Applet et frames

import java.awt.Dimension;

import javax.swing.JDesktopPane;
import javax.swing.JInternalFrame;
import javax.swing.JLayeredPane;

/**
*
* @author rabbit
*/
public class LinuxFrApplet extends javax.swing.JApplet {

public void init() {
initComponents();
}

private void initComponents() {
jDesktopPane1 = new JDesktopPane();

JInternalFrame internalFrame = new JInternalFrame() ;

jDesktopPane1.add(internalFrame, JLayeredPane.DEFAULT_LAYER);

internalFrame.setSize(new Dimension(100,100)) ; // <=
internalFrame.setVisible(true) ; // <=

getContentPane().add(jDesktopPane1, java.awt.BorderLayout.CENTER);
}
private javax.swing.JDesktopPane jDesktopPane1;
private javax.swing.JPanel jPanel1;

}

Ta JInternalFrame doit :
* Avoir une taille
* Etre rendue visible