@Override
  public GroupPrincipal lookupPrincipalByGroupName(String group) throws IOException {
    if (!supportsGroups) {
      throw new UserPrincipalNotFoundException(group); // required by spec
    }
    return createGroupPrincipal(group);
  }
{@link UserPrincipalLookupService} implementation.

@author Colin Decker
jimfs/src/main/java/com/google/common/jimfs/UserLookupService.java