一个学生对象如下：

班级对象

public class Grade {

    private Long id;
    private String name;
    private List<Student> users = new ArrayList<Student>();

    // 省略 setter、getter
    
    public void addStudent(Student student) {
        users.add(student);
    }

    @Override
    public String toString() {
        return "Grade{" +
                "id=" + id +
                ", name=‘" + name + ‘\‘‘ +
                ", users=" + users +
                ‘}‘;
    }
}

学生对象

public class Student {

    private Long   id;
    private String name;

    // 省略 setter、getter
    
    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name=‘" + name + ‘\‘‘ +
                ‘}‘;
    }
}

运行的 Main 函数

public class MainTest {

    public static void main(String[] args) {
        Grade group = new Grade();
        group.setId(0L);
        group.setName("admin");

        Student student = new Student();
        student.setId(2L);
        student.setName("guest");

        Student rootUser = new Student();
        rootUser.setId(3L);
        rootUser.setName("root");

        group.addStudent(student);
        group.addStudent(rootUser);

        // 转换为 JSON
        String jsonString = JSON.toJSONString(group);
        System.out.println("JSON字符串：" + jsonString);

        // 转换为 对象BEAN
        Grade grade = JSON.parseObject(jsonString, Grade.class);
        System.out.println("JavaBean对象：" + grade);
    }
}

最后的运行结果如下：

JSON字符串：
{"id":0,"name":"admin","users":[{"id":2,"name":"guest"},{"id":3,"name":"root"}]}

JavaBean对象：
Grade{id=0, name=‘admin‘, users=[Student{id=2, name=‘guest‘}, Student{id=3, name=‘root‘}]}