If you mean you have a Java applicaiton running that needs to display the PDF, then Java's Desktop provides one solution. Your code would do this:
get/download the pdf and save to a file:
File myPdf = saveToFile(pdfStream);
call desktop to display the PDF
java.awt.Desktop.getDesktop().open(myPdf);
Another option would be to launch a browser with a url to display the PDF. This can be done with the Desktop.browse(uri)
method.