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When my client after closure, the server will report errors, have who can help me to solve this problem under the

 package com. Java; 

Import the Java. IO. BufferedReader; 
import java.io.IOException; 
Import the Java. IO. InputStreamReader; 
Import the Java. IO. OutputStreamWriter; 
The import java.net.ServerSocket; 
The import java.net.Socket; 
import java.util.ArrayList; 
import java.util.List; 

/* * 
* the server 
* function: receive the client sends the message, and then sent to all online clients 
*/
Public class Server {
Public static List Sockets=new ArrayList (a); 
Public static void main (String [] args) throws IOException {
System. The out. Println (" the server to start "); 
ServerSocket server=new ServerSocket (2000); 
While (true) {
System. The out. Println (" waiting for the client connection "); 
The Socket s=server. The accept (); 
Sockets. The add (s); 
System. The out. Println (" the client is connected, the number of online "+ sockets. The size ()); 
New ServerThread (s). The start (); 
} 
} 

} 
The class ServerThread extends Thread {
Private Socket s; 
Public ServerThread (Socket s) {
Enclosing the s=s; 
} 

@ Override 
Public void the run () {
Try {
BufferedReader br=
New BufferedReader (
New InputStreamReader (s.g etInputStream ())); 
//get the logged in user's user name, and then send all clients welcome XXX login 
String username=br. ReadLine (); 
For (Socket Socket: Server sockets) {
OutputStreamWriter osw=
New OutputStreamWriter (socket. GetOutputStream ()); 
Osw. Write (" [system messages] "+ username +" login \ n "); 

Osw. Flush (); 
} 
//receive chat messages, and then forwarded to all the clients 
While (true) {
The String content=br. ReadLine (); 
For (Socket Socket: Server sockets) {
OutputStreamWriter osw=
New OutputStreamWriter (socket. GetOutputStream ()); 
Osw. Write (" : "username + + content +" \ n "); 

Osw. Flush (); 
} 
System. Out. Println (" log: "+ username +" : "+ content); 

} 
} the catch (IOException e) {

e.printStackTrace(); 
} 

} 
}

 package com. Java; 

The import javax.mail. Swing. *; 
import java.awt.*; 
Import the Java. The awt. Event. An ActionEvent; 
Import the Java. The awt. Event. ActionListener; 
Import the Java. The awt. Event. WindowAdapter; 
Import the Java. The awt. Event. WindowEvent; 
Import the Java. IO. BufferedReader; 
import java.io.IOException; 
Import the Java. IO. InputStreamReader; 
Import the Java. IO. OutputStreamWriter; 
The import java.net.Socket; 

/* * 
* interact with the server 
* receive server sends message 
* to the server sends the message 
*/
Public class Client extends JFrame {
Public static void main (String [] args) {
The new Login (); 
} 
Private Socket s; 
private String username; 

JPanel jp1, jp2; 
JTextArea jta.//multiline text, which is used to display content 
JTextField JTF;//a single text, which is used to input content 
The JScrollPane JSP;//the scroll bar 
Public Client (Socket s, String username) {
Enclosing the s=s; 
Enclosing the username=username; 
Jp1=new JPanel (); 
Jta=new JTextArea (20, 30);/line/column 
JSP=new JScrollPane (jta);//the jta has the function of the scroll bar 

Jp1. Add (JSP); 
//used to input content JTF panel 
Jp2=new JPanel (); 
JTF=new JTextField (30); 
Jp2. Add (JTF); 

Enclosing setTitle (" current user "+ username); 
Enclosing setLayout (new BorderLayout ()); 
This. The add (BorderLayout. CENTER, jp1); 
This. The add (BorderLayout. SOUTH, jp2); 
This pack (); 

this.setVisible(true); 
Enclosing addWindowListener (new WindowAdapter () {
@ Override 
Public void windowClosing (WindowEvent e) {
//the dispose (); 
//s.i sClosed (); 
System.exit(0); 
} 
}); 
JTF. AddActionListener (new MyListener ()); 
//start receiving server message thread 
New GetThread (). The start (); 
} 
The class MyListener implements ActionListener {

@ Override 
Public void actionPerformed (an ActionEvent e) {
/* jta. SetText (jta. GetText () + "\ n" + JTF. GetText ()); 
JTF. SetText (" "); */

//to the server sends the message 
The String content=JTF. GetText (); 
Try {
OutputStreamWriter osw=
New OutputStreamWriter (s.g etOutputStream ()); 
(osw. Write the content + "\ n"); 
Osw. Flush (); 
{} the catch (IOException IOException) 
IoException. PrintStackTrace (); 
} 
} 
} 

The class GetThread extends Thread {
Public void the run () {
Try {
BufferedReader br=
New BufferedReader (
New InputStreamReader (s.g etInputStream ())); 
While (true) {
The String content=br. ReadLine (); 
Jta. SetText (jta. GetText () + "\ n" + content); 
JTF. SetText (" "); 
} 
} the catch (IOException e) {
e.printStackTrace(); 
} 
} 
} 
}

 package com. Java; 

The import javax.mail. Swing. *; 
import java.awt.*; 
Import the Java. The awt. Event. An ActionEvent; 
Import the Java. The awt. Event. ActionListener; 
import java.io.IOException; 
Import the Java. IO. OutputStreamWriter; 
The import java.net.Socket; 

/* * 
* login window 
* records user name 
*/
Public class Login extends JFrame implements ActionListener {
JLabel label=new JLabel (" please enter the user name "); 
JTextField JTF=new JTextField (20); 
Public the Login () {
Enclosing setLayout (new FlowLayout ()); 
This. The add (label); 
This. The add (JTF); 
This pack (); 
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