Custom realm - why twice group object ?

Hello !

I am testing custom realm in glassfish and i am following "application -development-guide.pdf" from: https://glassfish.java.net/docs/4.0/application-development-guide.pdf
On page 48, the document explain that we must create 2 classes (derivated of AppservRealm and AppservPasswordLoginModule).

I don't understand why both in AppservPasswordLoginModule and AppservRealm we link user with group :

Ex:

public class CustomLoginModule extends AppservPasswordLoginModule {

    /*
     * Perform actual authentication
     */
    @Override
    protected void authenticateUser() throws LoginException {
        super.authenticateUser(); 
        
        _logger.info("Passing in authenticateUser of CustomLoginModule");
        _logger.log(Level.INFO, "user: {0}", _username);
        _logger.log(Level.INFO, "password: {0}", _passwd);  
        
        // .......some other code
        

        // Final step
        String[] grpList = {"userGroup"};
        // populate grpList with the set of groups to which
        // _username belongs in this realm, if any
        commitUserAuthentication(grpList);
    }
    
}

In the last part we populate group for current user, but in this case what are the use of : public Enumeration getGroupNames(String string) in AppservRealm class ?

public class CustomRealm extends AppservRealm {
   .......  
    
    /*
     * This method returns an Enumeration (of String objects) enumerating the groups 
     * (if any) to which the given username belongs in this realm.
     */            
    @Override
    public Enumeration getGroupNames(String string) throws InvalidOperationException, NoSuchUserException {        
        List groupNames = new LinkedList();
        return (Enumeration) groupNames;        
    }    
}

Some code over internet have put "List groupNames = new LinkedList();" but i don't understand why..
Can someone can explain me the link between these two class (in terms of groups) ?

Regards