Serialization - readObject writeObject overrides

Having written the code below, I now have to use custom readObject() and writeObject() methods in StudentData to read and write the object's variables without using the defaultWriteObject or defaultReadObject methods to do this.

I'm having trouble understanding what I am being asked to do. I have read Uses of readObject/writeObject in Serialization but I can't get my head around it. Can someone point me in the right direction?

My code:

import java.io.*; //importing input-output files

class Student implements java.io.Serializable {

    String name; // declaration of variables
    String DOB;
    int id;

    Student(String naam, int idno, String dob) // Initialising variables to user
                                                // data
    {
        name = naam;
        id = idno;
        DOB = dob;
    }

    public String toString() {
        return name + "\t" + id + "\t" + DOB + "\t";
    }

}

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

class StudentData                     //main class
{
    public static void main(String args[]) throws IOException                  //exception handling
    {
        System.out.println("Enter the numbers of students:");
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(in.readLine());

        Student[]  students = new Student[n];

        //Student[]  S=new Student[n];                      // array of objects declared and defined
        for (int i = 0; i < students.length; i++) {
            System.out.println("Enter the Details of Student no: " + (i + 1));             //reading data form the user
            System.out.println("Name: ");
            String naam = in.readLine();
            System.out.println("ID no: ");
            int idno = Integer.parseInt(in.readLine());
            System.out.println("DOB: ");               
            String dob = (in.readLine());

            students[i] = new Student(naam, idno, dob);                          

            File studentFile = new File("StudentData.txt");
            try {
                FileOutputStream fileOutput = new FileOutputStream(studentFile);
                ObjectOutputStream objectOutput = new ObjectOutputStream(fileOutput);
                objectOutput.writeObject(students);

                students = null;

                FileInputStream fileInput = new FileInputStream(studentFile);
                ObjectInputStream objectInputStream = new ObjectInputStream(fileInput);

                students = (Student[]) objectInputStream.readObject();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }

            for (Student student : students) {
                System.out.println(student);
            }
        }
    }
}

You have to do it like this:

import java.io.IOException;

class Student implements java.io.Serializable {

    String name;
    String DOB;
    int id;

    Student(String naam, int idno, String dob) {
        name = naam;
        id = idno;
        DOB = dob;
    }

    private void writeObject(java.io.ObjectOutputStream stream)
            throws IOException {
        stream.writeObject(name);
        stream.writeInt(id);
        stream.writeObject(DOB);
    }

    private void readObject(java.io.ObjectInputStream stream)
            throws IOException, ClassNotFoundException {
        name = (String) stream.readObject();
        id = stream.readInt();
        DOB = (String) stream.readObject();
    }

    public String toString() {
        return name + "\t" + id + "\t" + DOB + "\t";
    }

}

The readObject is invoked just after creating an instance of Student (bypassing the normal constructor).