@Nullable
  private static Set<String> getForbiddenApi(@NotNull LanguageLevel languageLevel) {
    String message = getShortMessage(languageLevel);
    if (message == null) return null;
    Reference<Set<String>> ref = ourForbiddenAPI.get(languageLevel);
    Set<String> result = dereference(ref);
    if (result == null) {
      String fileName = "api" + message + ".txt";
      URL resource = LanguageLevelUtil.class.getResource(fileName);
      if (resource != null) {
        result = loadSignatureList(resource);
      }
      else {
        Logger.getInstance(LanguageLevelUtil.class).error("File not found: " + fileName);
        result = Collections.emptySet();
      }
      ourForbiddenAPI.put(languageLevel, new SoftReference<>(result));
    }
    return result;
  }
@Nullable
  private static Set<String> getForbiddenApi(@NotNull LanguageLevel languageLevel)