带CRC的消息发送器

///* 来自 n o w j a v a . c o m*/

import java.io.*;

import java.net.*;


public class Reciever

{

     ServerSocket RecieverSocket;

     Socket connection = null;

     ObjectOutputStream out;

     ObjectInputStream in;

     String message,checksum;

     Reciever(){}

     void run()

     {

          try// 来自 时代Java公众号 - nowjava.com

          {

               BufferedReader br=new BufferedReader(new InputStreamReader(System.in));


               RecieverSocket = new ServerSocket(2004, 10);


               System.out.println("Waiting for connection");

               connection = RecieverSocket.accept();

               senderName = conncetion.getInetAddress().getHostName();

               System.out.println("Connection received from " + senderName);


               out = new ObjectOutputStream(connection.getOutputStream());

               out.flush();

               in = new ObjectInputStream(connection.getInputStream());

               sendMessage("Connection successful ...");


               try

               {

                    message = (String)in.readObject();

                    System.out.println("client>" + message);

                    checksum=checksum(message);

                    message=(checksum.equals("000"))?"correct":"corrupted";

                    sendMessage(message);

               }

               catch(ClassNotFoundException classnot)

               {

                    System.err.println("Data received in unknown format");

               }


          }

          catch(IOException ioException)

          {

               ioException.printStackTrace();

          }


          finally

          {

               try

               {

                    in.close();

                    out.close();

                    RecieverSocket.close();

               }

               catch(IOException ioException)

               {

                    ioException.printStackTrace();

               }

          }

     }


     String checksum(String str)

     {

          int[] a=new int[20];

          int n=str.length();


          for(int i=0;i<n;i++)

          {

               a[i]=Integer.valueOf(str.substring(i,i+1));

          }

          

          for(int i=0;i<n-3;i++) 

          {

               if(a[i]==1) 

               {

                    a[i]=(a[i]==1)?0:1;

                    a[i+1]=(a[i+1]==1)?1:0;

                    a[i+2]=(a[i+2]==1)?0:1;

                    a[i+3]=(a[i+3]==1)?0:1;

               }

          }


          str=String.valueOf(a[n-3]);

          str=str.concat(String.valueOf(a[n-2]));

          str=str.concat(String.valueOf(a[n-1]));

          return str;

     }


     void sendMessage(String msg)

     {

          try

          {