通过实例可以更好的了解java

        { Socket client=listen.accept();
           firstthread f=new firstthread(this, client);
           f.setPriority(Thread.MIN_PRIORITY);f.start();
          connections.addElement(f);
          }     
  } catch (IOException e)
  {System.err.println("Erro:"+e);
  System.exit(1);
 }
}

public static void main(String[] args)
{new chatserverthree();
}
public void broadcast(String msg)
{int i;firstthread you;
for (i=0; i<connections.size(); i++)
{you = (firstthread) connections.elementAt(i);
try{you.out.writeUTF(msg);} catch(IOException e) {}
 }
  }
      public void broadcast1(String msg)
{int i;String s1,s2,s3;
firstthread you; s1=new String("PEOPLE");s2=new String(msg.substring(4));
 s3=s1.concat(s2);
for (i=0; i<connections.size(); i++)
{you = (firstthread) connections.elementAt(i);
if(s3.startsWith(you.name))
 {try{you.out.writeUTF(msg);} catch(IOException e) {}}
  }
}
}
class firstthread extends Thread
{protected Socket client;String line,name;int k=0;
 protected DataOutputStream firstout,out;
 protected chatserverthree server;
  protected DataInputStream in;
public firstthread(chatserverthree server, Socket client)
{this.server=server; this.client=client;
try{in = new DataInputStream(client.getInputStream());
out = new DataOutputStream(client.getOutputStream());
firstout = new DataOutputStream(client.getOutputStream());
} 
catch (IOException e)
{try {server.connections.removeElement(this);
client.close(); } catch (IOException e2) 
{System.err.println("有问题哦:" + e);
 return;}
}
 if(this.client==null)  
 { server.broadcast("QUIT"+this.name); this.name=null;}
}
public void run()
 {  try
    {for(int i=0; i<server.connections.size(); i++)
       { firstthread c=(firstthread) (server.connections.elementAt(i));
           if(c.name!=null) 
          { try{firstout.writeUTF(c.name);}
            catch (IOException e){}
        } 
       } 
      }
   catch(ArrayIndexOutOfBoundsException e) {}
   catch(NullPointerException e) {}
    try {  while(true)
          {line=in.readUTF();
            if(line.startsWith("PEOPLE"))
{ try{
      { firstthread   
d=(firstthread)(
server.connections.elementAt(server.connections.indexOf(this)));
               if(d.name==null)
                { d.name=line; 
                 }
               else if(d.name!=null)
               { server.broadcast("QUIT"+this.name);
                d.name=line;      
             }
            }
           }
 catch(ArrayIndexOutOfBoundsException e) {}
 catch(NullPointerException e) {}
 finally {server.broadcast(line);}
  }
 else  if(line.startsWith("MSG"))
{server.broadcast(line);}
  else if(line.startsWith("QUIT"))
   {  server.broadcast("QUIT"+this.name); 
      server.connections.removeElement(this);
       this.client.close();this.yield();
    }  
else if(line.startsWith("BADQUIT"))
{  server.broadcast("MSG"+(this.name).substring(6)+"被踢出去了"); 
   server.broadcast("QUIT"+this.name);
  server.connections.removeElement(this);
  this.client.close();this.yield();
  } 
 else  if(line.startsWith("newlist"))
{ try{ for(int i=0; i<server.connections.size(); i++)
        {firstthread c=(firstthread) (server.connections.elementAt(i));
         if(c.name!=null)
         {  try{firstout.writeUTF(c.name);}
            catch (IOException e){}
          } 
   } 
  }
 catch(ArrayIndexOutOfBoundsException e) {}
catch(NullPointerException e) {}
}         
else  if(line.startsWith("悄悄地对"))
{ this.out.writeUTF(line+"*");
server.broadcast1(line);
 }
else  if(line.startsWith("坏悄悄地对"))
 { this.out.writeUTF(line.substring(1)+"*");
 server.broadcast(line.substring(1));
  }
 }
}
catch (IOException e){ server.connections.removeElement(this);}
 catch(NullPointerException e) {server.connections.removeElement(this);}
 }
}



21-例子8

import java.net.*; 
public class DomainName
{ public static void main(String args[])
  { try{InetAddress address_1=InetAddress.getByName("www.sina.com.cn");
        System.out.println(address_1.toString()); 
        InetAddress address_2=InetAddress.getByName("166.111.222.3");
        System.out.println(address_2.toString());
       }
    catch(UnknownHostException e)
       {System.out.println("无法找到 www.sina.com.cn");
       } 
  }
}




21-例子9   

import java.net.*; 
public class DomainName
{ public static void main(String args[])
  { try{InetAddress address=InetAddress.getByName("www.yahoo.com");
        String domain_name=address.getHostName();//获取 address所含的域名。
        String IP_name=address.getHostAddress(); //获取 address所含的IP地址。
        System.out.println(domain_name);        
        System.out.println(IP_name);
        }
    catch(UnknownHostException e)
       {System.out.println("无法找到 www.yahoo.com");
       } 
  }
}
运行结果：
www.yahoo.com
64.58.76.227




21-例子10
import java.net.*; 
public class DomainName
{ public static void main(String args[])
  { try{InetAddress address=InetAddress.getLocalHost();
        String domain_name=address.getHostName();//获取 address所含的域名。
        String IP_name=address.getHostAddress();//获取 address所含的IP地址。
        System.out.println(domain_name);        
        System.out.println(IP_name);
        }
    catch(UnknownHostException e)
       {
       } 
  }
}



21-例子11
(1)主机1：
import java.net.*;import java.awt.*; import java.awt.event.*;
 class Shanghai_Frame extends Frame implements Runnable,ActionListener
{ TextField out_message=new TextField("发送数据到北京：");
  TextArea in_message=new TextArea(); 
  Button b=new Button("发送数据包到北京");
  byte data[]=new byte[8192];
  DatagramPacket pack=null;
 Shanghai_Frame()
 { super("我是上海");
   setSize(200,200);
   setVisible(true);
   b.addActionListener(this);
   add(out_message,"South");
   add(in_message,"Center");
   add(b,"North");
   pack=new DatagramPacket(data,data.length);
   Thread thread=new Thread(this);
   thread.start();//线程负责接收数据包
 }

 //点击按扭发送数据包：
 public void actionPerformed(ActionEvent event)
 {byte buffer[]=out_message.getText().trim().getBytes();
   try{InetAddress address=InetAddress.getByName("localhost");
        //数据包的目标端口是888(那么收方(北京)需在这个端口接收):
DatagramPacket data_pack=
new DatagramPacket(buffer,buffer.length, address,888);
       DatagramSocket mail_data=new DatagramSocket();
       in_message.append("数据报目标主机地址:"+data_pack.getAddress()+"\n");
       in_message.append("数据报目标端口是:"+data_pack.getPort()+"\n");
       in_message.append("数据报长度:"+data_pack.getLength()+"\n");
       mail_data.send(data_pack);
      }
   catch(Exception e){}     
 } 
  //接收数据包：
 public void run()
 {DatagramSocket mail_data=null;
  try{//使用端口666来接收数据包(因为北京发来的数据报的目标端口是666)。
      mail_data=new DatagramSocket(666);
     }
  catch(Exception e){} 
  while(true)   
    { if(mail_data==null) break;
      else
      try{mail_data.receive(pack); 
          int length=pack.getLength(); //获取收到的数据的实际长度。
          InetAddress adress=pack.getAddress();//获取收到的数据包的始发地址。
          int port=pack.getPort();//获取收到的数据包的始发端口。
          String message=new String(pack.getData(),0,length);//获取收到的
//数据包中的数据。
            in_message.append("收到数据长度："+length+"\n");
            in_message.append("收到数据来自："+adress+"端口："+port+"\n");
            in_message.append("收到数据是："+message+"\n");
          
         }
      catch(Exception e){}
    } 
 }
}
public class Shanghai
{public static void main(String args[])
 { Shanghai_Frame shanghai_win=new Shanghai_Frame();
   shanghai_win.addWindowListener(new WindowAdapter()
     {public void windowClosing(WindowEvent e)
      {System.exit(0);
      }
     });
   shanghai_win.pack();
 }
} 

(2)主机2：
import java.net.*;import java.awt.*; import java.awt.event.*;
 class Beijing_Frame extends Frame implements Runnable,ActionListener
{ TextField out_message=new TextField("发送数据到上海：");
  TextArea in_message=new TextArea(); 
  Button b=new Button("发送数据包到上海");
  byte data[]=new byte[8192];
  DatagramPacket pack=null;
 Beijing_Frame()
 { super("我是北京"); 
   setSize(200,200);
   setVisible(true);
   b.addActionListener(this);
   add(out_message,"South");
   add(in_message,"Center");
   add(b,"North");
   pack=new DatagramPacket(data,data.length);//用来收取数据的数据包。
   Thread thread=new Thread(this);
    thread.start();//线程负责接收数据包
 }

//点击按扭发送数据包：
 public void actionPerformed(ActionEvent event)
 {byte buffer[]=out_message.getText().trim().getBytes();
   try{InetAddress address=InetAddress.getByName("localhost");
        //数据包的目标端口是666(那么收方(上海)需在这个端口接收):
DatagramPacket data_pack=
new DatagramPacket(buffer,buffer.length, address,666);
       DatagramSocket mail_data=new DatagramSocket();
       in_message.append("数据报目标主机地址:"+data_pack.getAddress()+"\n");
       in_message.append("数据报目标端口是:"+data_pack.getPort()+"\n");
       in_message.append("数据报长度:"+data_pack.getLength()+"\n");
       mail_data.send(data_pack);
      }
   catch(Exception e){}   
 } 
public void run()
 {DatagramSocket mail_data=null;
  try{//使用端口888来接收数据包(因为上海发来的数据报的目标端口是888)。
      mail_data=new DatagramSocket(888);
     }
  catch(Exception e){} 
  while(true)   
    { if(mail_data==null) break;
      else
      try{  mail_data.receive(pack); 
            int length=pack.getLength(); //获取收到的数据的实际长度。
            InetAddress adress=pack.getAddress();//获取收到的数据包的始发地址。
            int port=pack.getPort();//获取收到的数据包的始发端口。
String message=new String(pack.getData(),0,length);//获取收到
//的数据包中的数据。
            in_message.append("收到数据长度："+length+"\n");
            in_message.append("收到数据来自："+adress+"端口："+port+"\n");
            in_message.append("收到数据是："+message+"\n");
          
         }
      catch(Exception e){}
    }
 }
}
public class Beijing
{public static void main(String args[])
 {Beijing_Frame beijing_win=new Beijing_Frame();
   beijing_win.addWindowListener(new WindowAdapter()
     {public void windowClosing(WindowEvent e)
      {System.exit(0);
      }
     });
   beijing_win.pack();
 }
}