Error with writeobject and String in java

I have a client and server which I exchange messages and objects between.
My problem is when I send object with the help of toString and a string together. How do I pick out the object from the string and add the object to a list ; I wrote the code.

Server code:

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

public class Server  {

public static void main(String[] args) {
    ArrayList <PhoneBook> listPhone=new ArrayList<PhoneBook>(); 

    PhoneBook a = new PhoneBook("a","a","","",1);
    listPhone.add(a);
    PhoneBook pB = new PhoneBook();
    String ob;
    try{

        ServerSocket server = new ServerSocket(5555,50);

        System.out.println("Waiting Incoming Connection...");
        System.out.println("Local Address :"+server.getInetAddress()+" Port :"+server.getLocalPort());
        Socket sock = server.accept();

        ObjectOutputStream out =new ObjectOutputStream(sock.getOutputStream()); 
        ObjectInputStream  in =new ObjectInputStream(sock.getInputStream()); 

        String strin =(String) in.readObject();

        if (strin.equals("START")){ 
        out.writeObject("WAITING");
        out.flush();}
        strin =(String) in.readObject();
        String[] str=strin.split("\n");
        if(str[0].equals("REQUEST_SEARCH")){

               try{
                    // in this line is error
                   pB = (PhoneBook)in.readObject();  //String cannot be cast to PhoneBook
                   out.flush();
        }catch(ClassNotFoundException classnot){
          System.err.println("Data received in unknown format");}
               out.writeObject("RECORSDS"); 
               out.flush(); 
               String sName = pB.getsurName(); 
               for(int i=0;i<listPhone.size();i++) 
              if(listPhone.get(i).getsurName().equals(sName)){ 
              out.writeObject(pB.toString()); 
              out.flush(); 

        }else{
        out.writeObject("NXRECORD");
        out.flush();
        }}
        strin =(String) in.readObject();
        String[] st=strin.split("\n");

        if(st[0].equals("REQUEST_INSERT")){

            listPhone.add(pB); 
            System.out.println("The contact is add");
            out.writeObject("OK");
            out.flush();
        }
        out.flush();

        if(strin.equals("END")){ //bye = terminate the conversation

        in.close();
        out.close();
        sock.close();
        System.out.println("Connection Closing...");}
        }
        catch (Exception ex){
        System.out.println("Error during I/O");
        ex.getMessage();
        ex.printStackTrace();
        } } }

Client code:

     if (strin.equals("WAITING")) {
        System.out.println("The server says: " + strin);
        // out.writeObject("REQUEST_SEARCH\n");
        // out.flush();
        System.out.println("The server says: " + strin);
        System.out.print("Write the contact elements ");
        System.out.print("Write the name: ");
        String name = input.nextLine();
        System.out.print("Write the surname: ");
        String surName = input.nextLine();
        System.out.print("Write the job: ");
        String job = input.nextLine();
        System.out.print("Write the street: ");
        String street = input.nextLine();
        System.out.print("Write the phone number: ");
        int number = input.nextInt();
        PhoneBook p = new PhoneBook(name, surName, job, street, number);
        out.writeObject("REQUEST_SEARCH\n" + p.toString());
        out.flush();
    }

The error:

Connection reset
at java.net.SocketInputStream.read(Unknown Source)  at
java.net.SocketInputStream.read(Unknown Source)     at
java.net.SocketInputStream.read(Unknown Source)     at
java.io.ObjectInputStream$PeekInputStream.peek(Unknown Source)  at
java.io.ObjectInputStream$BlockDataInputStream.peek(Unknown Source)
    at java.io.ObjectInputStream$BlockDataInputStream.peekByte(Unknown Source) at 
       java.io.ObjectInputStream.readObject0(Unknown Source)    at
       java.io.ObjectInputStream.readObject(Unknown Source)     at Server.main

Application background:

This program is a PhooneBook with contacts (the server) and the client send first a START and the server take it and send WAITING after the client make a object phonebook which include name, surname, job, street and phone and sends it with the message REQUEST_SEARCH the server take the message and object (the contact) and search in array list with lastname if exist a contact, if exist send back the object and show the name, surname, job and other with toString() and message RECORDS then client send OK and server send back END and connection is closing if the contact is not exist the server send the message NXRECORD the client sent back a message REQUEST_INSERT the server take it and add the object in arraylist and sent OK And client send back END and closing connection.