import java.net.*;
import java.io.*;
import java.awt.*;
import java.awt.event.*;
public class Frame_Server extends Frame
{
    static TextArea t_1=new TextArea();
    public static void main(String args[])
    {
    //    System.out.println(server.in);
        Frame_Server fs=new Frame_Server("Frame_Server");
        fs.setSize(400,300);
        WindowClosing WC=new WindowClosing();
        fs.addWindowListener(WC);
        fs.panel();
        fs.setVisible(true);
        server.servers();
        //

    }
    public Frame_Server(String str)
    {
        super(str);
    }
    public void panel()
    {
        Panel p_1=new Panel();
        Label l_1=new Label("对方信息：");
        p_1.add("North",l_1);
        p_1.setBackground(Color.pink);
        add("North",p_1);
        t_1.setEditable(false);
        add("Center",t_1);
    }
}

class WindowClosing extends WindowAdapter
{
    public void windowClosing(WindowEvent e)
    {
        System.exit(1);
    }
}



class server extends Thread
{
    OutputStream os=null;
    InputStream is=null;
    static byte[] buf=new byte[100];
    static String in="haha";
    static int len=0;
    private Socket s;
    public server(Socket s)
    {
        this.s=s;
    }
    public void run()
    {
        try
        {
            os=s.getOutputStream();
            is=s.getInputStream();
            os.write("消息发送成功".getBytes());
            len=is.read(buf);
            System.out.println(new String(buf,0,len));
            in="hehe";
            Frame_Server.t_1.append(new String(server.in));
            Frame_Server.t_1.append(new String(buf,0,len));
            is.close();
            os.close();
            s.close();
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }
    public static void servers()
    {
        try
        {
            ServerSocket ss=new ServerSocket(6000);
            while(true)
            {
                Socket s=ss.accept();
                char[] buf=new char[100];
                new server(s).start();
                System.out.println("IP is:"+s.getInetAddress()+s.getPort());
                String s_3=new String(buf,0,30);
                System.out.println(s_3);
                String s_s="hello you !";
                Frame_Server.t_1.append(s_3);//new String(s_3));
            }
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }
}

------解决方案--------------------
如果只是想通过JAVA 程序来判断两台机器是否连通的话，可以用下面的测试程序，写的不太好，但可以达到你的目的。  

Frame_Server.java  
Java code
import java.net.*;
import java.io.*;
import java.awt.*;
import java.awt.event.*;
public class Frame_Server extends Frame
{
   static TextArea t_1=new TextArea();
   public static void main(String args[])
   {
   //    System.out.println(server.in);
       Frame_Server fs=new Frame_Server("Frame_Server");
       fs.setSize(400,300);
       WindowClosing WC=new WindowClosing();
       fs.addWindowListener(WC);
       fs.panel();
       fs.setVisible(true);
       server.servers();
       //

   }
   public Frame_Server(String str)
   {
       super(str);
   }
   public void panel()
   {
       Panel p_1=new Panel();
       Label l_1=new Label("对方信息：");
       p_1.add("North",l_1);
       p_1.setBackground(Color.pink);
       add(&qu