public class First extends Activity {
Button b;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    b = (Button) findViewById(R.id.button1);
    b.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            Thread t = new Thread(new Second());
            t.start();
        }
    });

public class Second implements Runnable {
Second()
{
    run();
}
public void run() {
    // TODO Auto-generated method stub
    try {

        String messageStr = "Hello Android!";
        int server_port = 9876;
        DatagramSocket s = new DatagramSocket();
        InetAddress local = InetAddress.getByName("127.0.0.1");
        int msg_length = messageStr.length();
        byte[] message = messageStr.getBytes();
        DatagramPacket p = new DatagramPacket(message, msg_length, local,
                server_port);
        s.send(p);
    } catch (Exception e) {

    }
}
}

public static void main(String args[]) throws Exception
    {
            DatagramSocket serverSocket = new DatagramSocket(9876);
            byte[] receiveData = new byte[1024];
            byte[] sendData = new byte[1024];
            while(true)
            {
                    DatagramPacket receivePacket = new     DatagramPacket(receiveData,receiveData.length);
                    serverSocket.receive(receivePacket);
                    String sentence = new String(receivePacket.getData(),0,receivePacket.getLength());
                    InetAddress IPAddress = receivePacket.getAddress();
                    int port = receivePacket.getPort();
                    System.out.println("MESSAGE RECEIVED  "+sentence+"  "+IPAddress+"         "+port);
            }
     }