Please fix these errors for me...

Question

Here is my code...
When I compiled it, it said
Exception in thread "main" java.lang.NullPointException
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at javax.swing.JFrame.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at SecondsCalc.<init>(SecondsCalc.java:43)
at SecondsCalc.main(SecondsCalc.java.166)
Please fix these errore for me..Thank you!!
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
public class SecondsCalc extends JFrame
private JPanel panel;   // To reference a panel
private JLabel messageLabel1; // To reference a label for days
private JLabel messageLabel2; // To reference a label for hours
private JLabel messageLabel3; // To reference a label for minutes
private JLabel messageLabel4; // To reference a label for minutes
private JTextField daysTextField; // To reference a text field for days
private JTextField hoursTextField; // To reference a text field for hours
private JTextField minutesTextField; // To reference a text field for minutes
private JTextField secondsTextField; // To reference a text field for seconds
private JButton calcButton;  // To reference a calculate button
private JButton exitButton;  // To reference a exit button
private final int WINDOW_WIDTH = 310; // Window width
private final int WINDOW_HEIGHT = 100; // Window height
  Constructor
public SecondsCalc()
  // Set the window title.
  setTitle("Seconds Calculator");
  // Set the size of the window.
  setSize(WINDOW_WIDTH, WINDOW_HEIGHT);
  // Specify what happens when the close button is clicked.
  setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  // Add a GridLayout manager to the content pane.
  setLayout(new GridLayout(6, 1));
  // Build the panel and add it to the frame.
  buildPanel();
  // Add the panel to the frame's content pane.
  add(panel);
  // Display the window.
  setVisible(true);
  The buildPanel method adds a label, text field, and
  add buttons to a panel.
private void buildPanel()
  // Create a label for days to display instructions.
  messageLabel1 = new JLabel("days");
  // Create a text field for days 10 characters wide.
  daysTextField = new JTextField(10);
  // Create a label for hours to display instructions.
  messageLabel2 = new JLabel("hours");
  // Create a text field for hours 10 characters wide.
  hoursTextField = new JTextField(10);
  // Create a label for minutes to display instructions.
  messageLabel3 = new JLabel("minutes");
  // Create a text field for minutes 10 characters wide.
  minutesTextField = new JTextField(10);
  // Create a label for seconds to display instructions.
  messageLabel4 = new JLabel("seconds");
  // Create a text field for seconds 10 characters wide.
  secondsTextField = new JTextField(10);
  // Create a calculate button with the caption "Calculate".
  calcButton = new JButton("Calculate");
  // Create a exit button with the caption "Exit".
  exitButton = new JButton("Exit");
  // Register the action listeners.
  calcButton.addActionListener(new CalcButtonListener());
  exitButton.addActionListener(new ExitButtonListener());
  // Create a JPanel object and let the panel
  // field reference it.
  JPanel panel1 = new JPanel();
  JPanel panel2 = new JPanel();
  JPanel panel3 = new JPanel();
  JPanel panel4 = new JPanel();
  JPanel panel5 = new JPanel();
  JPanel panel6 = new JPanel();
  // Add the label, text field, and button
  // components to the panel.
  panel1.add(messageLabel1);
  panel2.add(messageLabel2);
  panel3.add(messageLabel3);
  panel4.add(messageLabel4);
  panel1.add(daysTextField);
  panel2.add(hoursTextField);
  panel3.add(minutesTextField);
  panel4.add(secondsTextField);
  panel5.add(calcButton);
  panel6.add(exitButton);
  add(panel1);
  add(panel2);
  add(panel3);
  add(panel4);
  add(panel5);
  add(panel6);
  CalcButtonListener is an action listener class for
  the Calculate button.
private class CalcButtonListener implements ActionListener
        public void actionPerformed(ActionEvent e)
            int seconds; // The number of seconds
            int min;
            int day;
            int hour;
            long result;
            // Get the text entered by the user into the
            // text fields.
            day = Integer.parseInt(daysTextField.getText());
   hour = Integer.parseInt(hoursTextField.getText());
            min = Integer.parseInt(minutesTextField.getText());
            seconds = Integer.parseInt(secondsTextField.getText());
   //Actullly gettext method returns string and you need to convert it into int or double value using Integer.parseInt method.
            // Calculate to seconds.
   result = (day *((hour * 60 * 60) + (min * 60) +seconds));
   // Display the result.
             JOptionPane.showMessageDialog(null, result + " seconds");
  Private inner class that handles the event when
  the user clicks the Exit button.
private class ExitButtonListener implements ActionListener
  public void actionPerformed(ActionEvent e)
   System.exit(0);
public static void main(String[] args)
  SecondsCalc ac = new SecondsCalc();
}

Answer 1

fgb wrote:
Exception in thread "main" java.lang.NullPointException
at SecondsCalc.<init>(SecondsCalc.java:43)
at SecondsCalc.main(SecondsCalc.java.166)Something is null on line 43. What does that line do and why is it needed?That's right!!
I did not need to put line 43, which was
// Add the panel to the frame's content pane.
add(panel);
Thank you very much!!!