SORRY, I forgot to write the most important line!
Here is the correct code:

        public void paintComponent(Graphics gr) {
                Graphics2D gr2 = (Graphics2D)gr.create();
                if (isOpaque()) {
                        gr2.setColor(getBackground());
                        gr2.fillRect(0,0,getWidth(),getHeight());
                }
                imma.disegna(g2);
                gr2.dispose();
        }

I apologize for my absent-mindedness!

Daniele Simonelli

try this instead..  note that this example
is 'self-contained' in that you should be able
to copy/paste it into a Java file, compile it
and run it.  The reason I mention that particularly
is that I would appreciate if you added the extra
lines/changes to makes your examples the same (it
would have saved me some time..)

<sscce>
// imports omitted for brevity
// why, they only add three lines!
import java.awt.*;
import javax.swing.*;
import java.awt.geom.Ellipse2D;

class Image3D {
       private Shape shape;
       BasicStroke stroke;

       public Image3D(Shape sh) {
               shape = sh;
               // make the stroke *huge* so we
               // can see what's going on
               stroke = new BasicStroke(10);
       }

       public void disegna(Graphics gra) {

               Graphics2D gra2 = (Graphics2D)gra;
               gra2.setStroke( stroke );

               gra2.setColor(Color.RED);
               gra2.draw(shape);
               gra2.translate(6,0);
               gra2.setColor(Color.GREEN);

gra2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER,
0.5f));
               gra2.draw(shape);
       }
}

class ComponenteDiProva extends JComponent {
       private Image3D imma;
       public ComponenteDiProva() {
               setOpaque(true);
               //are you just making this up Danielle?
               //  setBackGround(Color.WHITE);
               setBackground(Color.WHITE);
               setPreferredSize(new Dimension(40,20));
               imma = new Image3D(new Ellipse2D.Float(5,5,50,40));
       }
        public void paintComponent(Graphics gr) {
                Graphics2D gr2 = (Graphics2D)gr.create();
                if (isOpaque()) {
                        gr2.setColor(getBackground());
                        gr2.fillRect(0,0,getWidth(),getHeight());
                }
                // and you miss-spelt g2 here..
                imma.disegna(gr2);
                gr2.dispose();
        }
}

class Prova extends JFrame {
       public Prova() {
           super();
           setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
           Container contents = getContentPane();
           contents.add(new ComponenteDiProva());
           pack();
           setSize(100,100);
       }

       public static void main(String[] args) {
           Prova test = new Prova();
           //test.show();  //deprecated in 1.5, use..
           test.setVisible(true);
       }
}
</sscce>

HTH