java.lang.ClassCastException：com.google.gson.internal.LinkedTreeMap无法转换为asd.Grade

我手头有一个大型项目，但在此示例中我模拟了遇到的问题：

我的第一堂课：

package asd;

import java.io.Serializable;

public class Grade implements Serializable{
String name;
Integer score;

public String getName() {
    return name;
}

public Grade() {}

public Grade(String name, Integer score) {
    this.name = name;
    this.score = score;
}

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

public Integer getScore() {
    return score;
}

public void setScore(Integer score) {
    this.score = score;
}
}

我的第二堂课：

package asd;

import java.io.Serializable;


public class Student implements Serializable {
private String name;
private Object grade;

public String getName() {
    return name;
}

public Student(String name, Object grade) {
    this.name = name;
    Grade = grade;
}

public Student() {}

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

public Object getGrade() {
    return Grade;
}

public void setGrade(Object grade) {
    Grade = grade;
}
}

这是我的主要课程：

package asd;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import java.io.*;

public class Test {
public static void main(String[] args){
    Student s1 = new Student();
    s1.setName("JeanPierre");
    s1.setGrade(new Grade("Math", 8));

    Gson gson = new GsonBuilder().create();
    String convertedToJson = gson.toJson(s1);
    System.out.println("Json string: " + convertedToJson);

    Student s2 = gson.fromJson(convertedToJson, Student.class);

    System.out.println("Student Name: " + s2.getName());
    System.out.println("Grade Name: " + ((Grade)s2.getGrade()).getName());
    System.out.println("Grade Score: " + ((Grade)s2.getGrade()).getScore());

}
}

输出：

Json字符串是：

{"name":"JeanPierre","Grade":{"name":"Math","score":8}}

Student Name: JeanPierre


Exception in thread "main" java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to asd.Grade
    at asd.Test.main(Test.java:24)

我的问题是当我打电话给：

System.out.println("Grade Name: " + ((Grade)s2.getGrade()).getName());

或者

System.out.println("Grade Score: " + ((Grade)s2.getGrade()).getScore());

我得到这个例外：

Exception in thread "main" java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to asd.Grade
    at asd.Test.main(Test.java:24)