我正在学习《Head First Java》一书,现在正在尝试构建一个聊天室。我有一个 ChatServer 和一个 ChatClient,它们可以相互连接。但是当我尝试发送消息时出现以下错误:
java.lang.NullPointerException
at VerySimpleChatServer.tellEveryone(VerySimpleChatServer.java:61)
at VerySimpleChatServer$ClientHandler.run(VerySimpleChatServer.java:29)
at java.lang.Thread.run(Unknown Source)
我检查了我的tellEveryone,但我看不出我哪里出错了。我没有看到任何地方有任何指向任何内容的指针。
谁能帮我一点忙吗?
客户端服务器代码:
import java.io.*;
import java.net.*;
import java.util.*;
public class VerySimpleChatServer {
ArrayList clientOutputStreams;
public class ClientHandler implements Runnable {
BufferedReader reader;
Socket sock;
public ClientHandler(Socket clientSocket) {
try {
sock = clientSocket;
InputStreamReader isReader = new InputStreamReader(sock.getInputStream());
reader = new BufferedReader(isReader);
} catch(Exception ex)
{
ex.printStackTrace();
}
}
public void run() {
String message;
try {
while (( message = reader.readLine()) != null) {
System.out.println(" read " + message);
tellEveryone(message);
}
}
catch(Exception ex)
{
ex.printStackTrace();
}
}
}
public static void main (String[] args) {
new VerySimpleChatServer(). go();
}
public void go() {
List<PrintWriter>clientOutputStreams = new ArrayList<>();
try {
ServerSocket serverSock = new ServerSocket( 5000);
while( true) {
Socket clientSocket = serverSock.accept();
PrintWriter writer = new PrintWriter( clientSocket.getOutputStream());
clientOutputStreams.add( writer);
Thread t = new Thread( new ClientHandler( clientSocket));
t.start(); System.out.println(" got a connection");
}
} catch(Exception ex)
{
ex.printStackTrace();
}
}
public void tellEveryone(String message) {
Iterator it = clientOutputStreams.iterator();
while(it.hasNext()) {
try {
PrintWriter writer = (PrintWriter) it.next();
writer.println(message);
writer.flush();
}
catch(Exception ex)
{
ex.printStackTrace();
}
}
}
}
ChatClient 的代码:
import java.io.*;
import java.net.*;
import java.util.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class SimpleChatClient {
JTextArea incoming;
JTextField outgoing;
BufferedReader reader;
PrintWriter writer;
Socket sock;
public static void main(String[] args) {
SimpleChatClient client = new SimpleChatClient();
client.go();
}
public void go()
{
JFrame frame = new JFrame("Ludicrously Simple Chat Client");
JPanel mainPanel = new JPanel();
incoming = new JTextArea(15, 30);
incoming.setLineWrap(true);
incoming.setWrapStyleWord(true);
incoming.setEditable(false);
JScrollPane qScroller = new JScrollPane(incoming);
qScroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
qScroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
outgoing = new JTextField(20);
JButton sendButton = new JButton("Send");
sendButton.addActionListener(new SendButtonListener());
mainPanel.add(qScroller);
mainPanel.add(outgoing);
mainPanel.add(sendButton);
setUpNetworking();
Thread readerThread = new Thread(new IncomingReader());
readerThread.start();
frame.add(mainPanel, BorderLayout.CENTER);
frame.setSize(400, 500);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
private void setUpNetworking()
{
try
{
sock = new Socket("127.0.0.1", 5000);
InputStreamReader streamReader = new InputStreamReader(sock.getInputStream());
reader = new BufferedReader(streamReader);
writer = new PrintWriter(sock.getOutputStream());
System.out.println("networking established");
}
catch(IOException ex)
{
ex.printStackTrace();
}
}
public class SendButtonListener implements ActionListener
{
public void actionPerformed(ActionEvent ev)
{
try
{
writer.println(outgoing.getText());
writer.flush();
}
catch(Exception ex)
{
ex.printStackTrace();
}
outgoing.setText("");
outgoing.requestFocus();
}
}
public class IncomingReader implements Runnable
{
public void run()
{
String message;
try
{
while ( (message = reader.readLine()) != null)
{
System.out.println("read " + message);
incoming.append(message + "\n");
}//close while
}
catch(Exception ex)
{
ex.printStackTrace();
}
}
}
}
编辑:
SimpleChatServerClient第60行到74行:
public void tellEveryone(String message) {
Iterator it = clientOutputStreams.iterator();
while(it.hasNext()) {
try {
PrintWriter writer = (PrintWriter) it.next();
writer.println(message);
writer.flush();
}
catch(Exception ex)
{
ex.printStackTrace();
}
}
}
}
最佳答案
您在 go
中使用的 clientOutputStreams
List<PrintWriter>clientOutputStreams = new ArrayList<>();
对于该方法来说是本地的
使用的clientOutputStreams
public void tellEveryone(String message) {
Iterator it = clientOutputStreams.iterator();
是从未初始化过的类变量
关于java - nullPointer 异常,我哪里出错了?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28624102/