Java图书管理系统，不知道哪错了

import java.io.*;
import java.util.*;
class Book implements Serializable{
    private static final long serialVersionUID = 1L;
    public String name;
    public int id;
    public String zuozhe;
    public String chubanshe;
    public int jiage;
    
    public Book(int id,String name,String zuozhe,String chubanshe,int jiage){
        this.id = id;
        this.name = name;
        this.zuozhe = zuozhe;
        this.chubanshe=chubanshe;
        this.jiage=jiage;        
    }
    public Book(){
        Scanner sca = new Scanner(System.in);
        System.out.println("请输入书的编号：");
        id = sca.nextInt();
        System.out.println("请输入书的名字：");
        name = sca.next();
        System.out.println("请输入书的作者：");
        zuzohe= sca.next();    
        System.out.println("请输入书的出版社：");
        chubanshe= sca.next();
        System.out.println("请输入书的价格：");
        jiage= sca.nextInt();    
    }
    public String toString() {
        String ret = null;
        ret = "name = " + name + "\n";
        ret += "id = " + id + "\n";
        ret += "zuozhe= " + zuozhe + "\n";
        ret += "chubanshe= " + chubanshe + "\n";
        ret += "jiage= " + jiage + "\n";
        return ret;
    } 
}



class Manage
{
    Vector<Book> vector = new Vector<Book>();
    Scanner sc = new Scanner(System.in);
    int a,tid;
    public Manage()  throws Exception
    {
        File f = new File("D:/c.txt");
        if (f.exists() == true)
        {
            FileInputStream in = new FileInputStream(f);
            ObjectInputStream oin = new ObjectInputStream(in);
            Book[] aa=(Book[])oin.readObject();
            vector.clear();
            for(Book x:aa)
                vector.add(x);
            oin.close();
        }
    }
    public void menu()
    {
        System.out.println("请输入你选择的功能：");
        System.out.println("1.添加");
        System.out.println("2.删除");
        System.out.println("3.修改");
        System.out.println("4.查询");
        System.out.println("5.列表");
        System.out.println("6.保存并退出系统");
        System.out.println("请输入你的选择：");
        a = sc.nextInt();
        switch(a)
        {
        case 1:
            vector.add(new Book());
            menu();
            break;
        case 2 :
            System.out.println("请输入你要删除的书籍编号：");
            tid = sc.nextInt();
            for(int i=0; i<vector.size(); i++)
            {
                if(vector.get(i).id==tid)
                    vector.remove(i);
            }
            menu();
            break;
        case 3:
            System.out.println("请输入你要修改的书籍编号：");
            tid = sc.nextInt();
            for(int i=0; i<vector.size(); i++)
            {
                if(vector.get(i).id==tid)
                    vector.set(i,new Book());
            }
            menu();
            break;
        case 4:
            System.out.println("请输入你要查询的书籍编号：");
            tid = sc.nextInt();
            for(int i=0; i<vector.size(); i++)
                if(vector.get(i).id==tid)
                    System.out.println(vector.get(i));
            menu();
            break;
        case 5:
            for(int i=0; i<vector.size(); i++)
                System.out.println(vector.get(i));
            menu();
            break;
        case 6:
            try  
            {
                File f = new File("D:/c.txt");
                FileOutputStream out = new FileOutputStream(f);
                ObjectOutputStream oout = new ObjectOutputStream(out);
                Book[] ob=vector.toArray(new Book[vector.size()]);
                oout.writeObject(ob);
                oout.close();
            }
            catch(Exception e)
            {
                e.printStackTrace();
            }
            System.exit(0);
        }
    }
}

public class Test {
    public static void main(String args[])
    {
        Manage mag;
        try {
            mag = new Manage();
            mag.menu();    
        } catch (Exception e) {
            e.printStackTrace();
        } 
    }
}
![img](https://img-mid.csdnimg.cn/release/static/image/mid/ask/559672191936112.png "#left")

Student类找不到？
你代码中没看到这个类，也没看到有引用的地方。

为什么不用map集合存储呢