add UDP apps

UDP/UDPReceiver.java

+import java.io.*;
+import java.net.*;
+class UDPReceiver {
+    public static void main(String[] args) {
+        try {
+            DatagramSocket socket = new DatagramSocket(4321);
+            byte[] buf = new byte[256];
+            for (int i = 0; i < 10; i++) {
+                DatagramPacket packet = new DatagramPacket(buf, buf.length);
+                socket.receive(packet);
+                System.out.println("receive DatagramPacket " + (new String(packet.getData())).trim() + " " + packet.getAddress() + ":" + packet.getPort());
+            }
+        } catch (Exception e) {
+            System.out.println("error " + e);
+        }
+    }
+}
\ No newline at end of file

UDP/UDPSender.java

+import java.io.*;
+import java.net.*;
+
+class UDPSender{
+  public static void main(String [] args){
+    try{
+      InetAddress address =InetAddress.getByName("isaac-VirtualBox");
+      DatagramSocket socket = new DatagramSocket();
+      for (int i=0;i<10;i++) {
+        byte[] buf = String.valueOf(i).getBytes();
+        DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 4321);
+        socket.send(packet);
+        System.out.println("send DatagramPacket " 
+        	              + new String(packet.getData()) 
+        	              + " "
+        	              + packet.getAddress() 
+        	              + ":"
+        	              + packet.getPort());
+        Thread.sleep(2000);
+        }
+    } catch(Exception e) {
+	  System.out.println("error");
+    }
+  }
+}