public class classes implements Serializable {

    private int grade;
    private String major;
    private int classnum;
    private ArrayList stulist;       //去掉之后读写无误,此容器用作存放students类

    public classes(int grade, String major, int classnum) {
        this.grade = grade;
        this.major = major;
        this.classnum = classnum;
        this.stulist = new ArrayList();       //去掉之后读写无误
    }

    public void addstudent(students a){
         this.stulist.add(a);                //添加学生
    }

    public void savaclass() {          //写入函数，无错误提示
        try {
            File classFile = new File("D:/TestData/Classes");
            
            FileOutputStream classdata = new FileOutputStream("D:/TestData/Classes/" + classnum + ".dat");
            ObjectOutputStream out = new ObjectOutputStream(classdata);
            out.writeObject(this);
            classdata.close();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public classes loadclass(int classnum) {
        classes a = new classes();
        try {
            FileInputStream classdata = new FileInputStream("D:/TestData/Classes/" + classnum + ".dat");
            ObjectInputStream in = new ObjectInputStream(classdata);
            if (a instanceof classes) {
                a = (classes) in.readObject();   //但classes不含对象数组或Arraylist时读写无误
                                                 //否则会抛出异常local class incompatible
            } else {
                System.out.print("Exception");
            }
            classdata.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return a;
    }

     public void static main(String[] args){
         classes a = new classes(2009,"softwareE",5);//产生一个不含学生的班级类
         a.addstudent(new students(10,"Tim",5));
         a.savaclass();
         classes c = new classes();//无参构造函数默认存在，且会对数组初始化，这里没贴上
         c = a.loadclass(5);      //classes含对象数组时出错，否则正常读出
}


class students implements Serializable {

    private String name;
    private int stunumber;
    private int classnum;

    public students(int stunumber, String name, int classnum) {
        this.name = name;
        this.stunumber = stunumber;
        this.classnum = classnum;
    }
}

import java.util.ArrayList;
import java.io.*;

public class Classes implements Serializable {

    private int grade;
    private String major;

    public int getGrade() {
        return grade;
    }

    public void setGrade(int grade) {
        this.grade = grade;
    }

    public String getMajor() {
        return major;
    }

    public void setMajor(String major) {
        this.major = major;
    }

    public int getClassnum() {
        return classnum;
    }

    public void setClassnum(int classnum) {
        this.classnum = classnum;
    }

    public ArrayList<Student> getStulist() {
        return stulist;
    }

    public void setStulist(ArrayList<Student> stulist) {
        this.stulist = stulist;
    }

    private int classnum;
    private ArrayList<Student> stulist;       //去掉之后读写无误,此容器用作存放students类

    public Classes(int grade, String major, int classnum) {
        this.grade = grade;
        this.major = major;
        this.classnum = classnum;
        this.stulist = new ArrayList();       //去掉之后读写无误
    }

    public void addStudent(Student a){
         this.stulist.add(a);                //添加学生
    }

    public void savaClass() {          //写入函数，无错误提示
        try {
//            File classFile = new File("D:/TestData/Classes");

            FileOutputStream classdata = new FileOutputStream("D:/TestData/Classes/" + classnum + ".dat");
            ObjectOutputStream out = new ObjectOutputStream(classdata);
            out.writeObject(this);
            classdata.close();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public Classes loadClass(int classnum) {
        Classes a = null;
        try {
            FileInputStream classdata = new FileInputStream("D:/TestData/Classes/" + classnum + ".dat");
            ObjectInputStream in = new ObjectInputStream(classdata);

            a = (Classes) in.readObject();   // 一切正常


            classdata.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return a;
    }

     public static void main(String[] args){
         Classes a = new Classes(2009,"softwareE",5);//产生一个不含学生的班级类
         a.addStudent(new Student(10,"Tim",5));
         a.savaClass();
         Classes c = a.loadClass(5);//无参构造函数默认存在，且会对数组初始化，这里没贴上
         System.out.println(c.getClassnum());
         System.out.println(c.getGrade());
         System.out.println(c.getMajor());
         ArrayList<Student> stus=c.getStulist();
         for(Student s:stus){
             System.out.println(s);
         }
               //classes含对象数组时出错，否则正常读出
}

}
class Student implements Serializable {

    private String name;
    private int stuNumber;

    public String getName() {
        return name;
    }

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

    public int getStuNumber() {
        return stuNumber;
    }

    public void setStuNumber(int stuNumber) {
        this.stuNumber = stuNumber;
    }

    public int getClassNum() {
        return classNum;
    }

    public void setClassNum(int classNum) {
        this.classNum = classNum;
    }

    private int classNum;

    public Student(int stuNumber, String name, int classNum) {
        this.name = name;
        this.stuNumber = stuNumber;
        this.classNum = classNum;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", stuNumber=" + stuNumber +
                ", classNum=" + classNum +
                '}';
    }
}