ObjectOutputStream - NotSerializableException???

this.oOutputStream.writeObject(sendOutputStreamObject);

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.InetAddress;
import java.net.Socket;

public class CHClient{

    private String serverName;
    private final int PORT = 8888;

    private ObjectInputStream oInputStream;
    private ObjectOutputStream oOutputStream;

    public CHClient(String serverName) throws IOException {

        this.serverName = serverName;

        Object[] inputStreamObject = null;
        String oInputStreamParam1;
        Object[] outputStreamObject = { this.serverName };

        InetAddress addr = InetAddress.getByName(null);
        System.out.println("addr = " + addr);
        Socket socket = new Socket(addr, PORT);
        try {
            System.out.println("socket = " + socket);

                this.oOutputStream = new ObjectOutputStream(socket.getOutputStream());
                this.oOutputStream.writeObject(outputStreamObject);

                this.oInputStream = new ObjectInputStream(socket.getInputStream());

                inputStreamObject = (Object[]) this.oInputStream.readObject();

                oInputStreamParam1 = inputStreamObject[0].toString();

        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } finally {
            System.out.println("closing...");
            socket.close();
        }
    }
}

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;

public class ChessServer {
    private static final int PORT = 8888;
    private Socket socket;

    private ServerSocket serverSocket = null;
    private ArrayList<String> servers = new ArrayList<String>();
    private boolean isServerExist;

    private ObjectInputStream oInputStream;
    private ObjectOutputStream oOutputStream;

    public static void main(String[] args) throws IOException {

        new ChessServer();
    }

    public ChessServer() {
        Object[] inputStreamObject;
        Object outputStreamServerNameObj = null;
        Object outputStreamChMultiServObj = null;
        String oInputStreamParam1;

        System.out.println("Server Started");

        try {
            serverSocket = new ServerSocket(PORT);
            do {
                this.socket = serverSocket.accept();
                this.oInputStream = new ObjectInputStream(socket.getInputStream());

                inputStreamObject = (Object[]) this.oInputStream.readObject();
                this.oOutputStream = new ObjectOutputStream(socket.getOutputStream());
                oInputStreamParam1 = inputStreamObject[0].toString();

                isServerExist = isServerExist(oInputStreamParam1);

                if (!isServerExist) {
                    ChessMultiServerThread serverOne = new ChessMultiServerThread(socket);
                    outputStreamServerNameObj = new String("newServer");
                    outputStreamChMultiServObj = serverOne;
                } else {
                    outputStreamServerNameObj = new String("serverExist");
                }

                Object[] sendOutputStreamObject = { outputStreamServerNameObj, outputStreamChMultiServObj };
                this.oOutputStream.writeObject(sendOutputStreamObject);

            } while (true);

        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } finally {
            try {
                this.serverSocket.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    private boolean isServerExist(String serverName) {

        boolean isExist = this.servers.contains(serverName);
        System.out.println("isExist: " + isExist);
        System.out.println("serverName: " + serverName);
        if (isExist) {
            return true;
        } else {
            this.servers.add(serverName);
            return false;
        }
    }
}

import java.io.*;
import java.net.Socket;

public class ChessMultiServerThread extends Thread{

    private Socket socket;
    private BufferedReader in;
    private PrintWriter out;


    public ChessMultiServerThread(Socket s) throws IOException {
        socket = s;
        in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())), true);
        start();
    }

    @Override
    public void run() {
        try {
            // while (true) {
            String str = in.readLine();
            // System.out.println("Echoing: " + str);
            out.println(str);
            // }
        } catch (IOException e) {
            System.err.println("IO Exception");
        } finally {
            try {
                socket.close();
            } catch (IOException e) {
                System.err.println("Socket not closed");
            }
        }
    }

}