Java Runtime Errors (GUI Class)

Question

I'm trying to make a simple GUI program to calculate theater revenue (gross and net for adult/kids/total). I keep getting a very long list of runtime errors and I'm following my book's instructions as close as possible so I have no idea what is causing them.

Here's the code:

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;

public class TheaterRevenue extends JFrame
{
  private JTextField adultTicketPriceTextField;
  private JTextField adultTicketSoldTextField;
  private JTextField childTicketPriceTextField;
  private JTextField childTicketSoldTextField;
  public TheaterRevenue()
  {
    setTitle("Theater Revenue");
    setSize(400, 175);
    JButton calcButton = new JButton("Calculate");
    //Need JButton's Listener
    calcButton.addActionListener(new calcButtonListener());
    //TextFields
    JTextField adultTicketPriceTextField = new JTextField(20);
    JTextField adultTicketSoldTextField = new JTextField(20);
    JTextField childTicketPriceTextField = new JTextField(20);
    JTextField childTicketSoldTextField = new JTextField(20);
    //TextField Labels (20 is field length, adjust as needed)
    JLabel adultTickPrice = new JLabel("Adult Ticket Price");
    JLabel adultTickSold = new JLabel ("Adult Tickets Sold");
    JLabel childTickPrice = new JLabel ("Child Ticket Price");
    JLabel childTickSold = new JLabel ("Child Tickets Sold");
    //Panel Setup
    JPanel mainPanel = new JPanel();
    add(mainPanel);
    mainPanel.add(adultTickPrice);
    mainPanel.add(adultTicketPriceTextField);
    mainPanel.add(adultTickSold);
    mainPanel.add(adultTicketSoldTextField);
    mainPanel.add(childTickPrice);
    mainPanel.add(childTicketPriceTextField);
    mainPanel.add(childTickSold);
    mainPanel.add(childTicketSoldTextField);
    mainPanel.add(calcButton);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setVisible(true);
  }
  private class calcButtonListener implements ActionListener
  {
    public void actionPerformed(ActionEvent e)
    {
      String input1, input2, input3, input4;
      double adultTickPrice, adultTickSold, childTickPrice, childTickSold;
      input1 = adultTicketPriceTextField.getText();
      adultTickPrice = Double.parseDouble(input1);
      input2 = adultTicketSoldTextField.getText();
      adultTickSold = Double.parseDouble(input2);
      input3 = childTicketPriceTextField.getText();
      childTickPrice = Double.parseDouble(input3);
      input4 = childTicketSoldTextField.getText();
      childTickSold = Double.parseDouble(input4);
      //Calculations
      double adultGross, adultNet, childGross, childNet, totalGross, totalNet;
      final double keep = 0.20;
      adultGross = (adultTickPrice*adultTickSold);
      adultNet = ((adultTickPrice*adultTickSold)*keep);
      childGross = (childTickPrice*childTickSold);
      childNet = ((childTickPrice*adultTickSold)*keep);
      totalGross = (adultGross+childGross);
      totalNet = (adultNet+adultGross);
      JOptionPane.showMessageDialog(null, adultGross); 
      JOptionPane.showMessageDialog(null, adultNet);
      JOptionPane.showMessageDialog(null, childGross);
      JOptionPane.showMessageDialog(null, childNet);
      JOptionPane.showMessageDialog(null, totalGross);
      JOptionPane.showMessageDialog(null, totalNet);
    }
  }
  public static void main(String[] args)
  {
    new TheaterRevenue();
  }
}

When I run it and hit the Calculate button I get the following errors:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at TheaterRevenue$calcButtonListener.actionPerformed(TheaterRevenue.java:53)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$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$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$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)

input1 = adultTicketPriceTextField.getText(); I'm trying to get the data from the text field — Alex
– Alex, Commented Oct 15, 2014 at 22:51

MadProgrammer · Accepted Answer · 2014-10-15 22:55:36Z

3

You have a variable shadowing issue, that is, you declare a bunch of instance fields....

public class TheaterRevenue extends JFrame {

    private JTextField adultTicketPriceTextField;
    private JTextField adultTicketSoldTextField;
    private JTextField childTicketPriceTextField;
    private JTextField childTicketSoldTextField;

But then redeclare them as local variables (within your constructor)

public TheaterRevenue() {
    //...
    //TextFields
    JTextField adultTicketPriceTextField = new JTextField(20);
    JTextField adultTicketSoldTextField = new JTextField(20);
    JTextField childTicketPriceTextField = new JTextField(20);
    JTextField childTicketSoldTextField = new JTextField(20);

This means that when the actionPerformed method tries to access the values from these instance fields

public void actionPerformed(ActionEvent e) {
    //..
    input1 = adultTicketPriceTextField.getText();

The field is still null.

To fix it, remove the declarations within you constructor and make sure you are initialising the instance fields correctly...

public TheaterRevenue() {
    //...
    //TextFields
    adultTicketPriceTextField = new JTextField(20);
    adultTicketSoldTextField = new JTextField(20);
    childTicketPriceTextField = new JTextField(20);
    childTicketSoldTextField = new JTextField(20);

answered Oct 15, 2014 at 22:55

MadProgrammer

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5 Comments

Alex Over a year ago

Now it says those fields cannot be resolved to a variable. Do I have some visibility issues?

MadProgrammer Over a year ago

Works fine for me...you did remove the JTextField .... from within the constructor and the instance fields?

Alex Over a year ago

I removed the declarations from the constructor entirely and then removed JTextField from the class body.

MadProgrammer Over a year ago

You will need to the instance fields. You need to initialize the fields within the constructor

MadProgrammer Over a year ago

"you it did fix it" Oh whoops, wasn't suppose to do that <nudge><nudge> ;)

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