I am creating a Java program that uses a GUI to display a mortgage payment. I am trying to output to a textArea the payments that will be made for the mortgage. Here is my code:
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import java.text.DecimalFormat;
public class MortgageGui extends JFrame implements ActionListener {
// Set two-places for decimal format
DecimalFormat twoPlaces = new DecimalFormat("$0.00");
// Declare variable for calculation
Double loanAmt;
double interestRate;
double monthlyPayment;
int payment;
String amount;
JTextField loanAmount;
JComboBox loanTypeBox;
JLabel paymentOutput;
JTextArea paymentList;
// Build arrays for mortgages
double[] mortgage1 = {7.0, 5.35, 0.0}; // (years, interest, monthly payment)
double[] mortgage2 = {15.0, 5.5, 0.0}; // (years, interest, monthly payment)
double[] mortgage3 = {30.0, 5.75, 0.0}; // (years, interest, monthly payment)
public MortgageGui() {
super("Mortgage Calculator");
setLookAndFeel();
setSize(350, 500);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// Loan Amount Panel
JPanel loanAmtPanel = new JPanel();
JLabel loanAmtLabel = new JLabel("Loan Amount: ", JLabel.LEFT);
JTextField loanAmount = new JTextField(10);
loanAmtPanel.add(loanAmtLabel);
loanAmtPanel.add(loanAmount);
// Loan Type Panel
JPanel loanTypePanel = new JPanel();
JLabel loanTypeLabel = new JLabel("Loan Type: ", JLabel.LEFT);
String[] items = {"7 years at 5.35%", "15 years at 5.5%", "30 years at 5.75%"};
JComboBox loanTypeBox = new JComboBox(items);
loanTypePanel.add(loanTypeLabel);
loanTypePanel.add(loanTypeBox);
// Calculate Button Panel
JPanel calculatePanel = new JPanel();
JButton calcButton = new JButton("Calculate Paytment");
calcButton.addActionListener(this);
calculatePanel.add(calcButton);
// Monthly Payment Panel
JPanel paymentPanel = new JPanel();
JLabel paymentLabel = new JLabel("Monthly Payment: ", JLabel.LEFT);
JLabel paymentOutput = new JLabel("Calculated Payment");
paymentPanel.add(paymentLabel);
paymentPanel.add(paymentOutput);
// View Payments Panel
JPanel viewPayments = new JPanel();
JTextArea paymentList = new JTextArea("", 17, 27);
paymentList.setEditable(false);
paymentList.setLineWrap(true);
viewPayments.add(paymentList);
// Add panels to win Panel
JPanel win = new JPanel();
BoxLayout box = new BoxLayout(win, BoxLayout.Y_AXIS);
win.setLayout(box);
win.add(loanAmtPanel);
win.add(loanTypePanel);
win.add(calculatePanel);
win.add(paymentPanel);
win.add(viewPayments);
add(win);
// Make window visible
setVisible(true);
}
private void setLookAndFeel() {
try {
UIManager.setLookAndFeel(
"com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel"
);
} catch (Exception exc) {
// ignore
}
}
public void actionPerformed(ActionEvent e) {
// Clear payment list
paymentList.setText("");
// Get loan amount from textfield
String amount = loanAmount.getText();
loanAmt = Double.valueOf(amount).doubleValue();
// Find which mortgate array to use from combobox
Object obj = loanTypeBox.getSelectedItem();
String loanSelected = obj.toString();
// Run the calculation based on the mortgage arrays
// 7-year loan
if (loanSelected.equals("7 years at 5.35%")) {
// Calculate payment, interest, and remaining
mortgage1[2] = (loanAmt + (loanAmt * (mortgage1[1] / 100))) / (mortgage1[0] * 12);
double interest1 = (loanAmt * (mortgage1[1] / 100)) / 84;
double amountRemaining1 = loanAmt + (loanAmt * (mortgage1[1] / 100));
// Loop through payments
for (int payment = 1; payment <=84; payment++) {
// Deduct one payment from the balance
amountRemaining1 = amountRemaining1 - mortgage1[2];
// Write payment to textArea
paymentList.append("Payment " + payment + ": $" + twoPlaces.format(mortgage1[2]) +
" / " + "Interest: $" + twoPlaces.format(interest1) + " / " +
"Remaining: $" + twoPlaces.format(amountRemaining1) + "\n");
}
} else {
// 15-year loan
if (loanSelected.equals("15 years at 5.5%")) {
// Calculate payment, interest, and remaining
mortgage2[2] = (loanAmt + (loanAmt * (mortgage2[1] / 100))) / (mortgage2[0] * 12);
double interest2 = (loanAmt * (mortgage2[1] / 100)) / 180;
double amountRemaining2 = loanAmt + (loanAmt * (mortgage2[1] / 100));
// Loop through payments
for (int payment = 1; payment <=180; payment++) {
// Deduct one payment from the balance
amountRemaining2 = amountRemaining2 - mortgage2[2];
// Write payment to textArea
paymentList.append("Payment " + payment + ": $" + twoPlaces.format(mortgage2[2]) +
" / " + "Interest: $" + twoPlaces.format(interest2) + " / " +
"Remaining: $" + twoPlaces.format(amountRemaining2) + "\n");
}
} else {
//30-year loan
//Calculate payment, interest, and remaining
mortgage3[2] = (loanAmt + (loanAmt * (mortgage3[1] / 100))) / (mortgage3[0] * 12);
double interest3 = (loanAmt * (mortgage3[1] / 100)) / 360;
double amountRemaining3 = loanAmt + (loanAmt * (mortgage3[1] / 100));
// Loop through payments
for (int payment = 1; payment <=360; payment++) {
// Deduct one payment from the balance
amountRemaining3 = amountRemaining3 - mortgage3[2];
// Write payment to textArea
paymentList.append("Payment " + payment + ": $" + twoPlaces.format(mortgage3[2]) +
" / " + "Interest: $" + twoPlaces.format(interest3) + " / " +
"Remaining: $" + twoPlaces.format(amountRemaining3) + "\n");
}
}
}
}
public static void main(String[] arguments) {
MortgageGui calc = new MortgageGui();
}
}
When I run the program I see the GUI but when I hit the button to calculate I get this in the console:
Exception in thread “AWT-EventQueue-0” java.lang.NullPointerException
at MortgageGui.actionPerformed(MortgageGui.java:100)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2713)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
at java.awt.EventQueue.access$000(EventQueue.java:101)
at java.awt.EventQueue$3.run(EventQueue.java:666)
at java.awt.EventQueue$3.run(EventQueue.java:664)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:680)
at java.awt.EventQueue$4.run(EventQueue.java:678)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
I cannot figure out where I am going wrong. Please help!
You’re redeclaring the paymentList variable in the class’s constructor
The variable inside of the constructor “shadows” the class field, so that the constructor doesn’t see the class field. This local variable is visible only within the constructor (since it was declared inside of it) and the class field will remain null.
Solution: don’t re-declare the variable in the constructor. It’s OK to initialize it there, but don’t re-declare it:
On a more general note, you should always inspect the variables on the line that throws the NPE because one of them is null. If you saw that it was paymentList, you’d quickly be able to identify what I just showed you and you wouldn’t need our help, which is kind of the goal of this forum — to get you to be able to solve things on your own.
Edit:
Also please note that you do this error more than once in your code. I’ll let you find the other occurrences of this.
Edit 2:
Consider putting your JTextArea in a JScrollPane. Consider using some decent layout managers and not setting the size of the GUI but rather letting the layout managers do this for you.