code: public static void checkRequiredPreconditions(final boolean required, final String ifMatch,
              final String ifUnmodifiedSince) {
          if (required && ifMatch == null && ifUnmodifiedSince == null) {
              throw new ClientErrorException(status(PRECONDITION_REQUIRED).build());
          }
      }
docs: Check whether conditional requests are required.
  @param required whether conditional requests are required
  @param ifMatch the If-Match header
  @param ifUnmodifiedSince the If-Unmodified-Since header