How do I only allow 1 internal frame to be added?

Question

Could you pls show me how can I do this, I need some code to learn how this code work.
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Test1a extends JFrame
     private JLabel label1;
//     private JLabel label2;
     private JLabel label3;
     private JLabel label4;
     private JLabel label5;
     private JDesktopPane theDesktop;
     public Test1a()
          super("TESTING JLABEL");
          setLayout(new BorderLayout(5, 5));
        JMenuBar bar = new JMenuBar();
      JMenu addMenu = new JMenu( "Add" );
      JMenuItem newFrame = new JMenuItem( "Internal Frame" );
      addMenu.add( newFrame );
      bar.add( addMenu );
      setJMenuBar( bar );
       theDesktop = new JDesktopPane();
      add( theDesktop, BorderLayout.CENTER );
      newFrame.addActionListener(
         new ActionListener()
            // display new internal window
            public void actionPerformed( ActionEvent event )
               // create internal frame
               JInternalFrame frame = new JInternalFrame("Internal Frame", true, true, true, true);
               MyJPanel panel = new MyJPanel();
               frame.add( panel, BorderLayout.CENTER );
               frame.pack(); // set internal frame to size of contents
               theDesktop.add( frame );
               frame.setVisible( true );
          label1= new JLabel ();
          label1.setText("Label 1");
          add(label1, BorderLayout.WEST);
     //     label2= new JLabel ("Label 2", SwingConstants.CENTER);
     //   add(label2, BorderLayout.CENTER);
          label3= new JLabel ();
          label3.setText("Label 3");
          add(label3, BorderLayout.EAST);
          label4= new JLabel ("Label 4", SwingConstants.CENTER);
          add(label4, BorderLayout.NORTH);
          label5= new JLabel ("Label 5", SwingConstants.CENTER);
          add(label5, BorderLayout.SOUTH);
=============================================
import java.awt.*;
import javax.swing.*;
// class to display an ImageIcon on a panel
class MyJPanel extends JPanel
   private ImageIcon imageIcon;
   // load image
   public MyJPanel()
      imageIcon = new ImageIcon("yellowflowers.png");
   // display imageIcon on panel
   public void paintComponent( Graphics g )
      super.paintComponent( g );
      imageIcon.paintIcon( this, g, 0, 0 );
   public Dimension getPreferredSize()
      return new Dimension( imageIcon.getIconWidth(), imageIcon.getIconHeight() ); 
========================================
import javax.swing.*;
public class Test1b
     public static void main (String args[])
          Test1a test1a = new Test1a();
          test1a.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          test1a.setSize(800, 600);
          test1a.setVisible(true);
}

Answer 1

You where given an answer in your original posting:
http://forum.java.sun.com/thread.jspa?threadID=5146301