code: public static <T extends Executable> Predicate<T> executableBelongsToClassAssignableTo(Class<?> reference) {
          return candidate -> candidate != null && reference.isAssignableFrom(candidate.getDeclaringClass());
      }
docs: Checks if a candidate executable does belong to a class assignable as the specified class.
  
  @param reference the class to check against.
  @return the predicate.