Сериализация на обекти през сокети

Пример
Класове: Person с два стринга - име и идентификатор, конструктор и преобразуване до стринг
Student наследява Person и добавя масив от оценки, конструктор, преобразуване до стринг
Оценките се извеждат до първа нулева оценка. Двата класа - в отделен пакет - stud

Сървър - в отделен пакет (server)поддържа ArrayList от студенти, които му се изпращат от клиенти от други машини
извежда съобщение за свързан и прекъснал връзката клиент, за получен студент и извежда на
конзола получените до момента студенти.

Клиент - в отделен пакет (client), въвежда студенти и ги изпраща на сървъра. Въвеждането на оценки (между 0 и 6) продължава до
отказ или до въвеждане на оценка 0

Пакет stud

package stud;
import java.io.*;
import javax.swing.*;

class Person implements Serializable{
    private static final long serialVersionUID = 1L;
    String name="";
    String id="";
    public Person(){
        do{
            name = JOptionPane.showInputDialog("Please enter person's name");
        } while((name == null)|| (name.length()==0));

        do{
            id = JOptionPane.showInputDialog("Please enter person' id");
        } while((id == null)|| (id.length()==0));

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

public class Student extends Person{
    private static final long serialVersionUID = 1L;
    int marks[];
    public Student(){
        marks = new int[5];
        for(int i=0;i<marks.length;i++){
            do{
                String s = JOptionPane.showInputDialog("Please enter marks "+(i+1)+" [0 - 6]");
                if(s==null)s="0";       //to terminate input with cancel
                try{
                    marks[i]= Integer.parseInt(s);
                }
                catch(Exception e){marks[i]=7;}
            } while((marks[i]<0)|| (marks[i]>6));
            if(marks[i]==0)break;
        }
    }
    public String toString(){
        String s = super.toString()+" marks: ";
        for(int i=0;i<marks.length;i++){
            if(marks[i]==0) break;
            s+=marks[i]+" ";
        }
        return s;
    }
    public String getName(){
        return this.name;
    }
}

пакет server

package server;
import java.io.*;
import java.net.*;
import java.util.*;
import stud.*;

class Students{
    private ArrayList<Student> st;
    public Students(){
        st = new ArrayList<Student>(10);
    }
    public synchronized void addS(Student p){
        st.add(p);
    }
    public synchronized void rmvS(Student p){
        st.remove(p);
    }
    public synchronized int nSt(){
        return st.size();
    }
    public synchronized void printSt(){
        Iterator<Student> itr = st.iterator();
        System.out.println("---------------------------");
        while(itr.hasNext()) {
            Student p=(Student)itr.next();
            System.out.println(""+p);
        }
        System.out.println(nSt() +" students total\n");
    }
}
//...............................................................
class ServeOneClient extends Thread {
    private static int number=0;
    private int num;
    private Socket socket;
    private BufferedReader in;
    private PrintWriter out;
    private ObjectInputStream iStr;
    private Students clt;
    public ServeOneClient(Socket s,Students clt) throws IOException {
        num= ++number;
        System.out.println("join a new client - with number "+ num);
        socket = s;
        this.clt =clt;
        in = new BufferedReader(
                new InputStreamReader(
                        socket.getInputStream()));
        // Enable auto-flush:
        out = new PrintWriter( new BufferedWriter(
                new OutputStreamWriter(
                        socket.getOutputStream()
                        )
                ),
                true);
        iStr = new ObjectInputStream(socket.getInputStream());

        // If any of the above calls throw an
        // exception, the caller is responsible for
        // closing the socket. Otherwise the thread
        // will close it.

        start();    // Calls run()
    }
    public void run() {
        try {
            while (true) {
                System.out.println("waiting for new student from client "+num);
                Student s=null;
                try{
                    s = (Student)iStr.readObject();
                }
                catch(ClassNotFoundException exc){}
                if (s == null) break;
                System.out.println("receive student from client "+num+"\t"+s);
                clt.addS(s);
                clt.printSt();
            }

        } catch (IOException e) { }
        finally {
            try {
                System.out.println("disconecting client "+num);
                socket.close();
            } catch(IOException e) {}
        }
    }
}

//................................................................................
public class StServer {
    static final int PORT = 9595;
    public static void main(String[] args) throws IOException {
        ServerSocket s = new ServerSocket(PORT);
        System.out.println("Server Started");
        Students clt = new Students();
        try {
            while(true) {
                // Blocks until a connection occurs:
                Socket socket = s.accept();
                try {
                    new ServeOneClient(socket,clt);

                } catch(IOException e) {
                    // If it fails, close the socket,
                    // otherwise the thread will close it:
                    socket.close();
                }
            }
        } finally {
            s.close();
        }
    }
}

пакет client

package client;

import java.net.*;
import java.io.*;
import javax.swing.*;
import stud.*;

public class Client {
    BufferedReader in;
    PrintWriter out;
    ObjectOutputStream oStr;
    InetAddress addr;
    Socket socket;
    String name;
    Client(){
        try{
            String server = null;
            InetAddress addr = InetAddress.getByName(server);
            System.out.println("addr = " + addr);
            socket = new Socket(addr, 9595);
            System.out.println("socket = " + socket);
            in = new BufferedReader(
                    new InputStreamReader(socket.getInputStream()));

            out = new PrintWriter(new BufferedWriter(
                    new OutputStreamWriter(socket.getOutputStream())),true);
            oStr = new ObjectOutputStream(socket.getOutputStream());

        }catch (Exception e){
            System.out.println("exception: "+e);
            System.out.println("closing...");
            try{
                socket.close();
            }catch (Exception e2){
                System.out.println("no server running");
                System.exit(5);
            }
        }

    }
    void send(Student s) throws IOException{
        oStr.writeObject(s);
    }
    public static void main(String[] args )throws IOException{
        Client cl= new Client();
        for(;;){
            if(JOptionPane.showConfirmDialog(null, "do you want to introduce a new student?")!=0)break;
            Student st = new Student();
            cl.send(st);
            JOptionPane.showMessageDialog(null,st+" is send");
        }
    }

}