请问爱和两个class怎么从第二个class里调用到第一个class中，一直在报错

Question

问题遇到的现象和发生背景

问题相关代码，请勿粘贴截图

运行结果及报错内容

我的解答思路和尝试过的方法

我想要达到的结果

```java
import java.util.ArrayList;
import java.util.Scanner;

public class shiyan2 {

    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        shiyan2 Xll=new shiyan2();    
        shiyan21 s1=new shiyan21("god","dateStructure",100,"000");
        shiyan21 s2=new shiyan21("Tom","Chinese",120,"001");
        shiyan21 s3=new shiyan21("Jerry","dateStructure",20,"002");
        shiyan21 s4=new shiyan21("Xll","dateStructure",233,"003");
        shiyan21 s5=new shiyan21("lipu","dateStructure",11,"004");
        System.out.println("1,显示全部记录");
        System.out.println("2，输入添加学生信息");
        System.out.println("3，删除学生信息（输入学号即可）");
        System.out.println("4，学号排序");
        System.out.println("5，课程排序");
        System.out.println("0，退出程序");
                boolean which=true;
                while(true) {
                    switch(n) {            
                    case 1: //我想把这个搞成第二class的show方法
                        ArrayList<shiyan21> Xinxi =new ArrayList<>();
                        if(Xinxi.size()!=0) {                                
                        Xinxi.add(s1);
                        Xinxi.add(s2);
                        Xinxi.add(s3);
                        Xinxi.add(s4);
                        Xinxi.add(s5);
                        for (int i = 0; i < Xinxi.size(); i++) {
                            System.out.println(Xinxi.get(i));
                        }}else {
                            System.out.println("快来添加几个学生吧！");
                        }
                        break;
                    case 2:
                    
                        System.out.println("请输入您的基本信息");
                        Xll.AddStu//这里出错。
                        break;
                    case 3:
                        
                        break;                    
                    case 4:
                        
                        break;
                    case 5:
                        
                        break;
                    case 0:
                        System.out.println("感谢你使用咸鲁力的编程，系统即将退出");
                        break;
                    }
                    
                }
            }
             
    }





```//第二个class，如果是接口请指教代码
import java.util.ArrayList;
import java.util.Scanner;

public class shiyan21 {
 /*public void show() {//用屏幕展示学生的记    无法用构造方法表示
            ArrayList<shiyan21> Xinxi = new ArrayList<>();
            for (int i = 0; i < shiyan21.s; i++) {
                System.out.println(shiyan21);
            }        
 }*/
public String Id;
public String Name;
public String learn;
public int grade;
public shiyan21(String Name,String learn,int grade,String Id) {
    this.grade=grade;
    this.Id=Id;
    this.learn=learn;
    this.Name=Name;
}
public void AddStu(ArrayList<shiyan21> ayy) {
    Scanner sc=new Scanner(System.in);
    System.out.println("请输入你的姓名");
    String name=sc.nextLine();
    System.out.println("请输入你的课程");
    String learn=sc.nextLine();
    System.out.println("请输入你的成绩");
    int grade=sc.nextInt();
    System.out.println("请输入你的姓名");
    String Id=sc.nextLine();
    shiyan21 t=new shiyan21(Name, learn, grade, Id);
    t.setGrade(grade);
    t.setLearn(learn);
    t.setName(name);
    t.setId(Id);
    
}
public String getId() {
    return Id;
}

public String getName() {
    return Name;
}
public void setName(String name) {
    Name = name;
}
public String getLearn() {
    return learn;
}
public void setLearn(String learn) {
    this.learn = learn;
}
public void setId(String Id) {
    this.Id = Id;
}
public int getGrade() {
    return grade;
}
public void setGrade(int grade) {
    this.grade = grade;
}
public String DelStu(String whichid) {
    return whichid;
}
public void SortByNo() {
    
}
public  void  SortByCourse() {
    
}
}

Answer 1

AddStu方法是shiyan21类中的。

shiyan2 Xll=new shiyan2(); 

你用shiyan2类的对象调用AddStu方法，这个类中并没有。