nullPointer異常，我在哪里出錯？

Question

我正在研究《 Head First Java》一書，現在嘗試建立一個Chatroom。 我有一個ChatServer和一個ChatClient，並且可以相互連接。 但是，當我嘗試發送消息時，出現以下錯誤：

java.lang.NullPointerException
    at VerySimpleChatServer.tellEveryone(VerySimpleChatServer.java:61)
    at VerySimpleChatServer$ClientHandler.run(VerySimpleChatServer.java:29)
    at java.lang.Thread.run(Unknown Source)

我檢查了tellEveryone，但看不到我要去哪里。 我看不到任何指向任何內容的指針。

誰能幫我一點忙？

ClientServer的代碼：

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的代碼：

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();
        }
    }
}

}

編輯：

第60到74行的SimpleChatServerClient：

    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();
        }
    } 
} 

}

Answer 1

該clientOutputStreams您正在使用中go

List<PrintWriter>clientOutputStreams = new ArrayList<>(); 

對於該方法而言是本地的

clientOutputStreams用於

public void tellEveryone(String message) {
    Iterator it = clientOutputStreams.iterator(); 

是從未初始化的類變量

Answer 2

感謝您的所有答復。 我更改了以下內容以使代碼正常工作：

在第7行更改

ArrayList clientOutputStreams;

成

ArrayList<PrintWriter> clientOutputStreams; 

在我的public void go

變

List<PrintWriter>clientOutputStreams = new ArrayList<>();

成

clientOutputStreams = new ArrayList<PrintWriter>();

謝謝您的幫助！