Sorry, you are right but the program is too big with too many UI
elements.
Thank to you, trying to reduce it to make a sscce I've found the line
of code that is responsible of the "flickering" of myWindow:

Button.addActionListener(new ActionListener()
{
 public void actionPerformed(ActionEvent e)
 {
   myWindow.update();  // <- this is the guilty line
   myWindow.setVisible(true);
 }
});

myWindow is a JFrame already built and initialized and the update()
method is full of setText() on a dozen of JTextField of myWindow.
it appears that the update routine happens after the setVisible,
giving me the "flickering" effect on the gui.
I have tried to put the two lines into a SwingUtilities.invokeLater
thread but with no result.
I have noted too that in all my other gui that are updated just before
being visible there is a flickering effect. The update are always some
JTextField.setText and some JTextBox.setSelected on the invoked
window.

Thank you,
Matteo

I've further analyzed the c