File - Quit no longer working in Java GUI
<p>in the process of tidying up the java gui code below i've managed to render the file/quit open inoperable - i can't see anything wrong, any suggestions please?</p>

<pre><code>import java.awt.FlowLayout;
import java.awt.Container;
import java.awt.event.ActionEvent;
import javax.swing.*;
import java.awt.*;


import java.awt.event.ActionListener; 


public class BookGUI extends JFrame implements ActionListener

{

    Book book = new Book("", "", 0, "", 0);
    String title  = "";
    String author  = "";
    int year = 0;
    String publisher  = "";
    double cost = 0;
    double total = 0;
    boolean goodInput = false;


    public BookShelf bookShelf = new BookShelf();
    public static final int WIDTH = 600;
    public static final int HEIGHT = 90;

    //Creates &amp; displays a window of the class FlowLayoutDemo
    public static void main(String[] args)
    {
        BookGUI gui = new BookGUI( );
        gui.setVisible(true);
    }

    public void setTitle(String title) 
    {
        this.title = title;
    }

    public void setAuthor(String author) 
    {
        this.author = author;
    }

    public void setYear(int year) 
    {
        this.year = year;
    }
    public void setPublisher(String publisher) 
    {
        this.publisher = publisher;
    }

    public void setCost(double cost) 
    {
        this.cost = cost;
    }

    public BookGUI( )
    {
        JFrame frame = this; 

        JMenuBar menubar = new JMenuBar();
        frame.setJMenuBar(menubar);

        frame.pack();
        frame.setVisible(true);

        JMenu fileMenu = new JMenu("File");
        menubar.add(fileMenu);
        JMenuItem quitItem = new JMenuItem("Quit");
        fileMenu.add(quitItem);;

        setSize(WIDTH, HEIGHT);
        addWindowListener(new WindowDestroyer( ));
        setTitle("GUI Assignment");
        Container content = getContentPane( );

        content.setLayout(new FlowLayout());

        JButton button1 = new JButton("Add Book");
        content.add(button1);
        button1.addActionListener(this); 

        JButton button2 = new JButton("Hightest Price Paid");
        content.add(button2);
        button2.addActionListener(this);

        JButton button3 = new JButton("Cost of BookShelf");
        content.add(button3);
        button3.addActionListener(this);

        JButton button4 = new JButton("Size of BookShelf");
        content.add(button4);
        button4.addActionListener(this);

        }

    public void actionPerformed(ActionEvent e)
    {

        if (e.getActionCommand().equals("Add Book"))

        {    
        title = JOptionPane.showInputDialog("Title");
        author = JOptionPane.showInputDialog("Author");
        publisher = JOptionPane.showInputDialog("Publisher");

        do{
        try { 
            cost = Double.parseDouble(JOptionPane.showInputDialog("Cost"));
            book.setCost(cost);
            goodInput = true;
            } 
        catch (NumberFormatException nfe){          
        JOptionPane.showMessageDialog(this, "Numerical entry required. Please try again");
            } 
        }while (!goodInput);


        book.setTitle(title);
        book.setAuthor(author);
        book.setPublisher(publisher);
        bookShelf.addBook(book);

        String message =  "The title of the book is :" + title + 
        "the Author of the Book is : " + author + " and it's published by " + publisher + "and it costs" + cost + "euro";
        JOptionPane.showMessageDialog(null, message, "Book Details", JOptionPane.PLAIN_MESSAGE);
        }
        else if (e.getActionCommand().equals("Size of BookShelf")) {
            int sizeOfBookShelf = bookShelf.sizeOfBookshelf();
            String message = "The book shelf has " + sizeOfBookShelf + " book(s)";
            JOptionPane.showMessageDialog(this, message);
        }
        else if (e.getActionCommand().equals("Cost of BookShelf")) 
        {
            double costOfBookshelf = bookShelf.costOfBookShelf();
            String message = "The book shelf value is " + total + costOfBookshelf + "Euro";
            JOptionPane.showMessageDialog(this, message);
        }

        else
        {
            if (e.getActionCommand().equals("Hightest Price Paid"))
            {          
                JOptionPane.showMessageDialog(this, "This facility is not currently available");
                    } 
            else
            {
                System.out.println("Error!");
            }
            //
        }
        //else
        {
     // Alows the class to quit.
        //System.exit( 0 ); 
        }




    }
    }
</code></pre>
