public class ObjectAndJson {

    public static void main(String[] args) {
        // 1、java对象转JSON
        Student student = new Student("张三", 18);
        Object studentJson = JSONObject.toJSON(student);
        System.out.println("student对象转json后：" + studentJson);
        // 2、java对象转JSONstring
        String stringJSON = JSON.toJSONString(student);
        System.out.println("student对象转jsonString后：" + studentJson);

        // 3、json转java对象
        JSONObject jsonObject = new JSONObject();
        jsonObject.put("name", "李四");
        jsonObject.put("age", 19);
        Student stu = JSONObject.parseObject(String.valueOf(jsonObject), Student.class);
        System.out.println("Json转为java对象后:" + stu.toString());

    }
}

class Student {
    private String name;
    private Integer age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    Student(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public Student() {
    }
}

运行结果 如下：

student对象转json后：{"name":"张三","age":18}
student对象转jsonString后：{"name":"张三","age":18}
Json转为java对象后:Student{name='李四', age=19}