Chcę uzyskać adres MAC komputera .. ale poniżej napisany kod wyświetla tylko adres MAC, gdy Internet jest podłączony do mojego komputera, inny zwróci wartość null ... Używam Windows 7

import java.net.InetAddress;
import java.net.NetworkInterface;
import java.net.SocketException;
import java.net.UnknownHostException;

class test
{
    public static void main(String[] args)
    {
        InetAddress ip;
        try {
            ip = InetAddress.getLocalHost();

            System.out.println("The mac Address of this machine is :" + ip.getHostAddress());

            NetworkInterface network = NetworkInterface.getByInetAddress(ip);

            byte[] mac = network.getHardwareAddress();

            System.out.print("The mac address is : ");

            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < mac.length; i++){
                sb.append(String.format("%02X%s", mac[i],(i< mac.length - 1)?"-":""));
            }

            System.out.println(sb.toString());

        } 
        catch (UnknownHostException e) {
            e.printStackTrace();
        } 
        catch (SocketException e) {
            e.printStackTrace();
        }
    }
}