Problema ao Minimizar InternalFrama

import javax.swing.JInternalFrame;
import javax.swing.JDesktopPane;
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.JMenuBar;
import javax.swing.JFrame;
import javax.swing.KeyStroke;

import java.awt.event.*;
import java.awt.*;

/*
 * InternalFrameDemo.java is a 1.4 application that requires:
 *   MyInternalFrame.java
 */
public class InternalFrameDemo extends JFrame
                               implements ActionListener {
    JDesktopPane desktop;

    public InternalFrameDemo() {
        super("InternalFrameDemo");

        //Make the big window be indented 50 pixels from each edge
        //of the screen.
        int inset = 50;
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        setBounds(inset, inset,
                  screenSize.width  - inset*2,
                  screenSize.height - inset*2);

        //Set up the GUI.
        desktop = new JDesktopPane(); //a specialized layered pane
        createFrame(); //create first "window"
        setContentPane(desktop);
        setJMenuBar(createMenuBar());

        //Make dragging a little faster but perhaps uglier.
        desktop.setDragMode(JDesktopPane.OUTLINE_DRAG_MODE);
    }

    protected JMenuBar createMenuBar() {
        JMenuBar menuBar = new JMenuBar();

        //Set up the lone menu.
        JMenu menu = new JMenu("Document");
        menu.setMnemonic(KeyEvent.VK_D);
        menuBar.add(menu);

        //Set up the first menu item.
        JMenuItem menuItem = new JMenuItem("New");
        menuItem.setMnemonic(KeyEvent.VK_N);
        menuItem.setAccelerator(KeyStroke.getKeyStroke(
                KeyEvent.VK_N, ActionEvent.ALT_MASK));
        menuItem.setActionCommand("new");
        menuItem.addActionListener(this);
        menu.add(menuItem);

        //Set up the second menu item.
        menuItem = new JMenuItem("Quit");
        menuItem.setMnemonic(KeyEvent.VK_Q);
        menuItem.setAccelerator(KeyStroke.getKeyStroke(
                KeyEvent.VK_Q, ActionEvent.ALT_MASK));
        menuItem.setActionCommand("quit");
        menuItem.addActionListener(this);
        menu.add(menuItem);

        return menuBar;
    }

    //React to menu selections.
    public void actionPerformed(ActionEvent e) {
        if ("new".equals(e.getActionCommand())) { //new
            createFrame();
        } else { //quit
            quit();
        }
    }

    //Create a new internal frame.
    protected void createFrame() {
        MyInternalFrame frame = new MyInternalFrame();
        frame.setVisible(true); //necessary as of 1.3
        desktop.add(frame);
        try {
            frame.setSelected(true);
        } catch (java.beans.PropertyVetoException e) {}
    }

    //Quit the application.
    protected void quit() {
        System.exit(0);
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Make sure we have nice window decorations.
        JFrame.setDefaultLookAndFeelDecorated(true);

        //Create and set up the window.
        InternalFrameDemo frame = new InternalFrameDemo();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Display the window.
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

import javax.swing.JInternalFrame;

import java.awt.event.*;
import java.awt.*;

/* Used by InternalFrameDemo.java. */
public class MyInternalFrame extends JInternalFrame {
    static int openFrameCount = 0;
    static final int xOffset = 30, yOffset = 30;

    public MyInternalFrame() {
        super("Document #" + (++openFrameCount), 
              true, //resizable
              true, //closable
              true, //maximizable
              true);//iconifiable

        //...Create the GUI and put it in the window...

        //...Then set the window size or call pack...
        setSize(300,300);

        //Set the window's location.
        setLocation(xOffset*openFrameCount, yOffset*openFrameCount);
    }
}