mortgage calculator help

hello. i am having trouble with my calculator that i need for an assignment. it only works once. it will calculate everything but wont calculate again after i clear it. any help will be appreciated.

import java.awt.*;
import java.awt.event.*;
import java.awt.datatransfer.*;
import java.text.DecimalFormat;
import javax.swing.JOptionPane;

public class calculator extends Frame implements ActionListener
{
private Button keys[];
private Panel userInterface, buttonInterface, result1Interface, result2Interface;
private TextField tfMortgagePayment, tfMortgageAmount;
private TextArea tfMonthlyPayments;
private Label label1, label2, label3, label4;
private Choice availableLoans;
private double mortgagePayment, loanBalance=0;
private boolean foundKey;
private DecimalFormat roundDecimal;
private double [] intRateAnnual = {5.35, 5.50, 5.75};
private int termMonths, arrayIndex=0;
private int [] termYears = {7, 15, 30};

public calculator()
{

roundDecimal = new DecimalFormat("########.##");

// create an instance of the menu
MenuBar mnuBar = new MenuBar();
setMenuBar(mnuBar);

// construct and populate the File menu
Menu mnuFile = new Menu("File", true);
mnuBar.add(mnuFile);
MenuItem mnuFileExit = new MenuItem("Exit");
mnuFile.add(mnuFileExit);

// add the ActionListener to menu item
mnuFileExit.addActionListener(this);

// assign an ActionCommand to menu item
mnuFileExit.setActionCommand("Exit");

// construct components and initialize beginning values
tfMortgageAmount = new TextField(20);
tfMortgageAmount.setEditable(true);
tfMortgagePayment = new TextField(20);
tfMortgagePayment.setEditable(false);
tfMonthlyPayments = new TextArea("",5,55);
tfMonthlyPayments.setEditable(false);

userInterface = new Panel();
buttonInterface = new Panel();
result1Interface = new Panel();
result2Interface = new Panel();

//These create the labels for the text fields
label1 = new Label();
label2 = new Label();
label3 = new Label();
label4 = new Label();

label1.setText("Mortgage Amount: ");
label2.setText("Available Loans: ");
label3.setText("Payment Summary: ");
label4.setText("Monthly Mortgage Payment: ");

//This creates the drop down
availableLoans = new Choice();

availableLoans.addItem("7 years at 5.35%");
availableLoans.addItem("15 years at 5.5%");
availableLoans.addItem("30 years at 5.75%");

//This is for the two buttons
keys = new Button[2];

// construct and assign captions to the Buttons
keys[0] = new Button("Calculate");
keys[1] = new Button("Clear");

for (int i=0; i<keys.length; i++)
keys[i].addActionListener(this);

// set Frame and keypad layout to grid layout
this.setLayout(new FlowLayout());
userInterface.setLayout(new GridLayout(5,2,5,5));
buttonInterface.setLayout(new FlowLayout());
result1Interface.setLayout(new GridLayout(1,2,5,5));
result2Interface.setLayout(new GridLayout(2,2,5,5));

userInterface.add(label1);
userInterface.add(tfMortgageAmount);
userInterface.add(label2);
userInterface.add(availableLoans);
userInterface.add(keys[0]);
userInterface.add(keys[1]);
userInterface.add(label4);
userInterface.add(tfMortgagePayment);
userInterface.add(label3);
result2Interface.add(tfMonthlyPayments);
this.add(userInterface, BorderLayout.NORTH);
this.add(result2Interface, BorderLayout.CENTER);

addWindowListener(
new WindowAdapter()
{
public void windowClosing(WindowEvent e)
{
System.exit(0);
}
}
);

} // end of constructor method

//Method to determine user selections within the Frame
public void actionPerformed(ActionEvent e)
{
// test for menu item clicks
String arg = e.getActionCommand();
if (arg == "Exit")
System.exit(0);

// search for the clicked key
for (int i=0; i<keys.length && !foundKey; i++)
{
if(e.getSource() == keys[i])
{
foundKey=true;
switch(i)
{
case 0:

if (isDecimal(tfMortgageAmount.getText(),"Mortgage Amount"))
{
arrayIndex = availableLoans.getSelectedIndex();
loanBalance = Double.parseDouble(tfMortgageAmount.getText());
termMonths = termYears[arrayIndex] * 12;
tfMortgagePayment.setText("");
tfMonthlyPayments.setText("");
mortgagePayment = calcMonthlyPayment(termYears[arrayIndex], intRateAnnual[arrayIndex], loanBalance);
tfMortgagePayment.setText(roundDecimal.format(mort gagePayment));

tfMonthlyPayments.append("Month # \t\tInterest Paid \tRemaining Balance\n");
tfMonthlyPayments.append("------ \t\t------------- \t---------------\n");

while (termMonths > 0)
{
//Call to Method calcMonthlyBalance
loanBalance = calcMonthlyBalance (loanBalance, intRateAnnual[arrayIndex], mortgagePayment, termMonths);

// decrement months
termMonths--;

} // end while
} //end if

break;
case 1:
tfMortgageAmount.setText("");
tfMortgagePayment.setText("");
tfMonthlyPayments.setText("");
availableLoans.select(0);
tfMortgageAmount.requestFocus();
break;

}
}
}
}

//Start of main
public static void main(String args[])
{
// set frame properties
calculator f = new calculator();
f.setTitle("Mortgage Calculator");
f.setBounds(200,200,420,305);
f.setVisible(true);

} // end of main

public static double calcMonthlyPayment(int termYears, double interestRate, double loanAmount)
{
//Variable declarations
double intRateMonthly, mthPayment;
int termMonths;

//Determine Monthly Values based on Annual Amounts
termMonths = termYears * 12;
intRateMonthly = (interestRate / 12) / 100;

//Formula to calculate monthly payment
mthPayment = (loanAmount * (intRateMonthly)) / (1 - Math.pow(1 / (1 + (intRateMonthly)), termMonths));
return mthPayment;
} // end of calcMonthlyPayment

public static boolean isDecimal(String number, String tfName)
{
boolean isValid = true;
try
{
Double.parseDouble(number);

}
catch (NumberFormatException ex)
{
JOptionPane.showMessageDialog(null, "Please enter "+ tfName +" value. Please try again.");
isValid = false;
}
return isValid;
}

/*
Method for validation of an integer.*/

public static boolean isInteger(String number, String tfName)
{
boolean isValid = true;

try
{
Integer.parseInt(number);

}
catch (NumberFormatException ex)
{
JOptionPane.showMessageDialog(null, "An integer must be entered for the "+ tfName +" value. Please try again.");
isValid = false;
}
return isValid;
}

/*
Method for calculation */
public double calcMonthlyBalance (double balance, double intRate, double payment, int month)
{
double monthlyInterestPayment, monthlyPrincipalPayment, monthlyBalance;
//Decimal Format
DecimalFormat roundDecimal = new DecimalFormat ("0.00");

// calculate interest and principal payments
monthlyInterestPayment = (balance * (intRate / 1200));
monthlyPrincipalPayment = (payment - monthlyInterestPayment);
monthlyBalance = balance - monthlyPrincipalPayment;

// This will display the month term, interest paid and Loan balance
tfMonthlyPayments.append(month +
"\t\t$" + roundDecimal.format(monthlyInterestPayment) +
"\t\t$" + roundDecimal.format(monthlyBalance) + "\n");

return (monthlyBalance);
}

} //end