Creating a JInternalFrame without a title bar

Try this:

   javax.swing.plaf.InternalFrameUI ui = frame.getUI();
   ((javax.swing.plaf.basic.BasicInternalFrameUI)ui).setNorthPane(null);

Main.java:

import java.awt.image.*;
import java.awt.event.*;
import java.applet.*;
import javax.swing.*;
import java.awt.*;
import java.net.*;

public class Main extends JFrame
{
   JDesktopPane desktop;
 
   public Main() {
      desktop = new JDesktopPane(); 
      setContentPane(desktop);
      setBackgroundImage();
 
      // create internal frame with title bar
      createInternalFrame(true);
 
      // create internal frame without title bar
      createInternalFrame(false);
 
      setSize(new Dimension(500, 300));
      setLocation(100, 100);
 
      addWindowListener(new WindowAdapter() {
         public void windowClosing(WindowEvent we) {
            System.exit(0); 
         }
      });
   }
 
   protected void setBackgroundImage() {
      try {
         ImageIcon icon = new ImageIcon(new URL("http://www.esus.com/images/mong.jpg"));
         JLabel label = new JLabel(icon);
         label.setBounds(0, 0, icon.getIconWidth(), icon.getIconHeight());
  
         desktop.add(label, new Integer(Integer.MIN_VALUE));
      } catch(MalformedURLException e) {
         System.out.println(e);
      }
   }
  
   protected void createInternalFrame(boolean titlebar) {
 
      JInternalFrame frame = new JInternalFrame("InternalFrame", 
         true,    // resizable
         true,    // closable
         true,    // maximizable
         true);   // iconifiable
      if (!titlebar) {
         javax.swing.plaf.InternalFrameUI ui = frame.getUI();
            ((javax.swing.plaf.basic.BasicInternalFrameUI)ui).setNorthPane(null);
      }
 
      frame.setVisible(true); 
      desktop.add(frame);
      frame.setSize(200, 200);
      try {
         frame.setSelected(true);
      } catch (java.beans.PropertyVetoException e) {}
   }
 
   public static void main(String []args) throws Exception {
      Main main = new Main();
      main.show();      
   }
}