Puzzling issue in casting a class

[GaryM]

Recently I asked the question on how to initialize a web app. The
advice I adpoted till I could learn a better approach was to declare
web.xml servlet that loads on startup.

I was browsing the Jetty API Doc and I noticed the class
WebApplicationContext inherits a loadclass method:

Class loadClass(String class)

To test it out I tried loading my intialization servlet as follows:

WebApplicationContext wac = new WebApplicationContext( ... );
Class class = wac.loadClass("InitializerServlet");
HttpServlet servlet = (HttpServlet) class.newInstance();
servlet.init();

And this worked fine. However there is no longer any need for that
class to be a servlet, so I changed it accordingly (i.e. removed the
'extends HttpServlet'). Changed the above lines accordingly
(HttpServer became Initializer). Immediately got a
ClassCastException. Created a constructor with a debug message and
saw that Initializer was being constructed. But it failed on the
casting line.

In the end I thought of a number of reasons why the approach was
unsound and since have moved on, but I am still wondering why the
heck I could not cast that instance as itself.

If someone could inch my java knowledge on a bit, would be most
grateful.