public class BusPayment extends JFrame{
private JButton paynow;
private JButton cancel;
private JComboBox<String> jComboBox1;
private JFrame frame;
private JLabel label1, label2, label3, label4;
private JPanel panel1, panel2,panel3;
private JRadioButton destination1;
private JRadioButton destination2;
private JRadioButton destination3;
private JRadioButton destination4;
private JRadioButton destination5;
private JRadioButton destination6;
private JComboBox company;
private JTextField passengerstextfield;

public BusPayment (){

    frame.setTitle("Express Bus Ticketing Management System");
    frame.setSize(1400,700);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
    buildPanel();
    panel1.add(label1);
    panel2.add(label2);
    panel2.add(passengerstextfield);
    panel2.add(label3);
    panel2.add(company);
    panel3.add(destination1);
    panel3.add(destination2);
    panel3.add(destination3);
    panel3.add(destination4);
    panel3.add(destination5);
    panel3.add(destination6);
    panel3.add(paynow);
    panel3.add(cancel);

}

private void buildPanel(){

    panel1 =new JPanel();
    label1=new JLabel("Welcome to the Express Bus Ticketing System! ");

    panel2=new JPanel();
    label2=new JLabel("Number of Passenger(s): ");
    passengerstextfield=new JTextField(8);

    label3=new JLabel("Selected Bus Company: ");
    final JComboBox<String> comp = new JComboBox<>(new String[] {"Company1", "Company2", "Company3", "Company4"});

    panel3=new JPanel();
    label4=new JLabel("Select a ticket: ");

    destination1=new JRadioButton ("Destination 1 RM15");
    JRadioButton des1 = new JRadioButton(Double.toString(15));
    des1.setSelected(true);   
    des1.add(des1);

    destination2=new JRadioButton ("Destination 2 RM18");
    JRadioButton des2 = new JRadioButton(Double.toString(18));
    des2.setSelected(true);    
    des2.add(des2);

    destination3=new JRadioButton ("Destination 3 RM15");
    JRadioButton des3 = new JRadioButton(Double.toString(15));
    des3.setSelected(true);    
    des3.add(des3);

    destination4=new JRadioButton ("Destination 4 RM12");
    JRadioButton des4 = new JRadioButton(Double.toString(12));
    des4.setSelected(true);    
    des4.add(des4);

    destination5=new JRadioButton ("Destination 5 RM16");
    JRadioButton des5 = new JRadioButton(Double.toString(16));
    des5.setSelected(true);   
    des5.add(des5);

    destination6=new JRadioButton ("Destination 6 RM13");
    JRadioButton des6 = new JRadioButton(Double.toString(13));
    des6.setSelected(true);   
    des6.add(des6);

    paynow=new JButton ("Proceed");
    paynow.addActionListener(new PayNowListener());
    cancel=new JButton ("Cancel");
    cancel.addActionListener(new CancelListener());

    panel1.add(label1);
    panel2.add(label2);
    panel2.add(passengerstextfield);
    panel2.add(label3);
    panel2.add(comp);
    panel3.add(des1);
    panel3.add(des2);
    panel3.add(des3);
    panel3.add(des4);
    panel3.add(des5);
    panel3.add(des6);
    panel3.add(paynow);
    panel3.add(cancel);

}

private void destination1ActionPerformed(ActionEvent e) {                                              
  String price = e.getActionCommand();
  System.out.println("Selected ticket price = RM " +  price);   
}                                             

private void destination2ActionPerformed(ActionEvent e) {                                              
  String price = e.getActionCommand();
  System.out.println("Selected ticket price = RM " +  price);       }                                             

private void destination3ActionPerformed(ActionEvent e) {                                              
    String price = e.getActionCommand();
    System.out.println("Selected ticket price = RM " +  price);   
}                                             

private void destination4ActionPerformed(ActionEvent e) {                                              
    String price = e.getActionCommand();
    System.out.println("Selected ticket price = RM " +  price);   
}                                             

private void destination5ActionPerformed(ActionEvent e) {                                           
    String price = e.getActionCommand();
    System.out.println("Selected ticket price = RM " +  price);   
}     

 private void destination6ActionPerformed(ActionEvent e) {                                           
    String price = e.getActionCommand();
    System.out.println("Selected ticket price = RM " +  price);   
} 

    private class PayNowListener implements ActionListener{
    @Override
    public void actionPerformed(ActionEvent ae){

        panel3.setBackground(Color.CYAN);

        double passengers;
        double price;
        double tax;
        double payment;
        DecimalFormat nf = new DecimalFormat("0.00");

        try{
        passengers=Double.parseDouble(passengerstextfield.getText());
        price=
        tax=0.06;//6%

        payment=passengers*price*tax+(passengers*price);
        JOptionPane.showMessageDialog(null, "Sub-Total: " + nf.format(passengers*price));
        JOptionPane.showMessageDialog(null, "Tax: " + nf.format(tax));
        JOptionPane.showMessageDialog(null, "Your total amount is " + nf.format(payment));
    } catch (NumberFormatException nfe)
    {
        JOptionPane.showMessageDialog(null, "Invalid Input", "Alert", JOptionPane.ERROR_MESSAGE);
    }
    }
}

        private class CancelListener implements ActionListener{
            @Override
            public void actionPerformed(ActionEvent ae){
                System.exit(0);
            }
        }

    public static void main (String [] args){
            BusPayment mycalc = new BusPayment();

        }

I got null pointer exception because you did not initialize frame.

frame = new JFrame();

commented: Oh I see. Is there anything else that is wrong in my code? +0

Your class is already extend to JFrame. So you don't need to create a new JFrame instance.

public class BusPayment extends JFrame {

    private JButton paynow;
    private JButton cancel;
    private JComboBox<String> jComboBox1;
    private JFrame frame;
    private JLabel label1, label2, label3, label4;
    private JPanel panel1, panel2, panel3;
    private JRadioButton destination1;
    private JRadioButton destination2;
    private JRadioButton destination3;
    private JRadioButton destination4;
    private JRadioButton destination5;
    private JRadioButton destination6;
    private JComboBox company;
    private JTextField passengerstextfield;

    public BusPayment() {
        buildPanel();
    }

    private void buildPanel() {
        panel1 = new JPanel();
        label1 = new JLabel("Welcome to the Express Bus Ticketing System! ");
        panel2 = new JPanel();
        label2 = new JLabel("Number of Passenger(s): ");
        company = new JComboBox();
        passengerstextfield = new JTextField(8);
        label3 = new JLabel("Selected Bus Company: ");
        final JComboBox<String> comp = new JComboBox<>(new String[]{"Company1", "Company2", "Company3", "Company4"});
        panel3 = new JPanel();
        label4 = new JLabel("Select a ticket: ");
        destination1 = new JRadioButton("Destination 1 RM15");
        JRadioButton des1 = new JRadioButton(Double.toString(15));
        des1.setSelected(true);
//        des1.add(des1);
        destination2 = new JRadioButton("Destination 2 RM18");
        JRadioButton des2 = new JRadioButton(Double.toString(18));
        des2.setSelected(true);
//        des2.add(des2);
        destination3 = new JRadioButton("Destination 3 RM15");
        JRadioButton des3 = new JRadioButton(Double.toString(15));
        des3.setSelected(true);
//        des3.add(des3);
        destination4 = new JRadioButton("Destination 4 RM12");
        JRadioButton des4 = new JRadioButton(Double.toString(12));
        des4.setSelected(true);
//        des4.add(des4);
        destination5 = new JRadioButton("Destination 5 RM16");
        JRadioButton des5 = new JRadioButton(Double.toString(16));
        des5.setSelected(true);
//        des5.add(des5);
        destination6 = new JRadioButton("Destination 6 RM13");
        JRadioButton des6 = new JRadioButton(Double.toString(13));
        des6.setSelected(true);
//    des6.add(des6);
        paynow = new JButton("Proceed");
        paynow.addActionListener(new PayNowListener());
        cancel = new JButton("Cancel");
        cancel.addActionListener(new CancelListener());

        panel1.add(label1);
        panel2.add(label2);
        panel2.add(passengerstextfield);
        panel2.add(label3);
        panel2.add(company);
        panel3.add(destination1);
        panel3.add(destination2);
        panel3.add(destination3);
        panel3.add(destination4);
        panel3.add(destination5);
        panel3.add(destination6);
        panel3.add(paynow);
        panel3.add(cancel);
        add(panel1);
        add(panel2);
        add(panel3);
    }

    private void destination1ActionPerformed(ActionEvent e) {
        String price = e.getActionCommand();
        System.out.println("Selected ticket price = RM " + price);
    }

    private void destination2ActionPerformed(ActionEvent e) {
        String price = e.getActionCommand();
        System.out.println("Selected ticket price = RM " + price);
    }

    private void destination3ActionPerformed(ActionEvent e) {
        String price = e.getActionCommand();
        System.out.println("Selected ticket price = RM " + price);
    }

    private void destination4ActionPerformed(ActionEvent e) {
        String price = e.getActionCommand();
        System.out.println("Selected ticket price = RM " + price);
    }

    private void destination5ActionPerformed(ActionEvent e) {
        String price = e.getActionCommand();
        System.out.println("Selected ticket price = RM " + price);
    }

    private void destination6ActionPerformed(ActionEvent e) {
        String price = e.getActionCommand();
        System.out.println("Selected ticket price = RM " + price);
    }

    private class PayNowListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent ae) {
            panel3.setBackground(Color.CYAN);
            double passengers;
            double price;
            double tax;
            double payment;
            DecimalFormat nf = new DecimalFormat("0.00");
            try {
                passengers = Double.parseDouble(passengerstextfield.getText());
                price
                        = tax = 0.06;//6%
                payment = passengers * price * tax + (passengers * price);
                JOptionPane.showMessageDialog(null, "Sub-Total: " + nf.format(passengers * price));
                JOptionPane.showMessageDialog(null, "Tax: " + nf.format(tax));
                JOptionPane.showMessageDialog(null, "Your total amount is " + nf.format(payment));
            } catch (NumberFormatException nfe) {
                JOptionPane.showMessageDialog(null, "Invalid Input", "Alert", JOptionPane.ERROR_MESSAGE);
            }
        }
    }

    private class CancelListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent ae) {
            System.exit(0);
        }
    }

    public static void main(String[] args) {
        JavaApplication5 mycalc = new JavaApplication5();
        mycalc.setTitle("Express Bus Ticketing Management System");
        mycalc.pack();
        mycalc.setLocationRelativeTo(null);
        mycalc.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mycalc.setVisible(true);

    }
}

Output

155776141993025236.png