① 客戶端程序（效果如圖11.6所示）
import java.net.*;  
import java.io.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class ClientFrame extends JFrame
 implements Runnable,ActionListener{
    JButton connection,computer;
    JTextField inputA,inputB,inputC;
    JTextArea showResult;
    Socket socket=null;
    DataInputStream in=null; 
    DataOutputStream out=null;
    Thread thread; 
    public ClientFrame(){
        socket=new Socket();               //待連接的套接字
        connection=new JButton("連接服務器");
        computer=new JButton("求方程的根");
        computer.setEnabled(false);         //沒有和服務器連接之前，該按鈕不可用
        inputA=new JTextField("0",12);
        inputB=new JTextField("0",12);
        inputC=new JTextField("0",12);
        Box boxV1=Box.createVerticalBox();
        boxV1.add(new JLabel("輸入2次項系數"));
        boxV1.add(new JLabel("輸入1次項系數"));
        boxV1.add(new JLabel("輸入常數項"));
        Box boxV2=Box.createVerticalBox();
        boxV2.add(inputA);
        boxV2.add(inputB);
        boxV2.add(inputC);
        Box baseBox=Box.createHorizontalBox();
        baseBox.add(boxV1);
        baseBox.add(boxV2);
        Container con=getContentPane();
        con.setLayout(new FlowLayout());
        showResult=new JTextArea(8,18);
        con.add(connection);
        con.add(baseBox);
        con.add(computer);
        con.add(new JScrollPane(showResult));
        computer.addActionListener(this);
        connection.addActionListener(this);
        thread = new Thread(this);
        setBounds(100,100,360,310);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);   
    }
    public void run(){
        while(true){
             try{  double root1=in.readDouble();  //堵塞狀態，除非讀取到信息
                  double root2=in.readDouble(); 
                  showResult.append("\n兩個根:\n"+root1+"\n"+root2);
                  showResult.setCaretPosition((showResult.getText()).length());
             }
             catch(IOException e){
                  showResult.setText("與服務器已斷開");
                  computer.setEnabled(false);
                  break;
             } 
        }
    }
    public void actionPerformed(ActionEvent e){
        if(e.getSource()==connection){
           try{  if(socket.isConnected()) {} 
                else{
                   InetAddress address=InetAddress.getByName("127.0.0.1");
                   InetSocketAddress socketAddress=new InetSocketAddress(address,4331);
                   socket.connect(socketAddress); 
                   in=new DataInputStream(socket.getInputStream());
                   out=new DataOutputStream(socket.getOutputStream());
                   computer.setEnabled(true);
                   thread.start();
                }
           } 
           catch (IOException ee){
        System.out.println(ee);
                socket=new Socket();
           }
        }
        if(e.getSource()==computer){
           try{  double a=Double.parseDouble(inputA.getText()),
                b=Double.parseDouble(inputB.getText()),
                c=Double.parseDouble(inputC.getText());
                double disk=b*b-4*a*c;
                if(disk>=0){
                      out.writeDouble(a);
                      out.writeDouble(b);
                      out.writeDouble(c);
                }
                else{
                      inputA.setText("此2次方程無實根");
                }
           }
           catch(Exception ee){
                inputA.setText("請輸入數字字符");
           }
        }
    }
    public static void main(String args[]){
        ClientFrame win=new ClientFrame();
    }
}
② 服務器端程序（效果如圖11.7所示）
import java.io.*;
import java.net.*;
import java.util.*;
public class MutiServer{
    public static void main(String args[]){
        ServerSocket server=null;
        ServerThread thread;
        Socket you=null;
        while(true){
            try{  server=new ServerSocket(4331);
            }
            catch(IOException e1){
                 System.out.println("正在監聽");   //ServerSocket對象不能重復創建
            } 
            try{  you=server.accept();
                 System.out.println("客戶的地址:"+you.getInetAddress());
            }
            catch (IOException e){
                 System.out.println("正在等待客戶");
            }
            if(you!=null) 
                 new ServerThread(you).start();     //為每個客戶啟動一個專門的線程 
            else 
                 continue;
        }
    }
}
class ServerThread extends Thread{
    Socket socket;
    DataOutputStream out=null;
    DataInputStream in=null;
    String s=null;
    ServerThread(Socket t){
       socket=t;
       try{   in=new DataInputStream(socket.getInputStream());
            out=new DataOutputStream(socket.getOutputStream());
       }
       catch (IOException e){}
    }  
    public void run(){
       while(true){
           double a=0,b=0,c=0,root1=0,root2=0;
           try{   a=in.readDouble();                   //堵塞狀態，除非讀取到信息
                 b=in.readDouble(); 
                 c=in.readDouble(); 
                 double disk=b*b-4*a*c;
                 root1=(-b+Math.sqrt(disk))/(2*a);
                 root2=(-b-Math.sqrt(disk))/(2*a);
                 out.writeDouble(root1);
                 out.writeDouble(root2);
           }
           catch (IOException e){
                 System.out.println("客戶離開");
                  break;
           }
       }
    }
}