Help with Head First Java simplechatclient -- NullPointerException

[Rafael Szlom]

Hi gyus,
first time poster here.

I tried to run the sample code from chapter 18 / Head first Java and get the following error message.

Exception in thread "Thread-0" java.lang.NullPointerException
at chatclient2.ChatClient2$MessageReader.run(ChatClient2.java:97)
at java.lang.Thread.run(Thread.java:722)


Here is the Full sourcecode

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package chatclient2;
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.*;
import java.net.*;
import javax.swing.*;

/**
 *
 * @author rambo
 */
public class ChatClient2 {
    JFrame chatframe;
    PrintWriter writer;
    public BufferedReader reader;
    Socket socket;
    JTextField messagefield;
    JTextArea messagearea;
    JPanel mainpanel;

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        ChatClient2 chatclient = new ChatClient2();
        chatclient.startClient();
    }
    public void startClient(){
        chatframe = new JFrame();
        chatframe.setTitle("Chat Client");
        messagefield = new JTextField(30);
        messagearea = new JTextArea(30,50);
        messagearea.setLineWrap(true);
        messagearea.setWrapStyleWord(true);
        messagearea.setEditable(false);
        JScrollPane scroller = new JScrollPane(messagearea);
        scroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        scroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
        JButton sendbutton = new JButton("Send");
        sendbutton.addActionListener(new SendListener());
        mainpanel = new JPanel();
        mainpanel.add(scroller);
        mainpanel.add(messagefield);
        mainpanel.add(sendbutton);
        chatframe.getContentPane().add(new BorderLayout().CENTER,mainpanel);
        startnetwork();
        
        Thread messagethread = new Thread(new MessageReader());
        messagethread.start();
        chatframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        chatframe.setSize(500,500);
        chatframe.setVisible(true);
        
    }
    
    private void startnetwork(){
        try{
            socket = new Socket("127.0.0.1",5000);
            InputStreamReader input = new InputStreamReader(socket.getInputStream());
            reader = new BufferedReader(input);
            writer = new PrintWriter(socket.getOutputStream());
        }
        catch(Exception ex){
            System.out.println("Problem: "+ex);
        }
        System.out.println("Network established");
      
    }
    
    class SendListener implements ActionListener{
        public void actionPerformed(ActionEvent event){
            
            try {
             
                writer.println(messagefield.getText());
                writer.flush();
                
            }
            catch(Exception ex){
                ex.printStackTrace();
            }
            messagefield.setText("");
            messagefield.requestFocus();
            
        }
    }
     class MessageReader implements Runnable {
        public void run() {
            String message;
            try {
                while ((message = reader.readLine()) != null) {
                    System.out.println("client read " + message);
                    messagearea.append(message + "\n");
                }
            }catch (IOException ex)
            {
                ex.printStackTrace();
            }
        }
    }
}

Could anyone please help me out. The problem lies in while((message = reader.readLine() != null) / Line 97

Don

[Rajesh Nagaraju]

If you run the program directly it gives first the following exception

Problem: java.net.ConnectException: Connection refused: connect


I am assuming you get this exception, this is because you dont have a
socket open at your local system at port 5000

This leads to a NullPointerException later

Hope this helps

[Rafael Szlom]

Thank you Rajesh ,

you will not believe it , I did not start the method responsible for setting up the networking in the server code.
by the way that was exactly the same error message that I got .