import java.io.BufferedOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;

public class Main {
    static DataOutputStream cout(Socket s) throws IOException {
        // replace those lines make the run very slow.
        //return new DataOutputStream( (s.getOutputStream()));
        return new DataOutputStream(new BufferedOutputStream(s.getOutputStream()));
    }
    static DataInputStream cin(Socket s) throws IOException {
        return new DataInputStream( (s.getInputStream()));
        //return new DataInputStream(new BufferedInputStream(s.getInputStream()));
    }
    public static void main(String[] args) throws IOException, InterruptedException {
        ServerSocket server = new ServerSocket(12345);
        new Thread(() -> {
            try {
                Thread.sleep(1000);
                Socket client = new Socket("127.0.0.1", 12345);
                DataOutputStream out = cout(client);
                DataInputStream in = cin(client);
                while (true) {
                    int a = in.readInt();
                    out.writeInt(a);
                    out.flush();
                }
            } catch (IOException | InterruptedException e) {
                e.printStackTrace();
            }
        }).start();
        Socket client=server.accept();
        DataOutputStream out = cout(client);
        DataInputStream in = cin(client);
        for (int i=0;i<10000;i++){
            out.writeInt(i);
            out.flush();
            if (i!=in.readInt()){
                System.out.println("ERROR");
            }
        }
        client.close();
        server.close();
        Thread.sleep(1000);
    }
}