Java client/server socket communication

Question

I wrote this code

 Random rand = new Random();
                int  n = rand.nextInt(50) + 1;
                DataOutputStream dos = new DataOutputStream(_socket.getOutputStream());
                dos.writeInt(n);
                dos.flush();

                DataInputStream in = new DataInputStream(_socket.getInputStream());
                int ServerNumber= in.readInt();
                System.out.println(ServerNumber);

Server code

DataInputStream in = new DataInputStream(socket.getInputStream());
    int ClientNumber= in.readInt();
    System.out.println(ClientNumber);
    System.out.println("----tessst----");

    ClientNumber++;
   DataOutputStream dos = new DataOutputStream(socket.getOutputStream());
   dos.writeInt(ClientNumber);
   dos.flush();

I want to send a number to the Server, increment it and send it again to the client. The first part work fine, i send the number to the server and he can read it, but in the second part i have a NullPointerError, i don't really know where