Try the following:

JTextPane edit=new JTextPane();
try {
    SimpleAttributeSet attrs=new SimpleAttributeSet();
    StyleConstants.setAlignment(attrs,StyleConstants.ALIGN_RIGHT);
    StyledDocument doc=(StyledDocument)edit.getDocument();
    doc.insertString(0,"test",attrs);
    doc.setParagraphAttributes(0,doc.getLength()-1,attrs,false);
}
catch (Exception ex) {
    ex.printStackTrace();
}

That came from
http://forum.java.sun.com/thread.jspa?forumID=57&threadID=390611