Need help with Java, have a few questions about my Applet

Question

Purpose of the program: To create a mortgage calculator that will allow the user to input the loan principal then select 1 of 3 choices from a combo-box (7 years @ 5.35%, 15 years @ 5.50%, 30 years @ 5.75%).
Problem: My program was working properly (so I thought). I can get the program to properly compile and run through TextPad. However, I noticed that when the user clicks the calculate more than once (with the same input), it slightly alters the output calculations. So that is my first question, Why is it doing that?  How can I get it only calculate that information once unless the user changes it etc?
My next question is regarding my exit button. I was told by my instructor (who has already stated he won't help any of us) that my exit button does not work for an applet and only for an application. So, how can I create an exit button that will work with an applet? I thought I did it right but I can't find any resources online for this.
Next question, why isn't my program properly validating invalid input? My program should only allow numeric input and nothing more.
And last question, when invalid input is entered and the user clicks calculate, why isn't my JOptionPane window for error messages not displaying?
I know my code is a little long so I have to post this in two messages. Please don't criticize me for what I am doing, that is why I am learning and have come to this forum for help. Thanks
import java.applet.Applet;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import java.text.NumberFormat;
import java.text.DecimalFormat;
// Creating the main class
public class test extends JApplet implements ActionListener
     // Defining of format information
     JLabel heading = new JLabel("McBride Financial Services Mortgage Calculator");
     Font newFontOne = new Font("TimesRoman", Font.BOLD, 20);
     Font newFontTwo = new Font("TimesRoman", Font.ITALIC, 16);
     Font newFontThree = new Font("TimesRoman", Font.BOLD, 16);
     Font newFontFour = new Font("TimesRoman", Font.BOLD, 14);
     JButton calculate = new JButton("Calculate");
     JButton exitButton = new JButton("Quit");
     JButton clearButton = new JButton("Clear");
     JLabel instructions = new JLabel("Please Enter the Principal Amount Below");
     JLabel instructions2 = new JLabel("and Select a Loan Type from the Menu");
     // Declaration of variables
     private double principalAmount;
     private JLabel principalLabel = new JLabel("Principal Amount");
     private NumberFormat principalFormat;
     private JTextField enterPrincipal = new JTextField(10);
     private double finalPayment;
     private JLabel monthlyPaymentLabel = new JLabel("  Monthly Payment    \t         Interest Paid    \t \t            Loan Balance");
     private NumberFormat finalPaymentFormat;
     private JTextField displayMonthlyPayment = new JTextField(10);
     private JTextField displayInterestPaid = new JTextField(10);
     private JTextField displayBalance = new JTextField(10);
     // Creation of the String of arrays for the ComboBox
     String [] list = {"7 Years @ 5.35%", "15 Years @ 5.50%", "30 Years @ 5.75%"};
     JComboBox selections = new JComboBox(list);
     // Creation of the textArea that will display the output
     private TextArea txtArea = new TextArea(5, 10);
     StringBuffer buff = null;
          Edited by: LoveMyAJ on Jan 19, 2009 1:49 AM

Answer 1

// Initializing the interface
     public void init()
          // Creation of the panel design and fonts
          JPanel upper = new JPanel(new BorderLayout());
          JPanel middle = new JPanel(new BorderLayout());
          JPanel lower = new JPanel(new BorderLayout());
          JPanel areaOne = new JPanel(new BorderLayout());
          JPanel areaTwo = new JPanel(new BorderLayout());
          JPanel areaThree = new JPanel(new BorderLayout());
          JPanel areaFour = new JPanel(new BorderLayout());
          JPanel areaFive = new JPanel(new BorderLayout());
          JPanel areaSix = new JPanel(new BorderLayout());
          Container con = getContentPane();
          getContentPane().add(upper, BorderLayout.NORTH);
          getContentPane().add(middle, BorderLayout.CENTER);
          getContentPane().add(lower, BorderLayout.SOUTH);
          upper.add(areaOne, BorderLayout.NORTH);
          middle.add(areaTwo, BorderLayout.NORTH);
          middle.add(areaThree, BorderLayout.CENTER);
          middle.add(areaFour, BorderLayout.SOUTH);
          lower.add(areaFive, BorderLayout.NORTH);
          lower.add(areaSix, BorderLayout.SOUTH);
          heading.setFont(newFontOne);
          instructions.setFont(newFontTwo);
          instructions2.setFont(newFontTwo);
          principalLabel.setFont(newFontThree);
          monthlyPaymentLabel.setFont(newFontFour);
          displayInterestPaid.setFont(newFontFour);
          displayBalance.setFont(newFontFour);
          areaOne.add(heading, BorderLayout.NORTH);
          areaOne.add(instructions, BorderLayout.CENTER);
          areaOne.add(instructions2, BorderLayout.SOUTH);
          areaTwo.add(principalLabel, BorderLayout.WEST);
          areaTwo.add(enterPrincipal, BorderLayout.EAST);
          areaThree.add(selections, BorderLayout.NORTH);
          areaFour.add(calculate, BorderLayout.CENTER);
          areaFour.add(exitButton, BorderLayout.EAST);
          areaFour.add(clearButton, BorderLayout.WEST);
          areaFive.add(monthlyPaymentLabel, BorderLayout.CENTER);
          areaSix.add(txtArea, BorderLayout.CENTER);
          // Using the ActionListener to determine when each button is clicked
          calculate.addActionListener(this);
          exitButton.addActionListener(this);
          clearButton.addActionListener(this);
          enterPrincipal.requestFocus();
          selections.addActionListener(this);
     // The method that will perform specific actions defined below
     public void actionPerformed(ActionEvent e)
          Object source = e.getSource();
          buff = new StringBuffer();
          // Outcome of pressing the calculate button
          if (source == calculate)
               // Used to call upon the user chosen selection
               int selection = selections.getSelectedIndex();
               // If statement to call upon Loan One's Method
               if (selection == 0)
                    Double data = (Double)calculateLoanOne();
                    String sourceInput = data.toString();
                    displayMonthlyPayment.setText(sourceInput);
                    txtArea.setText(buff.toString());
               // If statement to call upon Loan Two's Method
               else if (selection == 1)
                    Double data = (Double)calculateLoanTwo();
                    String sourceInput = data.toString();
                    displayMonthlyPayment.setText(sourceInput);
                    txtArea.setText(buff.toString());
               // If statement to call upon Loan Three's Method
               else if (selection == 2)
                    Double data = (Double)calculateLoanThree();
                    String sourceInput = data.toString();
                    displayMonthlyPayment.setText(sourceInput);
                    txtArea.setText(buff.toString());
               // Outcome of pressing the clear button
               else if (source == clearButton)
                    enterPrincipal.setText("");
                    displayMonthlyPayment.setText("");
                    selections.setSelectedIndex(0);
                    txtArea.setText("");
               // Outcome of pressing the quit button
               else if (source == exitButton)
                    System.exit(1);
     // Method used to validate user input
     private static boolean validate(JTextField in)
          String inText = in.getText();
          char[] charInput = inText.toCharArray();
          for(int i = 0; i < charInput.length; i++)
               int asciiVal = (int)charInput;
          if((asciiVal >= 48 && asciiVal <= 57) || asciiVal == 46)
          else
               JOptionPane.showMessageDialog(null, "Invalid Character, Please Use Numeric Values Only");
               return false;
               return true;