Thread: Code from textbook not working and I don't know why--Event-driven programming

Hello,

I am trying to learn event-driven programming, so I coded up an entire program from a textbook that is meant to simulate a monthly payment calculator for a loan. The loan amount, interest rate, and duration of the loan are given as inputs to JTextFields, and the payment amount is supposed to be output into a fourth JTextField.

But even though I copied the program line-for-line from the book, it will not work. The GUI looks fine, but when I try to calculate the payment, it gives me a LONG list of errors (which I confess I don't really know how to interpret--I would like to learn). I would really appreciate it if someone could tell me what is wrong with this code so that I have a correct program to learn from. Here is the code, followed by the errors:

public class LoanPayment    //a utility class that calculates monthly payment based on three //variables. The math is all based on a standard formula, also copied exactly from book.
{
    public static double getPayment(double amount, double interest, double years)
    {
        double payment = amount * ((interest/1200.0)/(1 - Math.pow(1 + interest/1200.0, -years * 12)));
        return (Math.round(payment * 100))/100.00;      //rounds to 2 decimal places
    }
}

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
 
public class LoanCalculator extends JFrame
{
    private JTextField amountField;
    private JTextField interestField;
    private JTextField yearsField;
    private JTextField paymentField;
    private JButton submitButton;
    private JButton clearButton;
    private JButton exitButton;
 
    public LoanCalculator()
    {
        super("Monthly Payment");
        setBounds(0, 0, 250, 200);
        JPanel panel = new JPanel();        //for text fields and labels
 
        //make a label for each text field; add the labels and text fields to the panel
        JLabel amountLabel = new JLabel();
        amountLabel.setFont(new Font("Courier", Font.BOLD, 12));
        amountLabel.setText("  Amount:");
        amountField = new JTextField(10);
        panel.add(amountLabel);
        panel.add(amountField);
 
        JLabel interestLabel = new JLabel();
        interestLabel.setFont(new Font("Courier", Font.BOLD, 12));
        interestLabel.setText("Interest:");
        interestField = new JTextField(10);
        panel.add(interestLabel);
        panel.add(interestField);
 
        JLabel yearsLabel = new JLabel();
        yearsLabel.setFont(new Font("Courier", Font.BOLD, 12));
        yearsLabel.setText("   Years:");
        yearsField = new JTextField(10);
        panel.add(yearsLabel);
        panel.add(yearsField);
 
        JLabel paymentLabel = new JLabel();
        paymentLabel.setFont(new Font("Courier", Font.BOLD, 12));
        paymentLabel.setText(" Payment:");
        JTextField paymentField = new JTextField(10);
        panel.add(paymentLabel);
        panel.add(paymentField);
        paymentField.setEditable(false);
 
        add(panel, BorderLayout.CENTER);
 
        //creat three buttons, add them to a new panel, add panel to bottom of frame
        JPanel buttonPanel = new JPanel();
        submitButton = new JButton("Submit");
        clearButton = new JButton("Clear");
        exitButton = new JButton("Exit");
        buttonPanel.add(submitButton);
        buttonPanel.add(clearButton);
        buttonPanel.add(exitButton);
        add(buttonPanel, BorderLayout.SOUTH);
 
        //register a listener with each button
        submitButton.addActionListener(new ButtonListener());
        clearButton.addActionListener(new ButtonListener());
        exitButton.addActionListener(new ButtonListener());
 
        setResizable(false);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
 
    private class ButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            if (e.getSource() == submitButton)
                try
                {
                    double amount = Double.parseDouble(amountField.getText());  //possible NumberFormatException
                    double interest = Double.parseDouble(interestField.getText());
                    double years = Double.parseDouble(yearsField.getText());
                    double payment = LoanPayment.getPayment(amount, interest, years);
                    //setText() requires a String reference; 'payment + ""' returns a String
                    paymentField.setText(payment + "");
                }
                catch (NumberFormatException excep)
                {
                    paymentField.setText("Illegal input");
                }
            else if (e.getSource() == clearButton)
            {
                amountField.setText("");
                interestField.setText("");
                yearsField.setText("");
                paymentField.setText("");
            }
            else
                System.exit(0);
        }        
    }
    public static void main(String[] args)
    {
        LoanCalculator frame = new LoanCalculator();
    }
}

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at LoanCalculator$ButtonListener.actionPerformed(LoanCalculator.java:85)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
	at javax.swing.AbstractButton$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:6288)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
	at java.awt.Component.processEvent(Component.java:6053)
	at java.awt.Container.processEvent(Container.java:2041)
	at java.awt.Component.dispatchEventImpl(Component.java:4651)
	at java.awt.Container.dispatchEventImpl(Container.java:2099)
	at java.awt.Component.dispatchEvent(Component.java:4481)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
	at java.awt.Container.dispatchEventImpl(Container.java:2085)
	at java.awt.Window.dispatchEventImpl(Window.java:2478)
	at java.awt.Component.dispatchEvent(Component.java:4481)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:643)
	at java.awt.EventQueue.access$000(EventQueue.java:84)
	at java.awt.EventQueue$1.run(EventQueue.java:602)
	at java.awt.EventQueue$1.run(EventQueue.java:600)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
	at java.awt.EventQueue$2.run(EventQueue.java:616)
	at java.awt.EventQueue$2.run(EventQueue.java:614)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:613)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Thanks!

Thanks for the reply. No, this did not fix the problem. Keep in mind that this is from a textbook, so I'm not looking for a redesign here. Assuming that the authors are not complete idiots, this has to be something simple that I'm just not seeing.

The method in LoanPayment is static because it is a utility class. Not to be argumentative, but I've seen it done this way before and it works fine. Again, I don't want to fix the aesthetics of the program that two computer scientists wrote--just the error.

Originally Posted by BloomingNutria

Thanks for the reply. No, this did not fix the problem. Keep in mind that this is from a textbook, so I'm not looking for a redesign here. Assuming that the authors are not complete idiots, this has to be something simple that I'm just not seeing.

The method in LoanPayment is static because it is a utility class. Not to be argumentative, but I've seen it done this way before and it works fine. Again, I don't want to fix the aesthetics of the program that two computer scientists wrote--just the error.

Alright.

Well I copyed & pasted your code, and I found the problem. You didn't create "paymentField = new JTextField();", in my IDE, your paymentField comes up as inactive.

You seem to have written "JTextField paymentField = new JTextField(10);" when you already have "private JTextField paymentField;" at the top as well.

Aha! Thanks, everyone. I just couldn't spot it. I guess the author just put that in twice (I double checked, and it is that way in the book), and I didn't notice. I think my main problem is that I just need to figure out how to read those error messages.

I just need to figure out how to read those error messages.

Too bad your helpers didn't help you find the problem instead finding the problem for you.

Originally Posted by Norm

Too bad your helpers didn't help you find the problem instead finding the problem for you.

I learned to read errors with someone pointing them out to me, and I find I can read errors myself fine now.
I'm sure they'll figure it out down the line.