In short – yes you can. There is nothing that prevents you from compiling the following:
String s1 = (String)null;
I even think you’ll have to do it to make your code compile for one for the methods of lotus.domino-package. I can’t of the top of my head remember which method it is, but there is one method where the compiler will force you to cast a null to a String. You’ll also sometimes end up casting a null pointer when doing dynamic classloading.
I told you it would be a short post.