boolean b = ((Object)null == 3);
   should not compile since it's not a numerical equality test
   (JLS15.21.1) nor a boolean equality test (15.21.2) nor a
   reference equality test (15.21.3). Indeed, it does not compile.

boolean b = (3 == (Object)null);
   should not compile for exactly the same reason, but does compile!
   I am told javac performs a boxing conversion followed by a
   widening reference conversion, which isn't a valid combination
   allowed under casting conversion (mentioned in 15.21.3), so I am
   not sure what javac is trying to do.