public static void main(String[] args) {



        //创建一个对象



        User user1 = new User("秦疆1号", 3, "男");

        User user2 = new User("秦疆2号", 3, "男");

        User user3 = new User("秦疆3号", 3, "男");

        User user4 = new User("秦疆4号", 3, "男");

        List list = new ArrayList();

        list.add(user1);

        list.add(user2);

        list.add(user3);

        list.add(user4);



        Map map =new HashMap<>();

        map.put("user1",user1);

        map.put("user2",user2);

        map.put("user3",user3);



        //JSON.toJSONString则是将对象转化为Json字符串。

        String mapString = JSON.toJSONString(map);

        System.out.println("json 格式的map"+mapString);

        //json格式的map{"user1":{"age":3,"name":"秦疆1号","sex":"男"},"user2":{"age":3,"name":"秦疆2号","sex":"男"},"user3":{"age":3,"name":"秦疆3号","sex":"男"}}



        //JSON.parseObject，是将Json字符串转化为相应的对象；

        JSONObject jsonObjectMap = JSON.parseObject(mapString);

        System.out.println("jsonObjectMap:"+jsonObjectMap);

//jsonObjectMap:{"user1":{"sex":"男","name":"秦疆1号","age":3},"user2":{"sex":"男","name":"秦疆2号","age":3},"user3":{"sex":"男","name":"秦疆3号","age":3}}



        //把json对象转换成map对象

        Map map1 = JSON.parseObject(mapString, Map.class);

        Object user11 = map1.get("user1");

        System.out.println("user11"+user11);



        System.out.println("*******Java对象 转 JSON字符串*******");

        String str1 = JSON.toJSONString(list);

        System.out.println("JSON.toJSONString(list)==>"+str1);

        //打印结果JSON.toJSONString(list)==>[{"age":3,"name":"秦疆1号","sex":"男"},{"age":3,"name":"秦疆2号","sex":"男"},{"age":3,"name":"秦疆3号","sex":"男"},{"age":3,"name":"秦疆4号","sex":"男"}]



        String str2 = JSON.toJSONString(user1);

        System.out.println("JSON.toJSONString(user1)==>"+str2);

        //JSON.toJSONString(user1)==>{"age":3,"name":"秦疆1号","sex":"男"}

         //为啥不是“{"age":3,"name":"秦疆1号","sex":"男"}” ？



        System.out.println("\n****** JSON字符串 转 Java对象*******");

        User jp_user1=JSON.parseObject(str2,User.class);

        System.out.println("JSON.parseObject(str2,User.class)==>"+jp_user1);



        System.out.println("\n****** Java对象 转 JSON对象 ******");

        JSONObject jsonObject1 = (JSONObject) JSON.toJSON(user2);

        System.out.println("(JSONObject) JSON.toJSON(user2)==>"+jsonObject1);

        //{"sex":"男","name":"秦疆2号","age":3}



        System.out.println("\n****** JSON对象 转 Java对象 ******");

        User to_java_user = JSON.toJavaObject(jsonObject1, User.class);

        System.out.println("JSON.toJavaObject(jsonObject1, User.class)==>"+to_java_user);



    }

}

JSON.toJSONString中序列化空字符串遇到的坑:

1.由json字符串转换成Map对象
如json字符串：{“contend”:[{“bid”:“22”,“carid”:“0”},{“bid”:“22”,“carid”:“0”}],“result”:100,“total”:2}

下面直接附代码：

//json字符串

String jsondata="{\"contend\":[{\"bid\":\"22\",\"carid\":\"0\"},{\"bid\":\"22\",\"carid\":\"0\"}],\"result\":100,\"total\":2}";

JSONObject obj= JSON.parseObject(jsondata);

//map对象

Map data =new HashMap<>();

//循环转换

 Iterator it =obj.entrySet().iterator();

 while (it.hasNext()) {

       Map.Entry entry = (Entry) it.next();

       data.put(entry.getKey(), entry.getValue());

 }

System.out.println("map对象:"+data.toString());

下面是输出内容：

{total=2, contend=[{“carid”:“0”,“bid”:“22”},{“carid”:“0”,“bid”:“22”}], result=100}

2.由Map对象转换成json字符串

//map对象

Map data =new HashMap<>();

String x =JSONObject.toJSONString(data);

System.out.println("json字符串:"+x);

下面是输出内容：

{“total”:2,“result”:100,“contend”:[{“carid”:“0”,“bid”:“22”},{“carid”:“0”,“bid”:“22”}]}