Java UDP Client Server Program Not Working

I am currently writing two java classes (client and server). The client takes an input number form keyboard and sends it to the server. The server then multiplies this number by two and sends it back to the client. The numbers should also be printed to screen along the way, for example if I input the number 3 I should get

"From Client: 3" "From Server: 6"

They should continuously do this unless a negative number is received by the client, say for example the number -3 is sent to the server and it returns -6.

The code I have for the two classes so far is:

import java.io.*;
import java.net.*;
import java.nio.ByteBuffer;
import java.util.Scanner;
class Client {
public static void main(String args[]) throws Exception {
DatagramSocket clientSocket = new DatagramSocket();

[Code] .....

Currently, when I run the program all I get is an output of the number first entered. I am aware it requires a loop but I don't know where and what the condition should be.

Also if I wanted to adapt this so that it would take the integer from client and subtract two at the server and return to client who sends back to server to keep subtracting two unless it reaches a negative number at which point the client will terminate the program - how might I do this.

I do realise there needs to be a while loop in the above code, but I wanted to test it sent the number from client to server and its not doing it. All I get is a print screen of 'enter number' and then the number I enter.