///// Java 用 DatagramSocket 做 UDP 接收/傳送  (connectionless)
magpie:network/> cat -n UDPReceive2.java 
     1	// UDPReceive2.java
     2	// This example is from the book _Java in a Nutshell_ by David Flanagan.
     3	// Written by David Flanagan.  Copyright (c) 1996 O'Reilly & Associates.
     4	// You may study, use, modify, and distribute this example for any purpose.
     5	// This example is provided WITHOUT WARRANTY either expressed or implied.
     6	// Modified by tsaiwn@csie.nctu.edu.tw to fix International problem
     7	
     8	import java.io.*;
     9	import java.net.*;
    10	 // This program waits to receive Datagrams sent to port 6010.  
    11	// When it receives one, it displays the sending host and port, 
    12	 // and prints the contents of the Datagram as a string.
    13	 /// ///  classes used: DatagramSocket, DatagramPacket
    14	public class UDPReceive2 {
    15	    static final int port = 6010;
    16	    public static void main(String args[]) throws Exception
    17	    {
    18	        byte[] buffer = new byte[1600];
    19	        String s;
    20	       // Create a socket to listen on the port. (use receive(packet); )
    21	        DatagramSocket socket = new DatagramSocket(port);
    22	        
    23	        for(;;) { // Create a packet with an empty buffer to receive data
    24	            // Bug workaround: create a new packet each time through the loop.
    25	            // If we create the packet outside of this loop, then it seems to
    26	            // loose track of its buffer size, and incoming packets are 
    27	            // truncated.
    28	            DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
    29	            socket.receive(packet);  // Wait to receive a Datagram
    30	             // Convert the contents to a string; note the Charset problem
    31	            ///s = new String(buffer, 0, 0, packet.getLength()); //deprecated
    32	            //s = new String(buffer, 0, packet.getLength(),"Big5"); //by tsaiwn
    33	            s = new String(buffer, 0, packet.getLength()); // default Charset
    34	             // Note that 網路上流動是 Byte Stream, Not character
    35	            // And display them on the Console
    36	            System.out.println("UDPReceive: received from " + 
    37	                       packet.getAddress().getHostName() + ":" +
    38	                       packet.getPort() + ": " + s);
    39	        } // for
    40	    } // main
    41	}

magpie:network/> cat -n UDPSend2.java
     1	// UDPSend2.java
     2	// This example is from the book _Java in a Nutshell_ by David Flanagan.
     3	// Written by David Flanagan.  Copyright (c) 1996 O'Reilly & Associates.
     4	// You may study, use, modify, and distribute this example for any purpose.
     5	// This example is provided WITHOUT WARRANTY either expressed or implied.
     6	// Modified by tsaiwn@csie.nctu.edu.tw to solve International problem
     7	
     8	import java.io.*;
     9	import java.net.*;
    10	
    11	 // This class sends the specified text as a datagram to port 6010 of the 
    12	// specified host.  Note: use quote "if the message contains special char "
    13	 //  Note:  若訊息(message)有空白或特殊字符請用雙引號"夾住" 
    14	
    15	public class UDPSend2 { 
    16	    static final int port = 6010;
    17	    public static void main(String args[]) throws Exception {
    18	        if (args.length != 2) {
    19	            System.out.println("Usage: java UDPSend host message");
    20	            System.exit(0);
    21	        }
    22		
    23	        // Get the internet address of the specified host
    24	        InetAddress address = InetAddress.getByName(args[0]);
    25		
    26	        // Convert the message to an array of bytes
    27	        int msglen = args[1].length();   // character length, not byte Len
    28	        byte[] message = new byte[msglen+msglen];    // double by tsaiwn
    29	        //args[1].getBytes(0, msglen, message, 0);
    30	        message = args[1].getBytes();  //use Default charset, by tsaiwn@csie
    31		
    32	        // Initilize the packet with data and address
    33	        DatagramPacket packet = new DatagramPacket(message,
    34	                       message.length, address, port);   //good form, tsaiwn
    35	        // Create a socket, and send the packet through it.
    36	        DatagramSocket socket = new DatagramSocket();
    37	        socket.send(packet);   // send it out anyway
    38	    } // main
    39	}