Question

I need to modify my Java program with the below requirements:

Use an array for the mortgage data for the different loans. Read the interest rates to fill the array from a sequential file. Display the mortgage payment amount followed by the loan balance and interest paid for each payment over the term of the loan. Add graphics in the form of a chart. Allow the user to loop back and enter a new amount and make a new selection or quit.

My program is as follows:

import java.io.*;
import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import java.awt.event.*;
import java.text.*;
import java.util.*;
import java.lang.*;
//GUI screen
public class Mortgage extends JApplet {
     private String[] terms={"7","15","30"};
     private double[] rates={0.0535,0.055,0.0575};
     private JLabel amountLabel=new JLabel("Loan amount: ");
     private JTextField amount=new JTextField();
     private JLabel termLabel=new JLabel("Term (Type Or pick from list): ");
     private JTextField term=new JTextField();
     private JLabel rateLabel=new JLabel("Interest rate (in decimal): ");
     private JTextField rate=new JTextField();
     private JComboBox termList = new JComboBox(terms);
     private JLabel payLabel=new JLabel("Monthly Payment: ");
     private JLabel payment=new JLabel();
     private JButton calculate=new JButton("Calculate");
     private JButton clear=new JButton("Clear");
     private JButton exit=new JButton("Exit");
     private JTextArea paymentSchedule=new JTextArea();
     private JScrollPane schedulePane=new JScrollPane(paymentSchedule);
     private Container cp = getContentPane();
     
     public void init() {
          // Take care of termList (Default terms)     
          termList.setSelectedIndex(0);
          termList.addActionListener(new ActionListener() {
               public void actionPerformed(ActionEvent e) {
                    JComboBox cb = (JComboBox)e.getSource();
                    // the source of the event is the combo box
                  String termYear = (String)cb.getSelectedItem();
                  term.setText(termYear);
                  int index=0;
                  switch (Integer.parseInt(termYear)) {
                       case 7: index=0; break;
                       case 15: index=1; break;
                       case 30: index=2; break;
                  }
                  rate.setText(rates[index]+"");
               }
          });
          
          // Assigning three buttons
          calculate.addActionListener(new ActionListener() {
               public void actionPerformed(ActionEvent e) {
                    try {// exception handler
                         // calculate monthly payment
                         double p=Double.parseDouble(amount.getText());
                         double r=Double.parseDouble(rate.getText())/12;
                         double n=Integer.parseInt(term.getText())*12;
                         double monthlyPayment=p*Math.pow(1+r,n)*r/(Math.pow(1+r,n)-1);
                         DecimalFormat df = new DecimalFormat("$###,###.00");
                         payment.setText(df.format(monthlyPayment));
                         // calculate the loan
                         double principal=p;
                         int month;
                         StringBuffer buffer=new StringBuffer();
                         buffer.append("Month\tAmount\tInterest\tBalance\n");
                         for (int i=0; i<n; i++) {
                              month=i+1;
                              double interest=principal*r;
                              double balance=principal+interest-monthlyPayment;
                              buffer.append(month+"\t");
                              buffer.append(new String(df.format(principal))+"\t");
                              buffer.append(new String(df.format(interest))+"\t");
                              buffer.append(new String(df.format(balance))+"\n");
                              principal=balance;
                         }
                         paymentSchedule.setText(buffer.toString());
                    } catch(NumberFormatException ex) { //Catch the exception error and show the error.
                         System.out.println("Please type the correct numerical number!Please avoid using other characters");
                    }
               }
          });
          clear.addActionListener(new ActionListener() {
               public void actionPerformed(ActionEvent e) {
                    amount.setText("");
                    payment.setText("");
                    paymentSchedule.setText("");
               }
          });
          exit.addActionListener(new ActionListener() {
               public void actionPerformed(ActionEvent e) {
                    System.exit(1);
               }
          });
          
          JPanel upScreen=new JPanel();
          upScreen.setLayout(new GridLayout(5,2));
          upScreen.add(amountLabel); upScreen.add(amount);     
          upScreen.add(termLabel); upScreen.add(term);
          upScreen.add(new Label()); upScreen.add(termList);
          upScreen.add(rateLabel); upScreen.add(rate);
          upScreen.add(payLabel); upScreen.add(payment);
          JPanel buttons=new JPanel();
          buttons.setLayout(new BoxLayout(buttons, BoxLayout.X_AXIS));
          buttons.add(calculate); buttons.add(clear); buttons.add(exit);
          JPanel up=new JPanel();
          up.setLayout(new BoxLayout(up, BoxLayout.Y_AXIS));
          up.add(upScreen); up.add(buttons);
          cp.add(BorderLayout.NORTH, up);
          cp.add(BorderLayout.CENTER, schedulePane);
     }
     public static void main(String[] args) {
          JApplet applet = new Mortgage();
          JFrame frame = new JFrame("Mortgage Calculator");
          frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          frame.setResizable(false);
          frame.getContentPane().add(applet);
          frame.setSize(380,400);
          applet.init();
          applet.start();
          frame.setVisible(true);
     }
}