UPD聊天程序.

import java.awt.BorderLayout;
import java.awt.Frame;
import java.awt.List;
import java.awt.Panel;
import java.awt.TextField;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class Message {

/**
* @param args
*/

DatagramSocket ds = null;

public Message(){
try{
ds = new DatagramSocket(3000);
}catch(Exception e){
e.printStackTrace();
}
}

public static void main(String[] args) {

final Message m = new Message();
//构造界面
final Frame f = new Frame();
final List l = new List(6);
final TextField tfip = new TextField(15);
final TextField tfmeg = new TextField(20);
f.add(l,"Center");
Panel p = new Panel();
f.add(p,"South");
p.setLayout(new BorderLayout());
p.add(tfip,"West");
p.add(tfmeg,"East");
f.setSize(300, 400);
f.setResizable(false);
f.setVisible(true);

//分出一个线程,单独执行接收方法.因为receive方法能够阻碍程序,只有接收到程序才能继续运行.
new Thread(new Runnable(){
public void run() {
//初始化接受的内存缓冲区
byte[] buf = new byte[1024];
DatagramPacket dp = new DatagramPacket(buf,1024);
while(true){
try {
m.ds.receive(dp);
String s = new String(dp.getData(),0,dp.getLength());
s += "--"+dp.getAddress().getHostAddress();
l.add(s,0);
} catch (IOException e) {
//如果Socket已经关闭,则不打印异常.
if(m.ds.isClosed()){

}else{
e.printStackTrace();
}

}
}

}

}).start();


//关闭界面监听
f.addWindowListener( new WindowAdapter(){
public void windowClosing(WindowEvent e){
m.ds.close();
f.dispose();
System.exit(0);
}
});


//按回城键发送数据.
tfmeg.addActionListener( new ActionListener(){
public void actionPerformed(ActionEvent e) {
String ip = tfip.getText();
String message = tfmeg.getText();
try {
DatagramPacket dp = new DatagramPacket(message.getBytes(),message.getBytes().length,InetAddress.getByName(ip),3000);
m.ds.send(dp);
tfmeg.setText("");
} catch (Exception e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
}

});
}

}

//在ip地址栏输入:127.0.0.1 ,可以接收到信息,因为127.0.0.1是本地ip,不经过网卡.

//输入本地ip可以也接收到信息.

//输入广播地址的ip,也可以收到信息.

//关于广播地址请看

http://baike.baidu.com/view/473043.htm