Problem with ActionEvent and JButton

I can not figure out what I am missing to make my exit button work in the below program. I am sure it is something simple but I am at a lost as to what it could be. The program complies without error and all my other buttons work correctly.  I am believe the problem revolves around the ActionEvent  for my  exit button but can't find it. What am I doing wrong?


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

class mortCalcWk3 extends JFrame implements ActionListener {

//Establish Container for User Input
JTextField principalText = new JTextField(15);
JTextField yearIntrateText = new JTextField(15);
JTextField numYearsText = new JTextField(15);
JTextField monthlyPaymentsText = new JTextField(15);
JLabel validateLabel = new JLabel();


mortCalcWk3() {
super("McBride Financial Services Mortgage Calculator");
setSize(390, 300);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Container pane = getContentPane();
FlowLayout flow = new FlowLayout(FlowLayout.RIGHT);
pane.setLayout(flow);

JPanel row1 = new JPanel();
JLabel principalLabel = new JLabel("Loan Amount:$");
row1.add(principalLabel);
row1.add(principalText);
pane.add(row1);

JPanel row2 = new JPanel();
JLabel yearIntrateLabel = new JLabel("Yearly Interest Rate:");
row2.add(yearIntrateLabel);
row2.add(yearIntrateText);
pane.add(row2);

JPanel row3 = new JPanel();
JLabel numYearsLabel = new JLabel("Term of Loan(Years):");
row3.add(numYearsLabel);
row3.add(numYearsText);
pane.add(row3);

JPanel row4 = new JPanel();
JButton calculate = new JButton("Calculate");
calculate.addActionListener(this);
row4.add(calculate);
pane.add(row4);
JButton reset = new JButton("Reset Calculator");
reset.addActionListener(this);
row4.add(reset);
pane.add(row4);
JButton exit = new JButton("Exit?");
exit.addActionListener(this);
row4.add(exit);
pane.add(row4);

JPanel row5 = new JPanel();
JLabel monthlyPaymentsLabel = new JLabel("Total Monthly Payment:");
row5.add(monthlyPaymentsLabel);
row5.add(monthlyPaymentsText);
pane.add(row5);
setContentPane(pane);
setVisible(true);

}

public void actionPerformed(ActionEvent event) {
String command = event.getActionCommand();
if (command == "Calculate")
compute();
if (command == "Reset Calculator")
clearAllFields();
if (command == "Exit")
exit();}
void compute()
{
try{
validateLabel.setText("");
validateLabel.validate();


Double yearIntrate = new Double (yearIntrateText.getText());
Double principal = new Double (principalText.getText());
Integer numYears = new Integer (numYearsText.getText());

double payFreq = 12;
double interestRateMonths;
double monthlyPayment;
double numPayments;


interestRateMonths = (yearIntrate.doubleValue() / 100) / payFreq;
numPayments = payFreq * numYears.intValue();

double temp = Math.pow(1 + interestRateMonths, (-1*numPayments));
monthlyPayment = principal.doubleValue() * interestRateMonths / (1 - (temp));

//Format used to set two decimal places.
DecimalFormat currency = new DecimalFormat("$0.00");

monthlyPaymentsText.setText(currency.format(monthlyPayment));

} catch(Exception error)
{
validateLabel.setText("Invalid Entry! Try again.");
}
}

void clearAllFields(){
principalText.setText("");
yearIntrateText.setText("");
numYearsText.setText("");
monthlyPaymentsText.setText("");
}
void exit(){
System.exit(0);

}

public static void main(String[] arguments)
{
mortCalcWk3 mortCalcWk3  = new mortCalcWk3 ();
      }
}// end program