Java Client-Server error

Question

I've made one server and client to send data one between another but that don't work and I don't know why...

A little earlier I have that error message from server:

java.net.SocketException: Broken pipe

and now I have null pointer exception from client on line 23:

java.lang.NullPointerException

at chat.Client.sendData(Client.java:23)

Here it's my code:

Server

public class Server {

    private ServerSocket server;
    public Socket socket;

    public Server() {
        try {
            this.server = new ServerSocket(Port_and_address.PORT);
            this.socket = server.accept();
        } catch (IOException ex) {
            Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public String reciveData() throws IOException, ClassNotFoundException {
        ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
        String result = (String) ois.readObject();
        ois.close();
        return result;
    }

    public void writeData(String message) throws IOException {
        ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
        oos.writeObject(message);
        oos.close();
    }

    public void closeSocket() throws IOException {
        socket.close();
    }

    public static void main(String args[]) throws IOException, ClassNotFoundException {
        while (true) {
            System.out.println("Waiting for client request");
            Server server = new Server();
            System.out.println(server.reciveData());
            server.writeData("TEST SERVER");
            server.closeSocket();
        }
    }
}

Client

public class Client extends Thread {

    Socket socket;

    public void Client() {
        try {
            this.socket = new Socket("localhost", Port_and_address.PORT);
        } catch (IOException ex) {
            Logger.getLogger(Client.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public void sendData(String message) throws IOException {
        ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
        oos.writeObject(message);
    }

    public String reciveData() throws IOException, ClassNotFoundException {
        ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
        ois.close();
        return (String) ois.readObject();
    }

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        Client client = new Client();
        client.sendData("TEST CLIENT");
        System.out.println(client.reciveData());
    }
}

From some reason the that line give null pointer exception:

    ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());

So if anyone of you guys knows what I've do wrong here please tell me. At the end I want to have one server how can receive any message from any client and response on it, theoretical I can have a infinite number of clients how can send messages.

Thx for help :)