Java applet redraw refresh

Question

My project is trying to implement java processing.core.Papplet to display a graphics with different color( Random color changing by times). It is so weird that I can only use:

  applet.mousePressed();

to refresh my applet. And I also tried:

        applet.redraw();
        applet.updatePixels();
        applet.loop();
        applet.redraw();
        applet.noLoop();
        applet.redraw();
        applet.draw();

But neither of them works in my project. This means I can refresh my Papplet by either Robot mouse click or mouse click to make my applet refresh. Does anybody know what's going on? Appreciate for your great help.

BTW, I have tried the invoke method:

        SwingUtilities.invokeLater(new Runnable(){

            @Override
            public void run() {
                // TODO Auto-generated method stub
                target.refresh();
                previewController.render(target);
                applet.redraw();    
            }

        });

It can force flash the applet, but with some error image. I don't know what's going on with Papplet mousePressed.