Adding an event listener to combo box

Question

I am working on a mortgage calculator and I cannot figure out how to add an event listener to a combo box.
I want to get the mortgage term and interest rate to calucate the mortgage using the combo cox. Here is my program.
Modify the mortgage program to allow the user to input the amount of a mortgage
and then select from a menu of mortgage loans: 7 year at 5.35%, 15 year at 5.50%, and
30 year at 5.75%. Use an array for the different loans. Display the mortgage payment
amount. Then, list the loan balance and interest paid for each payment over the term
of the loan. Allow the user to loop back and enter a new amount and make a new
selection, with resulting new values. Allow user to exit if running as an application
(can't do that for an applet though).
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.event.*;
import java.text.NumberFormat;
import java.util.Locale;
//creates class MortgageCalculator
public class MortgageCalculator extends JFrame implements ActionListener {
//creates title for calculator
     JPanel row = new JPanel();
     JLabel mortgageCalculator = new JLabel("MORTGAGE CALCULATOR", JLabel.CENTER);
//creates labels and text fields for amount entered          
     JPanel firstRow = new JPanel(new GridLayout(3,1,1,1));
     JLabel mortgageLabel = new JLabel("Mortgage Payment $", JLabel.LEFT);
     JTextField mortgageAmount = new JTextField(10);
     JPanel secondRow = new JPanel();
     JLabel termLabel = new JLabel("Mortgage Term/Interest Rate", JLabel.LEFT);
     String[] term = {"7", "15", "30"};
     JComboBox mortgageTerm = new JComboBox(term);
     JPanel thirdRow = new JPanel();
     JLabel interestLabel = new JLabel("Interest Rate (%)", JLabel.LEFT);
     String[] interest = {"5.35", "5.50", "5.75"};
     JComboBox interestRate = new JComboBox(interest);
     JPanel fourthRow = new JPanel(new GridLayout(3, 2, 10, 10));
     JLabel paymentLabel = new JLabel("Monthly Payment $", JLabel.LEFT);
     JTextField monthlyPayment = new JTextField(10);
//create buttons to calculate payment and clear fields
     JPanel fifthRow = new JPanel(new GridLayout(3, 2, 1, 1));
     JButton calculateButton = new JButton("CALCULATE PAYMENT");
     JButton clearButton = new JButton("CLEAR");
     JButton exitButton = new JButton("EXIT");
//Display area
     JPanel sixthRow = new JPanel(new GridLayout(2, 2, 10, 10));
     JLabel displayArea = new JLabel(" ", JLabel.LEFT);
     JTextArea textarea = new JTextArea(" ", 8, 50);
public MortgageCalculator() {
     super("Mortgage Calculator");                     //title of frame
     setSize(550, 350);                                             //size of frame
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     Container pane = getContentPane();
     GridLayout grid = new GridLayout(7, 3, 10, 10);
     pane.setLayout(grid);
     pane.add(row);
     pane.add(mortgageCalculator);
     pane.add(firstRow);
     pane.add(mortgageLabel);
     pane.add(mortgageAmount);
     pane.add(secondRow);
     pane.add(termLabel);
     pane.add(mortgageTerm);
     pane.add(thirdRow);
     pane.add(interestLabel);
     pane.add(interestRate);
     pane.add(fourthRow);
     pane.add(paymentLabel);
     pane.add(monthlyPayment);
     monthlyPayment.setEnabled(false);
     pane.add(fifthRow);
     pane.add(calculateButton);
     pane.add(clearButton);
     pane.add(exitButton);
     pane.add(sixthRow);
     pane.add(textarea); //adds texaarea to frame
     pane.add(displayArea);
     setContentPane(pane);
     setVisible(true);
     //Adds Listener to buttons
     calculateButton.addActionListener(this);
     clearButton.addActionListener(this);
     exitButton.addActionListener(this);
     mortgageTerm.addActionListener(this);
     interestRate.addActionListener(this);
public void actionPerformed(ActionEvent event) { 
     Object command = event.getSource();
     JComboBox mortgageTerm = (JComboBox)event.getSource();
     String termYear = (String)mortgageTerm.getSelectedItem();
if (command == calculateButton) //calculates mortgage payment
     int year = Integer.parseInt(mortgageTerm.getText());
     double rate = new Double(interestRate.getText()).doubleValue();
     double mortgage = new Double(mortgageAmount.getText()).doubleValue();
     double interest = rate /100.0 / 12.0;
     double monthly = mortgage *(interest/(1-Math.pow(interest+1,-12.0 * year)));
               NumberFormat myCurrencyFormatter;
               myCurrencyFormatter = NumberFormat.getCurrencyInstance(Locale.US);
               monthlyPayment.setText(myCurrencyFormatter.format(monthly));
     if(command == clearButton) //clears all text fields
               mortgageAmount.setText(null);
               //mortgageTerm.setText(null);
               //interestRate.setText(null);
               monthlyPayment.setText(null);
          if(command == exitButton) //sets exit button
                    System.exit(0);
     public static void main(String[] arguments) {
          MortgageCalculator mor = new MortgageCalculator();

Answer 1

The OP already did this to both JComboBoxes.
mochatay, here is a new actionPerformed method for you to use.
I've improved a few things here and there...
1) You can't just cast the ActionEvent's source into a JComboBox!
What if it was a JButton that fired the event? Then you would get ClassCastExceptions (I'm sure you did)
So check for all options, what the source of the ActionEvent actually was...
2) You can't assume the user will always type in valid data.
So enclose the Integer and Double parse methods in try-catch brakcets.
Then you can do something when you know that the user has entered invalid input
(like tell him/her what a clumsy idiot they are !)
3) As soon as user presses an item in any JComboBox, just re-calculate.
I did this here by programmatically clicking the 'Calculate' button.
Alternatively, you could have a 'calculate' method, which does everything inside the
if(command==calculateButton) if-block.
This will be called when:
a)calculateButton is pressed
b)when either of the JComboBoxes are pressed.
public void actionPerformed (ActionEvent event)
        Object command = event.getSource ();
        if (command == calculateButton) //calculates mortgage payment
            int year = 0;
            double rate = 0;
            double mortgage = 0;
            double interest = 0;
            /* If user has input invalid data, tell him so
            and return (Exit from this method back to where we were before */
            try
                year = Integer.parseInt (mortgageTerm.getSelectedItem ().toString ());
                rate = new Double (interestRate.getSelectedItem ().toString ()).doubleValue ();
                mortgage = new Double (mortgageAmount.getText ()).doubleValue ();
                interest = rate / 100.0 / 12.0;
            catch (NumberFormatException nfe)
                /* Display a message Dialogue box with a message */
                JOptionPane.showMessageDialog (this, "Error! Invalid input!");
                return;
            double monthly = mortgage * (interest / (1 - Math.pow (interest + 1, -12.0 * year)));
            NumberFormat myCurrencyFormatter;
            myCurrencyFormatter = NumberFormat.getCurrencyInstance (Locale.US);
            monthlyPayment.setText (myCurrencyFormatter.format (monthly));
        else if (command == clearButton) //clears all text fields
            /* Better than setting it to null (I think) */
            mortgageAmount.setText ("");
            //mortgageTerm.setText(null);
            //interestRate.setText(null);
            monthlyPayment.setText ("");
        else if (command == exitButton) //sets exit button
            System.exit (0);
        else if (command == mortgageTerm)
            /* Programmatically 'clicks' the button,
            As is user had clicked it */
            calculateButton.doClick ();
        else if (command == interestRate)
            calculateButton.doClick ();
        //JComboBox mortgageTerm = (JComboBox) event.getSource ();
        //String termYear = (String) mortgageTerm.getSelectedItem ();
    }Hope this solves your problems.
I also hope you'll be able to learn from what I've indicated, so you can use similar things yourself
in future!
Regards,
lutha