Question

below I have got the code for the connector, then i have code for another class called viewHistory. now, what i was trying to do is to enable a user to enter a studentID number, and then the relevant attendance history should come up. It’s however compiloing but giving me an error message when i type in a number to check if it shows anything. Any suggestion?

public class Connector {
Connection con;
PreparedStatement stmt;
ResultSet rs;

Connector()
{
    try{


        Class.forName("com.mysql.jdbc.Driver");

        con=DriverManager.getConnection("jdbc:mysql://localhost/luxmidatabase","root","");
                    stmt=con.prepareStatement("select * from login where username=? and password=?");
                    }

    catch (Exception e) 
    {
        System.out.println(e);
    }
}
}

2nd CLASS

public class AttendanceHistory extends Connector
{

public AttendanceHistory() {
    initialize();
}

public JFrame frmAttendanceHistory;

Connector con;
private JTextField textField;

// initialise the frame
private void initialize() {
    frmAttendanceHistory = new JFrame();
    frmAttendanceHistory.setTitle("LEC AdminPro: Attendance History");
    frmAttendanceHistory.setBounds(100, 100, 323, 149);
    frmAttendanceHistory.setDefaultCloseOperation(JFrame.HIDE_ON_CLOSE);
    frmAttendanceHistory.setLocationRelativeTo(null);
    // Initialising a Grey Border, which I am going to be using later to add borders to JLabels.
    Border border = LineBorder.createGrayLineBorder();
    frmAttendanceHistory.getContentPane().setLayout(null);
    con = new Connector();

    JPanel panel_1 = new JPanel();
    panel_1.setBounds(0, 0, 307, 113);
    frmAttendanceHistory.getContentPane().add(panel_1);
    panel_1.setBorder(border);
    panel_1.setLayout(null);

    JLabel lblRegister = new JLabel("View Attendance History");
    lblRegister.setFont(new Font("Tahoma", Font.BOLD, 12));
    lblRegister.setBounds(10, 11, 193, 20);
    panel_1.add(lblRegister);

    JLabel lblStudentId = new JLabel("Student ID");
    lblStudentId.setFont(new Font("Arial", Font.PLAIN, 11));
    lblStudentId.setBounds(10, 45, 69, 14);
    panel_1.add(lblStudentId);

    // Created a Cancel button along with an OptionDialog which pops up to confirm whether user wants to cancel registration or not.
    JButton btnNewButton = new JButton("Cancel");
    btnNewButton.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
            Object[] options = {"Yes", "No"};
            Component form = null;
            int n = JOptionPane.showOptionDialog(form, "Would you like to cancel the progress?", "Exit Confirmation", JOptionPane.YES_NO_CANCEL_OPTION, JOptionPane.QUESTION_MESSAGE,  null, options, options);
            if(n == JOptionPane.YES_OPTION) {
                frmAttendanceHistory.setVisible(false);
            }

        }
    });
    btnNewButton.setBounds(159, 73, 75, 23);
    panel_1.add(btnNewButton);

    JButton button = new JButton("Submit");
    button.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {


        //String studentID = textField.getText();   

        try {


            con.stmt.executeQuery("SELECT StudentID, date FROM attendance WHERE StudentID = 21457");            
            while ( rs.next() ) {
                String date = rs.getString("date");
                System.out.println(date);
            }

            JOptionPane.showMessageDialog(frmAttendanceHistory, "Attendance has been registered.");
            frmAttendanceHistory.setVisible(false);

        } catch (SQLException e) {
        //System.out.println("Record couldn't be added!");
        JOptionPane.showMessageDialog(frmAttendanceHistory, "Attendance couldn't be registered. Please try again!");
        e.printStackTrace();
        }
        }
        });



    button.setBounds(81, 73, 75, 23);
    panel_1.add(button);

    textField = new JTextField();
    textField.setBounds(81, 42, 212, 20);
    panel_1.add(textField);
    textField.setColumns(10);




}




public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {

        public void run() {
            try {
            AttendanceHistory window = new AttendanceHistory();
            window.frmAttendanceHistory.setVisible(true);
            } catch (Exception e) {
            e.printStackTrace();
            }
        }
    });
}



// method to make this frame visible (may be used in other classes for retrieval)
public void setVisible(boolean b) {
    frmAttendanceHistory.setVisible(true);

}    
}

Main Issue is the code inside the Submit Button. Thats where I try to filter the data.

This is the error Message:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at AttendanceHistory$2.actionPerformed(AttendanceHistory.java:85)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

In the comments you said that the line where the error happens is

while ( rs.next() ) {

The NullPointerException happens because rs is null. I don’t see that you’re initializing rs anywhere in your code. Probably the line above should have been:

ResultSet rs = con.stmt.executeQuery(...);

(Where are you declaring the variable rs?).

Another problem: You’re re-using the Statement object that you had prepared in the Connector object with the select * from login ... SQL. You should create a new statement to execute the query instead. Something like this:

Statement st = con.con.createStatement();
ResultSet rs = st.executeQuery("SELECT StudentID, date from ...");
while (rs.hasNext()) {
    // ...
}

Note that you should also properly discard the Statement and ResultSet objects by calling close() on them. (Preferably in a finally block so that it happens even in case of an exception).