如何从java中的父对象获取子对象？

Question

I am getting trouble in an application developement. My task is i have a service object containing set of person objects. the person object has two derived objects called student and teacher. This service object i am getting from client to server as a json object. At server side i am able to get the List<Person> object from the service by using service.getPersonList() method. But i am not able to get the child classes of Person . My code is like this

//person

public class Person {

    private String name;
    private Integer type;

    public static final Integer TYPE_STUDENT = 1 ;
    public static final Integer TYPE_TEACHER = 2 ;

    public Integer getType() {
        return type;
    }

    public void setType(Integer type) {
        this.type = type;
    }

    public String getName() {
        return name;
    }

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

}

//student

public class Student extends Person{

    private Integer year;

    public Integer getYear() {
        return year;
    }

    public void setYear(Integer year) {
        this.year = year;
    }
}

//teacher

public class Teacher extends Person{

    private String dept;

    public String getDept() {
        return dept;
    }

    public void setDept(String dept) {
        this.dept = dept;
    }
}

//service

public class Service {

    private List<Person> personList;

    public List<Person> getPersonList() {
        return personList;
    }

    public void setPersonList(List<Person> personList) {
        this.personList = personList;
    }
}

// my json object is like this

[{"year":"4","name":"sreenu","type":1},{"dept":"physics","name":"somehow","type":2}]

I am trying to find the child class by using type property. I am getting type value means parent object but not able to fetch child from that. How can i solve this problem?

In controller i am getting the json object like this

//controller public String saveService(@RequestBody String service) {

List<Person> personList = (List<Person>) gson.fromJson(
    data, new TypeToken<List<Person>>(){}.getType()
);
for(Person person : personList) {
    if(person.getType() ==  Person.TYPE_STUDENT) {
        Student student = (Student) person;
        System.out.println(student.getYear());
    }
    if(person.getType() ==  Person.TYPE_TEACHER) {
        Teacher teacher = (Teacher) teacher;
        System.out.println(teacher.getDept());
    }
}

}

I am getting class cast exception. Even i tried with instance of operator.

Answer 1

You are deserializing to a simpler type (Person) rather than a more complex type (Student/Teacher). This means that additional subtype information (like Year or Department) is being discarded. A Student can be cast as a Person, but a Person cannot be cast as a Student.

This reflects a design problem. Either you will need to change your Person class so that it contains all the possible data that a Subtype needs, or you will need to preemptively segregate your data into Teachers and Students, before creating your objects from Json.