网络的停止等待ARQ的模拟，可以实现信息的交换

发送方：

import java.io.*;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
class Cl extends Frame implements ActionListener
{
  TextArea ta=new TextArea(5,10);
  TextField tf=new TextField(10);
  Socket c;
  double m;
  int i=0;
  int q=0;
  static int n;
  String ss="";
  static char abc[];
  DataInputStream in=null;
  DataOutputStream out=null;
  String s;
  double m1;
  public Cl()
  {
   super("发送方");
   s=null;
   setSize(250,250);
   tf.addActionListener(this);
   add("North",tf);
   add("Center",ta);
   addWindowListener(new WindowAdapter(){public void windowClosing(WindowEvent e){System.exit(0);}});
   show();
  
   }
  public void actionPerformed(ActionEvent e)
  {
  if(e.getSource()==tf)
  {
  String sss=tf.getText();
  abc=sss.toCharArray();
  try
  {
  c=new Socket("200.200.202.58",8888);
  in=new DataInputStream(c.getInputStream());
  out=new DataOutputStream(c.getOutputStream());
  while(true)
  {
  n=0;
  m=Math.random()*100;
  m1=Math.random()*100;
  if((m1>=0)&&(m1<50))
  {
   q=1;
  }
  else
  {
   q=0;
  }
  ss=String.valueOf(abc[i]);
  String ss1=String.valueOf(q);
  ss1=ss1.concat(" ");
  ss1=ss1.concat(ss);
  ta.append("发送:"+ss1);
  ta.append("\n");
  if(m>20&&m<=100)
  {
  out.writeUTF(ss1);
  ss1="";
  }
  System.out.println("启动超时计时器:");
  for(int zz=0;zz<3;zz++)
  {System.out.println(n++);
   Thread.sleep(1000);
  if(m>20&&m<=100)
  {
  break;
           
  }
  }
  if(n==3)
  {  
  ta.append("超时重发");
  ta.append("\n");
  continue;
  }
  while(true)
  {
  s=in.readUTF();
  if(s.equals("ack"))
  {
  ta.append("发送正确继续发"+"\n");
  i=i+1;
  break;
  }
  if(s.equals("nak"))
  {
  ta.append("发送错误重新发"+"\n");
  break;
  }
  }
  if(i==abc.length){break;}
  }
  }
  catch(InterruptedException e1){}
  catch(IOException e2){}
  }
    }
}
public class fa
{
public static void main(String args[])
{
new Cl();
}
}

接收方：

import java.io.*;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
class Se extends Frame
{
  TextField tf=new TextField(10);
  TextArea ta=new TextArea(5,10);
  Socket c;
  public Se()
  {
   super("接收方");
   setSize(250,250);
   ServerSocket sever=null;
   Socket you=null;
   String s=null;
   DataOutputStream out=null;
   DataInputStream in=null;
   double m1,m2;
   String s1,s2,s3;
   s1="";s2="";s3="";
   int i=0;
   int q=0;
   int n;
   add("North",tf);
   add("Center",ta);
   addWindowListener(new WindowAdapter(){public void windowClosing(WindowEvent e){System.exit(0);}});
   show();
   try
   {
    sever=new ServerSocket(8888);
    you=sever.accept();

in=new DataInputStream(you.getInputStream());
out=new DataOutputStream(you.getOutputStream());
while(true)
{   
s=in.readUTF();
m1=Math.random()*100;
m2=Math.random()*100;
if(m2>0&&m2<50)
{
q=1;
}
else
{
q=0;
}
char a[]=s.toCharArray();
if((m1>20&&m1<=100)&&((a[0]-'0')==q))
{
ta.append("收到:"+s);
ta.append("\n");
            s2=a[2]+"";
            s3=s3.concat(s2);
            tf.setText(s3);
out.writeUTF("ack");
}
if(m1>=0&&m1<=20)
{
ta.append("CRC检测错误!");
ta.append("\n");
out.writeUTF("nak");
}
else if((a[0]-'0')!=q)
{
ta.append("序号不对应丢弃帧");
ta.append("\n");
out.writeUTF("nak");
}
        
}
}catch(IOException e1){}
       
}
}
public class jie
{
public static void main(String args[])
{
new Se();
}
}