Question

I made a small program that listens and sends lines on a tcp socket and appends the received info to a JTextArea. I use this to chat on a Minecraft server without having the game open.

I was working fine last night, but when I got up it wasn’t working. When I opened netbeans and ran it, it gave this error:

Exception in thread "main" java.lang.NullPointerException
    at com.xxx.mcchat.chat.main(chat.java:333)

Can anyone explain what’s wrong?

Here is the code (http://pastebin.com/FPNty0qf):

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package com.xxx.mcchat;

import java.io.*;
import java.net.*;
import net.sf.json.*;
import org.apache.commons.beanutils.*;
import org.apache.commons.collections.*;
import org.apache.commons.lang.*;
import net.sf.ezmorph.*;
import org.apache.commons.logging.*;
import java.awt.event.*;
import javax.swing.UIManager;
/**
 *
 * @author xxx
 */
public class chat extends javax.swing.JFrame {

    /**
     * Creates new form chat
     */

    public chat() {
        initComponents();
    }

    public void send(String user, String message){
        Socket socket = null;
        PrintWriter out = null;
        BufferedReader in = null;


        try {
            socket = new Socket("mc.xxx.net", 20060);
            out = new PrintWriter(socket.getOutputStream(), true);
            in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        } catch (UnknownHostException e) {
            System.err.println("Don't know about host");
            System.exit(1);
        } catch (IOException e) {
            System.err.println("Couldn't get I/O for the connection");
            System.exit(1);
        }

        BufferedReader read = new BufferedReader(new InputStreamReader(System.in));

                //System.out.println(in.readLine()); //Uncomment to debug
        if(username != null){
            out.println("/api/call?method=broadcast&args="+"[\"§7[Web] §b"+username+"§7:§f "+message+"\"]"+"&key=f0e2ad47a9a43c783d2c54f396f655c9279829c8c69ae9f52934648098dec993");
            chatArea.append(username + ": " + message + "\n\r");
            if(autoscrollCheck.isSelected()){
                chatArea.setCaretPosition(chatArea.getText().length() - 1);
            }
        }else{
            chatArea.append("You must set your username!!" + "\n\r");
            if(autoscrollCheck.isSelected()){
                chatArea.setCaretPosition(chatArea.getText().length() - 1);
            }

        }
    }

    /**
     * This method is called from within the constructor to initialize the form. WARNING: Do NOT modify this code. The content of this method is always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                       
    private void initComponents() {

        jCheckBoxMenuItem1 = new javax.swing.JCheckBoxMenuItem();
        jToggleButton1 = new javax.swing.JToggleButton();
        jScrollPane1 = new javax.swing.JScrollPane();
        chatArea = new javax.swing.JTextArea();
        input = new javax.swing.JTextField();
        send = new javax.swing.JButton();
        user = new javax.swing.JTextField();
        userset = new javax.swing.JButton();
        autoscrollCheck = new javax.swing.JCheckBox();
        jLabel1 = new javax.swing.JLabel();

        jCheckBoxMenuItem1.setSelected(true);
        jCheckBoxMenuItem1.setText("jCheckBoxMenuItem1");

        jToggleButton1.setText("jToggleButton1");

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("Minecraft Chat");
        addWindowListener(new java.awt.event.WindowAdapter() {
            public void windowOpened(java.awt.event.WindowEvent evt) {
                formWindowOpened(evt);
            }
            public void windowClosing(java.awt.event.WindowEvent evt) {
                formWindowClosing(evt);
            }
        });

        chatArea.setEditable(false);
        chatArea.setBackground(new java.awt.Color(0, 0, 0));
        chatArea.setColumns(20);
        chatArea.setFont(new java.awt.Font("Consolas", 0, 14)); // NOI18N
        chatArea.setForeground(new java.awt.Color(255, 255, 255));
        chatArea.setLineWrap(true);
        chatArea.setRows(5);
        jScrollPane1.setViewportView(chatArea);

        input.setToolTipText("Enter message here");
        input.addKeyListener(new java.awt.event.KeyAdapter() {
            public void keyPressed(java.awt.event.KeyEvent evt) {
                inputKeyPressed(evt);
            }
        });

        send.setText("Send");
        send.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                sendActionPerformed(evt);
            }
        });

        user.setToolTipText("");
        user.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                userActionPerformed(evt);
            }
        });
        user.addKeyListener(new java.awt.event.KeyAdapter() {
            public void keyPressed(java.awt.event.KeyEvent evt) {
                userKeyPressed(evt);
            }
        });

        userset.setText("Set");
        userset.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                usersetActionPerformed(evt);
            }
        });

        autoscrollCheck.setSelected(true);
        autoscrollCheck.setText("Auto Scroll");
        autoscrollCheck.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                autoscrollCheckActionPerformed(evt);
            }
        });

        jLabel1.setText("Enter Username:");

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(10, 10, 10)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                        .addComponent(jLabel1)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(user, javax.swing.GroupLayout.PREFERRED_SIZE, 218, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(userset)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(autoscrollCheck))
                    .addComponent(jScrollPane1)
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                        .addComponent(input, javax.swing.GroupLayout.PREFERRED_SIZE, 649, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(send)))
                .addGap(10, 10, 10))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(11, 11, 11)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addGap(1, 1, 1)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(user, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jLabel1)))
                    .addComponent(userset)
                    .addComponent(autoscrollCheck))
                .addGap(6, 6, 6)
                .addComponent(jScrollPane1, javax.swing.GroupLayout.DEFAULT_SIZE, 316, Short.MAX_VALUE)
                .addGap(6, 6, 6)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addGap(1, 1, 1)
                        .addComponent(input, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                    .addComponent(send))
                .addGap(11, 11, 11))
        );

        pack();
    }// </editor-fold>                      

    String username = null;
    private void inputKeyPressed(java.awt.event.KeyEvent evt) {                              
           int key = evt.getKeyCode();
           if (key == KeyEvent.VK_ENTER) {  
                send(username, input.getText());
                input.setText("");
              }
    }                               

    private void sendActionPerformed(java.awt.event.ActionEvent evt) {                                   
        send(username, input.getText());
        input.setText("");
    }                                   

    private void usersetActionPerformed(java.awt.event.ActionEvent evt) {                                       
        if(username == null){
                    if(!"".equals(user.getText())){
                        username = user.getText();
                        chatArea.append("Username set!"+"\n\r");
                        if(autoscrollCheck.isSelected()){
                            chatArea.setCaretPosition(chatArea.getText().length() - 1);
                        }
                    }else{
                        chatArea.append("Username can not be blank."+"\n\r");
                        if(autoscrollCheck.isSelected()){
                            chatArea.setCaretPosition(chatArea.getText().length() - 1);
                        }
                    }
                }else{
                    send(username, "§7changed name to " + user.getText());
                    username = user.getText();
                }
    }                                      

    private void userActionPerformed(java.awt.event.ActionEvent evt) {                                   
        // TODO add your handling code here:
    }                                   

    private void userKeyPressed(java.awt.event.KeyEvent evt) {                              
           int key = evt.getKeyCode();
           if (key == KeyEvent.VK_ENTER) {  
                if(username == null){
                    if(!"".equals(user.getText())){
                        username = user.getText();
                        chatArea.append("Username set!"+"\n\r");
                        if(autoscrollCheck.isSelected()){
                            chatArea.setCaretPosition(chatArea.getText().length() - 1);
                        }
                    }else{
                        chatArea.append("Username can not be blank."+"\n\r");
                        if(autoscrollCheck.isSelected()){
                            chatArea.setCaretPosition(chatArea.getText().length() - 1);
                        }
                    }
                }else{
                    send(username, "§7changed name to " + user.getText());
                    username = user.getText();
                }
              }
    }                              

    private void formWindowClosing(java.awt.event.WindowEvent evt) {                                   

    }                                 

    private void formWindowOpened(java.awt.event.WindowEvent evt) {                               
        // TODO add your handling code here:
    }                                

    private void autoscrollCheckActionPerformed(java.awt.event.ActionEvent evt) {                                               
        // TODO add your handling code here:
    }                                              

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) throws IOException {

        /* Set the system look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            javax.swing.UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(chat.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(chat.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(chat.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(chat.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                new chat().setVisible(true);
            }
        });
        Socket socket = null;
        PrintWriter out = null;
        BufferedReader in = null;


        try {
            socket = new Socket("mc.xxx.net", 20060);
            out = new PrintWriter(socket.getOutputStream(), true);
            in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        } catch (UnknownHostException e) {
            System.err.println("Don't know about host");
            System.exit(1);
        } catch (IOException e) {
            System.err.println("Couldn't get I/O for the connection");
            System.exit(1);
        }

        BufferedReader read = new BufferedReader(new InputStreamReader(System.in));

                //System.out.println(in.readLine()); //Uncomment to debug

        out.println("/api/subscribe?source=chat&key=1e287587f5d1d45255f4708467eeaf8a71085f9ccfd8a354523d233cf5a77be4&show_previous=true");
        out.println("/api/subscribe?source=connections&key=e410592b70c0288654e6c1040edb0f21811dcb3f2ee11051163f36be9be00788&show_previous=false");

        while(true){
            String jsonString = in.readLine();
            JSONObject obj = JSONObject.fromObject(jsonString); 
            JSONObject success = obj.getJSONObject("success");
            if(success.get("message") != null){
                chatArea.append("<" + success.get("player") + "> " + success.get("message") + "\n\r");
                if(autoscrollCheck.isSelected()){
                    chatArea.setCaretPosition(chatArea.getText().length() - 1);
                }
            }else if (success.get("action") != null){
                chatArea.append(success.get("player") + " " + success.get("action") + "\n\r");
                if(autoscrollCheck.isSelected()){
                    chatArea.setCaretPosition(chatArea.getText().length() - 1);
                }
            }
        }
    }

    // Variables declaration - do not modify                     
    public static javax.swing.JCheckBox autoscrollCheck;
    public static javax.swing.JTextArea chatArea;
    private javax.swing.JTextField input;
    private javax.swing.JCheckBoxMenuItem jCheckBoxMenuItem1;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JToggleButton jToggleButton1;
    private javax.swing.JButton send;
    private javax.swing.JTextField user;
    private javax.swing.JButton userset;
    // End of variables declaration                


}

(P.S Please don’t get grumpy because I’m using a GUI generator, this is my first program, I promise I will learn to do it by hand )

Answer 1

It is probably a race condition which makes it work sometimes. The variable chatArea is not guaranteed to be initialized by the time the main thread gets to line 333. This is due to the deferred initialization of the GUI via invokeLater() some lines before that:

java.awt.EventQueue.invokeLater(new Runnable() {
    @Override
    public void run() {
        new chat().setVisible(true);
    }
});

You need some synchronization between those threads, or, what also should work, just initialize the GUI in the main thread:

final chat chatObject = new chat();
java.awt.EventQueue.invokeLater(new Runnable() {
    @Override
    public void run() {
        chatObject.setVisible(true);
    }
});