Entering a time into textbox

I am doing a homework assignment. Although the assignment says to pass
parameters to a method, I wanted to extend the capabilities by entering
the time values. I modified some code I found and it "works", kind of,
but I do have a few questions. Perhaps you have some clarifications.

1) If I supply a default value to the text boxes, or leave as blank and
enter a value, it inserts the values, not overwrites the default. IOW,
if I enter 0 as a default, and go to that text box and enter 17, my
value is 170. I would expect a text box to overwrite the values else
press the Ins key to insert a value. How do I permit overwrite?

2) I can move between the fields using the Tab key. But if I enter a
value and hit the Enter key, I stay within the same field. How does one
move to the next field if one hits the Enter key?

3) Same holds true with the Exit button. If I use the mouse on the
Exit button, the window closes. But if I tab to the Exit button then
press Enter, nothing.

4) Is there a reliable way to enter a time (hour/minute/maybe ampm)
field into a listbox? I haven't been exposed to what I would consider
an input mask in Java.

5) I used System.exit(0) to "close" the window. Is that what one does
to close windows?

/**
    MeterTime class
    Inputs the hour and minutes
*/

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.util.*;

class MeterTime extends JFrame
{

     private JTextField hourTF = new JTextField(2);
     private JTextField minutesTF = new JTextField(2);
     private JTextField apTF = new JTextField(1);

     private int hour;
     private int minutes;

     public MeterTime()
     {

         // Create/initialize components
         JButton exit = new JButton("Exit");
         exit.addActionListener(new ExitBtnListener());

         //Create hmContent panel, set layout
         JPanel hmContent = new JPanel();
         hmContent.setLayout(new FlowLayout());

         //Add the components to the hmContent panel.
         hmContent.add(new JLabel("Hour: "));
         hmContent.add(hourTF); // Add input field
         hmContent.add(new JLabel("Minutes: "));
         hmContent.add(minutesTF); // Add input field
         hmContent.add(new JLabel("AmPm: "));
         hmContent.add(apTF); // Add input field
         hmContent.add(exit); // Add button

         //Set this window's attributes, and pack it.
         setContentPane(hmContent);
         pack(); // Layout components.
         setTitle("Enter Time");
         setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         setLocationRelativeTo(null); // Center window.
     }

     class ExitBtnListener implements ActionListener {
         public void actionPerformed(ActionEvent e) {
             String myStr;

                //get the hours
             myStr = hourTF.getText();
                try
             {
                hour = Integer.parseInt(myStr);
             }
             catch (NumberFormatException f)
             {
                 hour = 0;
        }

        //get the minutes
             myStr = minutesTF.getText();
    try
             {
                minutes = Integer.parseInt(myStr);
             }
             catch (NumberFormatException f)
             {
                 minutes = 0;
                }

        //get the ampm flag
             myStr = apTF.getText();
   myStr = myStr.toUpperCase();
             System.out.println(myStr);

             System.exit(0);

         }
     }

     public static void main(String[] args)
     {
         MeterTime window = new MeterTime();
         window.setVisible(true);
     }
}