＆＃39;此代码可能存在多个问题，但我无法解决此第一个错误。我是一个新手，这项任务超出了我的想法。看了将近两个星期，我很绝望。任何帮助将不胜感激。在我尝试在服务器的textField中输入内容并按下回车之前，它不会给出错误。代码编译，但给出：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Server$1.actionPerformed(Server.java.71).
以下是代码：
import java.io.*;
import java.net.*;
import java.util.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Server extends JFrame {
//Global variables
private static ServerSocket serverSocket = null;
private static Socket connection = null;
private static PrintStream output = null;
private static BufferedReader input = null;
public static ArrayList<threadFromClient> clientList = new ArrayList<threadFromClient>();
private static JTextField textField;
private static JTextArea msgWindow;
private static int port = 3333;
//Main program
public static void main(String[] args){
Server server = new Server();
if(args.length < 1){
displayMsg("Usage: java Server <port> \n" + "Now using port number=" + port);
}else{
port = Integer.valueOf(args[0]).intValue();
}
server.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
try{
serverSocket = new ServerSocket(port);
allowInput(true);
}catch(EOFException e){
displayMsg("Client terminated connection");
}catch(IOException ioe){
displayMsg(ioe.getMessage());
}
//Create a client socket for each connection and pass it to a new client thread
while(true){
try{
connection = serverSocket.accept();
for(int i = 0; i< clientList.size();i++){
if(clientList.get(i) == null){
(clientList.set(i, new threadFromClient(connection,clientList))).start();
break;
}
}
if(clientList.size() == 100){
PrintStream os = new PrintStream(connection.getOutputStream());
displayMsg("Server too busy.");
os.close();
connection.close();
}
}catch(Exception e){
displayMsg(e.getMessage());
}
}
}//end main()
public Server(){
super("Messenger Server");
textField = new JTextField();
textField.setEditable(false);
textField.addActionListener(
new ActionListener(){
public void actionPerformed(ActionEvent e){
String event = e.getActionCommand();
output.println(event);
output.flush();
msgWindow.append(event);
textField.setText("");
}
}
);
add(textField, BorderLayout.NORTH);
msgWindow = new JTextArea();
add(new JScrollPane(msgWindow));
setSize(500,350);
setVisible(true);
}
//Enable textField
private static void allowInput(boolean bool){
SwingUtilities.invokeLater(
new Runnable(){
public void run(){
textField.setEditable(true);
}
}
);
}//end allowInput()
//Sends msg to the client
private static void sendMsg(String msg){
output.println("Server: " + msg);
output.flush();
displayMsg("\n Server:" + msg);
}//end sendMsg()
//Updates msgWindow
private static void displayMsg(final String msg){
SwingUtilities.invokeLater(
new Runnable(){
public void run(){
msgWindow.append(msg);
}
}
);
}//end displayMsg()
}//end Server class
import java.io.*;
import java.net.*;
import java.util.ArrayList;
public class threadFromClient extends Thread implements Runnable{
private String userName;
private BufferedReader input;
private PrintStream output;
private Socket connection;
private ArrayList<threadFromClient> threads;
public threadFromClient(Socket connection, ArrayList<threadFromClient> threads){
this.connection = connection;
this.threads = threads;
}//end constructor
public void run(){
ArrayList<threadFromClient> threads = this.threads;
try{
//create input and output for client
output = new PrintStream(connection.getOutputStream());
output.flush();
input = new BufferedReader(new InputStreamReader(connection.getInputStream()));
output.println("\n Streams are now connected! \n");
//communicating
String name;
while(true){
output.println("Enter your name");
name = input.readLine().trim();
if(name.indexOf('@') == -1){
break;
}else{
output.println("No @ allowed");
}
}
//welcome the new client
output.println("Welcome " + name + "!");
synchronized (this){
for(int i = 0;i < threads.size();i++){
if(threads.get(i) != null && threads.get(i) == this){
userName = "@" + name;
break;
}
}
for(int i = 0;i<threads.size();i++){
if(threads.get(i) != null && threads.get(i) != this){
threads.get(i).output.println("A new user " + name + " entered the network");
}
}
}
//starting conversation
while(true){
String line = input.readLine();
if(line.equals("END")) break;
//if the message is private send it to the given client
if(line.startsWith("@")){
String[] words = line.split("\\s", 2);
if (words.length > 1 && words[1] != null) {
words[1] = words[1].trim();
if (!words[1].isEmpty()) {
synchronized (this) {
for (int i = 0; i < threads.size(); i++) {
if (threads.get(i) != null && threads.get(i) != this
&& threads.get(i).userName != null
&& threads.get(i).userName.equals(words[0])) {
threads.get(i).output.println("<" + name + "> " + words[1]);
/*
* Echo this message to let the client know the private
* message was sent.
*/
this.output.println(">" + name + "> " + words[1]);
break;
}
}
}
}
}
}else {
/* The message is public, broadcast it to all other clients. */
synchronized (this) {
for (int i = 0; i < threads.size(); i++) {
if (threads.get(i) != null && threads.get(i).userName != null) {
threads.get(i).output.println("<" + name + "> " + line);
}
}
}
}
}
synchronized (this) {
for (int i = 0; i < threads.size(); i++) {
if (threads.get(i) != null && threads.get(i) != this
&& threads.get(i).userName != null) {
threads.get(i).output.println("*** The user " + name
+ " is leaving the chat room !!! ***");
}
}
}
output.println("*** Bye " + name + " ***");
/*
* Clean up. Set the current thread variable to null so that a new client
* could be accepted by the server.
*/
synchronized (this) {
for (int i = 0; i < threads.size(); i++) {
if (threads.get(i) == this) {
threads.set(i, null);
}
}
}
/*
* Close the output stream, close the input stream, close the socket.
*/
input.close();
output.close();
connection.close();
}catch(Exception e){
output.println(e.getMessage());
}
}//end run()
} //结束threadFromClient类