Query from mySQL java.lang.NullPointerException

Question

I have compile and run the following code but it was an error (java.lang.NullPointerException) when I click the loginButton. Please help me, I don't know how to solve the problem.
/* * Main.java * * Created on February 19, 2008, 2:50 AM */ package dataprotect; import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.SQLException; import java.sql.Statement; import javax.swing.*; import javax.swing.JOptionPane; import dataprotect.SecretQuest; public class Main extends javax.swing.JFrame {         /** Creates new form Main */     private Connection connection;     private Statement statement;     private ResultSet rs; /** * * @author  Aiman */     public void connectToDB() {         try {             connection= DriverManager.getConnection("jdbc:mysql://localhost/data_protect?user=root&password=password");             statement = connection.createStatement();                     } catch (SQLException connectException) {             System.out.println(connectException.getMessage());             System.out.println(connectException.getSQLState());             System.out.println(connectException.getErrorCode());             System.exit(1);         }     }     private void init() {         connectToDB();     }             /** Creates new form Main */     public Main() {         initComponents();     }         /** 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.     */     // <editor-fold defaultstate="collapsed" desc=" Generated Code ">                              private void initComponents() {         loginPanel = new javax.swing.JPanel();         userField = new javax.swing.JTextField();         passLabel = new javax.swing.JLabel();         passField = new javax.swing.JPasswordField();         loginButton = new javax.swing.JButton();         userLabel = new javax.swing.JLabel();         jLabel1 = new javax.swing.JLabel();         registerPanel = new javax.swing.JPanel();         regLabel = new javax.swing.JLabel();         registerButton = new javax.swing.JButton();         setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);         setTitle("Data Protector");         loginPanel.setBorder(javax.swing.BorderFactory.createLineBorder(new java.awt.Color(0, 0, 0)));         passLabel.setText("Password:");         loginButton.setText("Login");         loginButton.addActionListener(new java.awt.event.ActionListener() {             public void actionPerformed(java.awt.event.ActionEvent evt) {                 loginButtonActionPerformed(evt);             }         });         userLabel.setText("Username:");         javax.swing.GroupLayout loginPanelLayout = new javax.swing.GroupLayout(loginPanel);         loginPanel.setLayout(loginPanelLayout);         loginPanelLayout.setHorizontalGroup(             loginPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)             .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, loginPanelLayout.createSequentialGroup()                 .addContainerGap()                 .addGroup(loginPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)                     .addComponent(userLabel, javax.swing.GroupLayout.DEFAULT_SIZE, 89, Short.MAX_VALUE)                     .addComponent(passLabel, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, 89, Short.MAX_VALUE))                 .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)                 .addGroup(loginPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)                     .addComponent(loginButton)                     .addComponent(passField, javax.swing.GroupLayout.DEFAULT_SIZE, 134, Short.MAX_VALUE)                     .addComponent(userField, javax.swing.GroupLayout.DEFAULT_SIZE, 134, Short.MAX_VALUE))                 .addContainerGap())         );         loginPanelLayout.setVerticalGroup(             loginPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)             .addGroup(loginPanelLayout.createSequentialGroup()                 .addContainerGap()                 .addGroup(loginPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)                     .addComponent(userField, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)                     .addComponent(userLabel))                 .addGap(16, 16, 16)                 .addGroup(loginPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)                     .addComponent(passLabel)                     .addComponent(passField, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))                 .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 21, Short.MAX_VALUE)                 .addComponent(loginButton)                 .addContainerGap())         );         jLabel1.setFont(new java.awt.Font("Tahoma", 1, 18));         jLabel1.setText("Data Protector");         registerPanel.setBorder(javax.swing.BorderFactory.createEtchedBorder());         regLabel.setText("If you are first user? Please register first.");         registerButton.setText("Register");         registerButton.addActionListener(new java.awt.event.ActionListener() {             public void actionPerformed(java.awt.event.ActionEvent evt) {                 registerButtonActionPerformed(evt);             }         });         javax.swing.GroupLayout registerPanelLayout = new javax.swing.GroupLayout(registerPanel);         registerPanel.setLayout(registerPanelLayout);         registerPanelLayout.setHorizontalGroup(             registerPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)             .addGroup(registerPanelLayout.createSequentialGroup()                 .addGroup(registerPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)                     .addGroup(registerPanelLayout.createSequentialGroup()                         .addGap(93, 93, 93)                         .addComponent(registerButton))                     .addGroup(registerPanelLayout.createSequentialGroup()                         .addGap(28, 28, 28)                         .addComponent(regLabel)))                 .addContainerGap(18, Short.MAX_VALUE))         );         registerPanelLayout.setVerticalGroup(             registerPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)             .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, registerPanelLayout.createSequentialGroup()                 .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)                 .addComponent(regLabel)                 .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)                 .addComponent(registerButton)                 .addContainerGap())         );         javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());         getContentPane().setLayout(layout);         layout.setHorizontalGroup(             layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)             .addGroup(layout.createSequentialGroup()                 .addContainerGap(72, Short.MAX_VALUE)                 .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)                     .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()                         .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING, false)                             .addComponent(registerPanel, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)                             .addComponent(loginPanel, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))                         .addGap(67, 67, 67))                     .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()                         .addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, 146, javax.swing.GroupLayout.PREFERRED_SIZE)                         .addGap(114, 114, 114))))         );         layout.setVerticalGroup(             layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)             .addGroup(layout.createSequentialGroup()                 .addContainerGap()                 .addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, 33, javax.swing.GroupLayout.PREFERRED_SIZE)                 .addGap(15, 15, 15)                 .addComponent(loginPanel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)                 .addGap(25, 25, 25)                 .addComponent(registerPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)                 .addGap(27, 27, 27))         );         pack();     }// </editor-fold>                            private void registerButtonActionPerformed(java.awt.event.ActionEvent evt) {                                              // TODO add your handling code here:   //      registerPanel a = new MainFrame();       //      a.show();                        this.setVisible(false);     }                                                  private void loginButtonActionPerformed(java.awt.event.ActionEvent evt) {                                                            Connection conn = null;         Statement stmt = null;         ResultSet rs = null;           try {         Statement statement = connection.createStatement();         rs = statement.executeQuery("select username, password from system");                         // create a statement             stmt = conn.createStatement();             // extract data from the ResultSet                 String username = userField.getText();         String password = new String(passField.getPassword());         if(username.equalsIgnoreCase(password)) {             JOptionPane.showMessageDialog(this, "Login Successful");             SecretQuest a = new SecretQuest();             a.show();                        this.setVisible(false);   }         else{     JOptionPane.showMessageDialog(this, "Incorrect username/password combination.", "Login Error", JOptionPane.ERROR_MESSAGE);                 } } catch (Exception e) {             e.printStackTrace();             System.exit(1);         }     /*  try {                 java.sql.Statement s = conn.createStatement();                 java.sql.ResultSet r = s.executeQuery                 ("SELECT username, password FROM system");                 while(r.next()) {                         System.out.println (                                 r.getString("username") + " " +                                 r.getString("password") );                         }         }         catch (Exception e) {                 System.out.println(e);                 System.exit(0);                 } */     }                                                  /**     * @param args the command line arguments     */     public static void main(String args[]) {         java.awt.EventQueue.invokeLater(new Runnable() {             public void run() {                 new Main().setVisible(true);             }         });     }         // Variables declaration - do not modify                        private javax.swing.JLabel jLabel1;     private javax.swing.JButton loginButton;     private javax.swing.JPanel loginPanel;     private javax.swing.JPasswordField passField;     private javax.swing.JLabel passLabel;     private javax.swing.JLabel regLabel;     private javax.swing.JButton registerButton;     private javax.swing.JPanel registerPanel;     private javax.swing.JTextField userField;     private javax.swing.JLabel userLabel;     // End of variables declaration                      }
I'm using mysql-connector-java-5.0.8 and mysql version 5.0...Any ideas?
Thanks a lot for your help =)

Answer 1

Its look can't solve the error..I try to change from:
// create a statement
        stmt = conn.createStatement();
        // extract data from the ResultSetto this one as you said:
// create a statement
        stmt = connection.createStatement();
        // extract data from the ResultSetBut the same problem was happen.
After the error it appear this on output windows of NetBean 5.5:
at dataprotect.Main.loginButtonActionPerformed(Main.java:198)
        at dataprotect.Main.access$000(Main.java:17)
        at dataprotect.Main$1.actionPerformed(Main.java:76)
        at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
        at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
        at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
        at java.awt.Component.processMouseEvent(Component.java:6038)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
        at java.awt.Component.processEvent(Component.java:5803)
        at java.awt.Container.processEvent(Container.java:2058)
        at java.awt.Component.dispatchEventImpl(Component.java:4410)
        at java.awt.Container.dispatchEventImpl(Container.java:2116)
        at java.awt.Component.dispatchEvent(Component.java:4240)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
        at java.awt.Container.dispatchEventImpl(Container.java:2102)
        at java.awt.Window.dispatchEventImpl(Window.java:2429)
        at java.awt.Component.dispatchEvent(Component.java:4240)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)Edited by: Aiman on Feb 25, 2008 5:43 AM