New to Java

Hi all,
I want to add a watermark(text or image) at the runtime, I am creating pdf file with using iText 1.3 api.
It created fine. I have following code for add watermark for an existining file.

<code>
try {
PdfReader reader = new PdfReader("ConfirmBillPDF.pdf");
int n = reader.getNumberOfPages();

PdfStamper stamp = new PdfStamper(reader, new FileOutputStream("NewPDFWithWatermarkImage.pdf"));
int i = 0;
PdfContentByte under;
Image img = Image.getInstance("ValidityExpWaterMark.png");
img.setAbsolutePosition(100, 400);
while (i < n)
{
i++;
under = stamp.getUnderContent(i);
under.addImage(img);
}
stamp.close();
}
catch (Exception de)
{
de.printStackTrace();
}

</code>

But I want to add this with runtime.

Plese help me..