You've come a long way in a day.
Short answer, you are ignoring the namespace of your XML Document. Change the line:
List list = xmlfile.getChildren("url");
to
Namespace ns = Namespace.getNamespace("http://www.sitemaps.org/schemas/sitemap/0.9");
List list = xmlfile.getChildren("url", ns);
For your convenience, you may also want to simplify the whole build process to:
org.jdom2.Document doc = saxBuilder.build("http://www.someurl.info/sitemap.xml");