package apibook.c5.s1;import java.net.*;import java.io.*;//測試DatagramPacket類public class TestDatagramPacket {  public TestDatagramPacket() {  }  public static String dgExchange(String msg, InetAddress dst, int port) {      byte[] outbuf = msg.getBytes();      byte[] inbuf = new byte[256];  // default size      try {          //發送datagram          DatagramPacket request =              new DatagramPacket(outbuf, outbuf.length, dst, port);          DatagramSocket sock = new DatagramSocket();          sock.send(request);          //等待回應          DatagramPacket reply = new DatagramPacket(inbuf, inbuf.length);          sock.receive(reply);          System.out.println(              "Received packet from:" + reply.getAddress() +              " port: " + reply.getPort() +              " length: " + reply.getLength());          sock.close();          return (new String(reply.getData()));      } catch (SocketException e) {          e.printStackTrace();      } catch (IOException e) {          e.printStackTrace();      }      return (null);  }  public static void main(String[] args) {    try {        String msg = "\n";        int port = 2121;        InetAddress dst = InetAddress.getLocalHost();        if (args.length > 0) {            port = Integer.parseInt(args[0]);            if (args.length >= 2)                msg = args[1];            if (args.length == 3)                dst = InetAddress.getByName(args[2]);        }        System.out.println(dgExchange(msg, dst, port));    } catch (UnknownHostException e) {        e.printStackTrace();    }  }}