Problem with Amortizing in Mortgage Calculator

Hey guys! I have been working out the kinks in this thing all day and I can't seem to get my Amortization to work. I got everything else to work how I want it. I've double checked all my formulas, and I think it's in the looping. But I'm not sure what to change from here. Could you take a look at it and tell me what you think? Thanks in advance!

import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.DecimalFormat;
import java.text.NumberFormat;
import java.util.Locale;

import javax.swing.BorderFactory;
import javax.swing.BoxLayout;
import javax.swing.ButtonGroup;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JRadioButton;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.border.Border;
import javax.swing.border.EmptyBorder;

public class MortgageCalculator extends JFrame implements ActionListener {

      int term = 0;
      double principal = 0;
      double rate = 0;
      double monthlyPayment = 0;
      double interest = 0;
      int notePeriod = 0;

      String mTerm[] = {"7", "15", "30"};
      String mInterst[] = {"5.35", "5.50", "5.75"};

      JPanel row1 = new JPanel();
      JLabel mortgageLabel = new JLabel("MORTGAGE PAYMENT CALCULATOR", JLabel.CENTER);

      JPanel row2 = new JPanel(new GridLayout(1, 2));
      JLabel principalLabel = new JLabel("Mortgage Principal $",JLabel.LEFT);
      JTextField principalTxt = new JTextField(10);

      JPanel row3 = new JPanel(new GridLayout(1, 2));
      JLabel termLabel = new JLabel("Mortgage Term (Yrs)",JLabel.LEFT);
      JTextField termTxt = new JTextField(10);

      JPanel row4 = new JPanel(new GridLayout(1, 2));
      JLabel rateLabel = new JLabel("Interest Rate (%)", JLabel.LEFT);
      JTextField rateTxt = new JTextField(10);

      JPanel radioPanel = new JPanel();
      JRadioButton buttonA = new JRadioButton("7 Years at 5.35%" , false);
      JRadioButton buttonB = new JRadioButton("15 Years at 5.50%" , false);
      JRadioButton buttonC = new JRadioButton("30 Years at 5.75%", false);

      JPanel row5 = new JPanel(new GridLayout(1, 2));
      JLabel monthlyPaymentLabel = new JLabel("Monthly Payment $", JLabel.LEFT);
      JTextField monthlyPaymentTxt = new JTextField(10);

      //create buttons
      JPanel button = new JPanel(new FlowLayout(FlowLayout.CENTER, 10, 10));
      JButton amortizeButton = new JButton("Amortize Payments");
      JButton clearButton = new JButton("Clear");
      JButton exitButton = new JButton("Exit");
      JButton calculateButton = new JButton("Calculate");

      //create textarea to diplay payments
      JTextArea displayArea = new JTextArea(10, 45);
      JScrollPane scroll = new JScrollPane(displayArea);

      public MortgageCalculator()
      {
            super ("Mortgage Payment Calculator by S Kemen");
            setSize(550, 500);
            setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            Container pane = getContentPane();

            Border rowborder = new EmptyBorder( 3, 10, 3, 10 );

            pane.add(row1);
            row1.add(mortgageLabel);
            row1.setMaximumSize( new Dimension( 10000, row1.getMinimumSize().height));
            row1.setBorder( rowborder);

            pane.add(row2);
            row2.add(principalLabel);
            row2.add(principalTxt);
            row2.setMaximumSize( new Dimension( 10000, row2.getMinimumSize().height));
            row2.setBorder( rowborder);

            pane.add(row3);
            row3.add(termLabel);
            row3.add(termTxt);
            row3.setMaximumSize( new Dimension( 10000, row3.getMinimumSize().height));
            row3.setBorder( rowborder);

            pane.add(row4);
            row4.add(rateLabel);
            row4.add(rateTxt);
            row4.setMaximumSize( new Dimension( 10000, row4.getMinimumSize().height));
            row4.setBorder( rowborder);

            ButtonGroup bgroup = new ButtonGroup();
            bgroup.add(buttonA);
            bgroup.add(buttonB);
            bgroup.add(buttonC);

            radioPanel.setLayout(new FlowLayout(FlowLayout.RIGHT, 4, 4 ));
            radioPanel.add(buttonA);
            radioPanel.add(buttonB);
            radioPanel.add(buttonC);
            pane.add(radioPanel);
            radioPanel.setMaximumSize( new Dimension( 10000, radioPanel.getMinimumSize().height));
            radioPanel.setBorder( rowborder);

            pane.add(row5);
            row5.add(monthlyPaymentLabel);
            row5.add(monthlyPaymentTxt);
            monthlyPaymentTxt.setEnabled(false);  //set payment amount uneditable
            row5.setMaximumSize( new Dimension( 10000, row5.getMinimumSize().height));
            row5.setBorder( rowborder);

            button.add(calculateButton);
            button.add(clearButton);
            button.add(exitButton);
            button.add(amortizeButton);
            pane.add(button);
            button.setMaximumSize( new Dimension( 10000, button.getMinimumSize().height));

            scroll.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));
            pane.add(scroll);

            pane.setLayout(new BoxLayout( pane, BoxLayout.Y_AXIS));
            setVisible(true);
            setContentPane(pane);

            //add listeners
            clearButton.addActionListener(this);
            exitButton.addActionListener(this);
            calculateButton.addActionListener(this);
            amortizeButton.addActionListener(this);
            buttonA.addActionListener(this);
            buttonB.addActionListener(this);
            buttonC.addActionListener(this);
    }
    public void actionPerformed(ActionEvent event)
    {
        Object command = event.getSource();
           if(command == calculateButton)
      {
      try
                       {
            principal = Double.parseDouble(principalTxt.getText());
        }
      catch(NumberFormatException e)
        {
            //catch null pointer exception if Principal is null
            JOptionPane.showMessageDialog(null, "Invaild Entry!  Please Try Again", "ERROR", JOptionPane.ERROR_MESSAGE);
        }
      try
         {
            term = Integer.parseInt(termTxt.getText());
             rate = Double.parseDouble(rateTxt.getText());
         }
            catch(NumberFormatException e)
            {
       //Set rate and term based on which item in the combobox is selected
       if(buttonA.isSelected() == true)
       {
            rate = 5.35;
            term = 7;
      }
      else if(buttonB.isSelected() == true)
      {
            rate = 5.5;
            term = 15;
      }
      else if (buttonC.isSelected() == true)
      {
            rate = 5.75;
            term = 30;
      }
      else
      {
      //If no button is selected, this is an actual error.  Throw an exception
      JOptionPane.showMessageDialog(null, "Invaild Entry!  Please Try Again", "ERROR",      JOptionPane.ERROR_MESSAGE);
      }
               }
              double interest = rate / 100 / 12;               //Monthly interst rate
              double notePeriod= term * 12;                    //Number of months over which loan is amortized

              //calculation formula
              double monthlyPayment = (principal * interest) / (1 - Math.pow(1 + interest, -notePeriod));

              //formatting variables
              DecimalFormat df = new DecimalFormat("\u00A4#,##0.00"); //currency
              DecimalFormat pf = new DecimalFormat("#,##0.00%");      //percentages
              DecimalFormat mi = new DecimalFormat("#,##0.000%");     //percentages
              monthlyPaymentTxt.setText("" + df.format(monthlyPayment));
        }
        if(command == clearButton)
        {
             principalTxt.setText(null);
             monthlyPaymentTxt.setText(null);
             displayArea.setText(null);
        }
        if(command == exitButton)
        {
             System.exit(0);
        }
        if (command == amortizeButton)
         {
      //formatting variables
      DecimalFormat df = new DecimalFormat("\u00A4#,##0.00"); //currency
      DecimalFormat pf = new DecimalFormat("#,##0.00%");      //percentages
      DecimalFormat mi = new DecimalFormat("#,##0.000%");     //percentages

      //Amoritization variables
      double loanBalance = notePeriod * monthlyPayment;
      double interestPaid = 0;                                       //Amount of interest paid on the loan
      double monthlyPrincipal = 0;                                //Amount of principal in each monthly payment
      double principalBalance = principal;                    //runing total of principal after payment
      int y = 0;                                                 //Counter

       //This loop is used to calculate and display the payment schedule information
      for(y = 1; y <= notePeriod; y++)
      {                                                                                         //start loop
                            displayArea.append("");                             //Inserts a blank line

            //start inner loop
            interestPaid = principalBalance * interest;
            monthlyPrincipal = monthlyPayment - interestPaid;
            loanBalance = loanBalance - monthlyPayment;
            principalBalance = principalBalance - monthlyPrincipal;

            displayArea.append("Month "+y+"\t\t"+df.format(monthlyPrincipal)+"\t\t"
                  +df.format(interestPaid)+"\t\t"+df.format(principalBalance)+"\n");
                displayArea.setCaretPosition(0);
            }
      }
    }
    public static void main (String[] arguments) //Main Method
    {
        MortgageCalculator smc = new MortgageCalculator();
        smc.setVisible(true);
        smc.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
 } //End of program