Help with opening a text file upon closing a program.

I need some helping in figuring out something I want to do. I have this program that is a swimming pool volume calculator. It stored that data on a output.txt file but I want that file to open whenever the user exits the program.

My code so far:

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

public class PoolVolume { private JFrame mainFrame; private JButton calcButton, exitButton; private JTextField length, width, depth, volume; private JLabel lengthLabel, widthLabel, depthLabel, volumeLabel; public PoolVolume() { mainFrame = new JFrame("Swimming Pool Volume Calculator"); calcButton = new JButton("Calculate Volume"); exitButton = new JButton("Exit"); length = new JTextField(5); width = new JTextField(5); depth = new JTextField(5); volume = new JTextField(5); lengthLabel = new JLabel(" Enter the length of the swimming pool: "); widthLabel = new JLabel(" Enter the width of the swimming pool: "); depthLabel = new JLabel(" Enter the depth dept of the swimming pool: "); volumeLabel = new JLabel(" Swimming pool volume: "); JPanel c=new JPanel(new GridLayout(5,2)); c.add(lengthLabel); c.add(length); c.add(widthLabel); c.add(width); c.add(depthLabel); c.add(depth); c.add(volumeLabel); c.add(volume); c.add(calcButton);

            c.add(exitButton);
            calcButton.setMnemonic('C');
            exitButton.setMnemonic('x');
            mainFrame.setSize(500,200);
            mainFrame.add(c);
        mainFrame.addWindowListener(new WindowAdapter() 
        {
            public void windowClosing(WindowEvent e) {System.exit(0);}});
            calcButtonHandler chandler =new calcButtonHandler();
            calcButton.addActionListener(chandler);
            exitButtonHandler ehandler =new exitButtonHandler();
            exitButton.addActionListener(ehandler);
            FocusHandler fhandler =new FocusHandler();
            length.addFocusListener(fhandler);
            width.addFocusListener(fhandler);
            depth.addFocusListener(fhandler);
            volume.addFocusListener(fhandler);
            mainFrame.setVisible(true);
        }
class calcButtonHandler implements ActionListener 
{
    public void actionPerformed(ActionEvent e) 
        {
            DecimalFormat num =new DecimalFormat(",###.##");
            Double cLength, cWidth, cdepth, cVolume, Total;
            String sLength, sWidth, sdepth, sVolume;
            sLength =length.getText();
            cLength = Double.parseDouble(sLength);
            sWidth =width.getText();
            cWidth = Double.parseDouble(sWidth);
            sdepth =depth.getText();
            cdepth = Double.parseDouble(sdepth);
            if(e.getSource() == calcButton) {
            Total = cLength * cWidth * cdepth;
            volume.setText(num.format(Total));
            try{
            String value=volume.getText();
            File file = new File("output.txt");
            FileWriter fstream = new FileWriter(file,true);
            BufferedWriter out = new BufferedWriter(fstream);
            out.write("Length= "+sLength+", Width= "+sWidth+", Depth= "+sdepth+" so the volume of Swimming Pool is "+value);
            out.newLine();
            out.close();
        }
        catch(Exception ex){}
        }
    }
}
class exitButtonHandler implements ActionListener 
    {
        public void actionPerformed(ActionEvent e)
            {
                System.exit(0);
            }
    }
class FocusHandler implements FocusListener 
{
    public void focusGained(FocusEvent e) 
        {
        }
    public void focusLost(FocusEvent e) 
        {
        }
}

public static void main(String args[]) {