code: @SuppressWarnings("rawtypes")
  	public static Class<?> getMapKeyType(Class<? extends Map> mapClass) {
  		return ResolvableType.forClass(mapClass).asMap().resolveGeneric(0);
  	}
docs: Determine the generic key type of the given Map class
  (if it declares one through a generic superclass or generic interface).
  @param mapClass the map class to introspect
  @return the generic type, or {@code null} if none