code: public static String toDefault(String stringEntry) {
      if (stringEntry == null) {
        throw new IllegalArgumentException("Input acl string is null");
      }
      List<String> components = Arrays.stream(stringEntry.split(":")).map(String::trim).collect(
          Collectors.toList());
      if (components != null && components.size() > 0 && components.get(0).equals(DEFAULT_KEYWORD)) {
        return stringEntry;
      } else {
        return DEFAULT_PREFIX + stringEntry;
      }
    }
docs: Convert a normal ACL to a string representing a default ACL.
  @param stringEntry normal ACL, if it is already default ACL, then return the default ACL
  @return a default ACL