New to Java

I am in dire need of some expertise.

I am writing an applet that contains a mortgage calculator. My problem is that
I can't get my payments to amortize properly. Can someone please take a look
at this code and let me know where I'm going wrong? This was done in JCreator.

import java.awt.*;
import java.text.*;
import java.applet.*;
import java.awt.event.*;
import java.awt.Container;
import javax.swing.*;
import javax.swing.JLabel;
import javax.swing.JTextField;
 
  public class MortgageWk31 extends JApplet implements ActionListener
 
{
    //Declaring Labels, TexFields and Greeting
    JLabel greetingLabel = new JLabel("Welcome to My Mortgage Calculator");
    JLabel amountLabel = new JLabel("Amount");
    JLabel termLabel = new JLabel("Select your Terms");
    JLabel payLabel = new JLabel("You're Mortgage Payment is:");
    JLabel scheduleLabel = new JLabel("Below is your Amortization Schedule:");
    JButton calculateButton = new JButton("Calculate");
    JTextField amountField = new JTextField("", 10);
    JTextField paymentField = new JTextField("", 10);
    JTextField tableField = new JTextField("", 40);
 
 
    public void init() {
 
        //Setting up the Layout Manager
        SpringLayout layout = new SpringLayout();
        getContentPane().setLayout(layout);
        paymentField.setEditable(false);
 
 
        //Adjusting constraints for labels
        layout.putConstraint(SpringLayout.WEST, greetingLabel,
        130,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, greetingLabel,
        0,
        SpringLayout.NORTH, getContentPane());
        layout.putConstraint(SpringLayout.WEST, amountLabel,
        25,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, amountLabel,
        50,
        SpringLayout.NORTH, getContentPane());
        layout.putConstraint(SpringLayout.WEST, termLabel,
        25,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, termLabel,
        100,
        SpringLayout.NORTH, getContentPane());
        layout.putConstraint(SpringLayout.WEST, payLabel,
        130,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, payLabel,
        188,
        SpringLayout.NORTH, getContentPane());
        layout.putConstraint(SpringLayout.WEST, scheduleLabel,
        135,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, scheduleLabel,
        235,
        SpringLayout.NORTH, getContentPane());
 
 
        //Adjusting constraints for textfields
        layout.putConstraint(SpringLayout.WEST, amountField,
        75,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, amountField,
        48,
        SpringLayout.NORTH, getContentPane());
        layout.putConstraint(SpringLayout.WEST, paymentField,
        300,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, paymentField,
        186,
        SpringLayout.NORTH, getContentPane());
 
 
        //Adjusting constraints for combo boxes
        layout.putConstraint(SpringLayout.WEST, terms,
        25,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, terms,
        125,
        SpringLayout.NORTH, getContentPane());
        layout.putConstraint(SpringLayout.WEST, tableField,
        25,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, tableField,
        250,
        SpringLayout.NORTH, getContentPane());
 
 
        //Adjusting constraint for button
        layout.putConstraint(SpringLayout.WEST, calculateButton,
        25,
        SpringLayout.WEST, getContentPane());
        layout.putConstraint(SpringLayout.NORTH, calculateButton,
        185,
        SpringLayout.NORTH, getContentPane());
 
        //Displays Labels, TextFields and Combo box in the Applet
        getContentPane().add(greetingLabel);
        getContentPane().add(amountLabel);
        getContentPane().add(amountField);
        getContentPane().add(termLabel);
        getContentPane().add(terms);
        getContentPane().add(tableField);
        getContentPane().add(calculateButton);
        getContentPane().add(payLabel);
        getContentPane().add(paymentField);
        getContentPane().add(scheduleLabel);
 
        calculateButton.addActionListener(this);
 
    }
 
    //Terms for combo box menu
    String[] termsArray = {"7 years at 5.35%", "15 years at 5.5%", "30 years at 5.75%"};
    JComboBox terms = new JComboBox(termsArray);
    JComboBox amortizationCombo = new JComboBox();
 
 
    public void actionPerformed(ActionEvent thisEvent) {
        //Establishing local variables
        double amt=0, rt=0, tm=0;
        double monthlyInterest;
        double monthlyPrinciple;
        double balance = amt;
        double payment=0;
 
        try{
           amt = Double.parseDouble(amountField.getText());
        }catch(NumberFormatException nfe){
           JOptionPane.showMessageDialog(null, "Non Numeric value entered for Amount", "Input Error", JOptionPane.WARNING_MESSAGE);
        }
 
        int menuNum = terms.getSelectedIndex();
                switch(menuNum){
                  case 0:
                  rt = 5.35;
                  tm = 84;
                  break;
                  case 1:
                  rt = 5.5;
                  tm = 180;
                  break;
                  case 2:
                  rt = 5.75;
                  tm = 360;
                  break;
                }
 
        //Formulas for determining payment amount
        double monthlyAPR = (rt /1200);
        double exponentCalculation = (1 - Math.pow((1 + monthlyAPR), ( -tm)));
     	payment = (amt * monthlyAPR) / (exponentCalculation);
        System.out.println("Monthly Payment is: "+payment);
        monthlyInterest = (amt * monthlyAPR);
        System.out.println("Monthly Interest is: "+monthlyInterest);
		monthlyPrinciple = (payment - monthlyInterest);
	    System.out.println("Monthly Principle is: "+monthlyPrinciple);
        
        
        NumberFormat currencyFormatter = NumberFormat.getCurrencyInstance();
        String currencyOut = currencyFormatter.format(payment);
        paymentField.setText(currencyOut);
        
    for(int i=1;i<=tm;i++)
    
	{
	
	//Reduces Principle each month
	double newBalance = (amt - monthlyPrinciple);
    double remainingBalance = (newBalance);
	monthlyPrinciple = (payment - monthlyInterest);
	monthlyInterest = (newBalance * monthlyAPR);
 
   	//Determining monthly payment
	payment = (amt * monthlyAPR) / (1 - Math.pow ((1 + monthlyAPR), - tm));
 
    System.out.println("Payment "+i+"\tInterest Paid: "+currencyFormatter.format(monthlyInterest)+"\tRemaining Balance: "+currencyFormatter.format(remainingBalance));
	}
}
}