code: private URI createFileSystemURI(URI configKey) throws URISyntaxException, IOException {
      // Validate the scheme
      String configKeyScheme = configKey.getScheme();
      if (!configKeyScheme.startsWith(getSchemePrefix())) {
        throw new IllegalArgumentException(
            String.format("Scheme for configKey \"%s\" must begin with \"%s\"!", configKey, getSchemePrefix()));
      }
  
      if (Strings.isNullOrEmpty(configKey.getAuthority())) {
        return new URI(getPhysicalScheme(), getDefaultStoreFsLazy().getUri().getAuthority(), "", "", "");
      }
      String uriPhysicalScheme = configKeyScheme.substring(getSchemePrefix().length(), configKeyScheme.length());
      return new URI(uriPhysicalScheme, configKey.getAuthority(), "", "", "");
    }
docs: Creates a Hadoop FS {@link URI} given a user-specified configKey. If the given configKey does not have an authority,
  a default one is used instead, provided by the default root path.