Can't set Printing Attributes for DocPrintJob Job

Hi, everyone

i 've been trying to set some printing attributes for a .ps file using
PrintRequestAttributeSet class
but nothing happens.What i want to do is to set the tray on a network
printer(HP LaserJet 4100 Series PCL ). I am able to print fine but
can't set the tray through my prog.
Also notice that i don't want any user interaction(silent print), so
don't wanna use the printDialog().
Any ideas suggestions?

Thanx in advance..

public static void main(String[] args)
{
 PrintService defaultPrintService =
PrintServiceLookup.lookupDefaultPrintService();
 DocPrintJob printerJob = defaultPrintService.createPrintJob();
 PrintRequestAttributeSet aset = new HashPrintRequestAttributeSet();

 Doc simpleDoc = null;
 String psFile = "F:/test.ps";
 FileInputStream in = null;

 try
{
in = new FileInputStream(psFile);
simpleDoc = new SimpleDoc(in, DocFlavor.INPUT_STREAM.AUTOSENSE,null);

aset.add(PrintQuality.HIGH);
aset.add(new Copies(2));
aset.add(MediaTray.MAIN);

//I would like to print from the buttom(<=>main) tray even when there
are papers in
//another e.g.manual feed,not to print from manual when paper is
present in manual feed

printerJob.print(simpleDoc,aset);

 }
    catch(Exception e){System.out.println(e.getMessage());}
}