how to retrieve the right object (user or group) with a param UserOrGroupDataType inside a plugin (function) ?

Like in the next reply 

private EntityIdentity retrieveIdentityType(TypedValue userorGroup, UserService us, GroupService gs)
throws InvalidLocalObjectTypeException {
Object obj = userorGroup.getValue();
if (AppianType.GROUP == userorGroup.getInstanceType().intValue()) {
Long groupId = (Long) obj;
if (gs.doesGroupExist(groupId))
return new GroupIdentity((Long) obj);
else
throw new InvalidLocalObjectTypeException(
"The grop with the following groupId doesn't exist:" + groupId);
// Long groupId=(Long) obj;
}

if (AppianType.USERNAME == userorGroup.getInstanceType().intValue()) {
String username = (String) obj;
if (us.doesUserExist(username))
return new UserIdentity(username);
else
throw new InvalidLocalObjectTypeException(
"The user with the following username doesn't exist:" + username);
}
throw new InvalidLocalObjectTypeException(
"the userorGroup param is invalid : it should be either a user or a group");

}

Where TypedValue userorGroup is an incoming parameter for the custom function 

@Function
public Boolean hasAdminRightOnDocument(ContentService _cs, UserService us, GroupService gs,
@Parameter(required = true) @DocumentDataType @Name("document_to_check") Long documentId,
@Parameter(required = false) @UserOrGroupDataType @Name("user_or_group") TypedValue userorGroup