我得到错误'java.lang.String'并且不知道为什么

我是Java的新手。所以问题可能听起来很简单，但我被卡住了，无法弄清楚为什么我得到错误 java.lang.String 虽然我所做的只是正常地写出 setter 和 getter。所以下面是我的代码：

    package PerSion;

public class Student extends Persion{
    String RollNum, Class;
    double Score;

    public String getRollNum() {
        return this.RollNum;
    }

    public void setRollNum(String RollNum) {
        this.RollNum = RollNum;
    }

    public String getClass() {  //this is where the error show up
        return this.Class;
    }

    public void setClass(String Class) {
        this.Class = Class;
    }

    public double getScore() {
        return this.Score;
    }

    public void setScore(double Score) {
        this.Score = Score;
    }

    public Student() {
    }

    public Student(String RollNum, String Class, double Score) {
        this.RollNum = RollNum;
        this.Class = Class;
        this.Score = Score;
    }

    public Student(String RollNum, String Class, double Score, String ID, String Name, String Address, int Age) {
        super(ID, Name, Address, Age);
        this.RollNum = RollNum;
        this.Class = Class;
        this.Score = Score;
    }

    @Override
    public void Display() {
        System.out.println(RollNum+"\t"+Name+"\t"+Class+"\t"+Score);
    }
    
}