code: public com.sun.javadoc.Type superclassType() {
          if (isInterface() || isAnnotationType() ||
                  (tsym == env.syms.objectType.tsym))
              return null;
          Type sup = env.types.supertype(type);
          return TypeMaker.getType(env,
                                   (sup.hasTag(TypeTag.NONE)) ? env.syms.objectType : sup);
      }
docs: Return the superclass of this class.  Return null if this is an
  interface.  A superclass is represented by either a
  <code>ClassDoc</code> or a <code>ParameterizedType</code>.