code: public Set<String> getKeys(Class<?> type) {
          return data.entrySet().stream().filter(val -> type.isAssignableFrom(val.getValue()
                  .getClass())).map(Map.Entry::getKey).collect(Collectors
                  .toSet());
      }
docs: Get the present keys of all entries with a given type, checking hereby if assignable.
  
  @param type The attribute type, not null.
  @return all present keys of attributes being assignable to the type, never null.