Avoiding using instanceof operator

Question

I've heard that using the instanceof operator isn't such a great code technique, and since I'm in the design phase, I feel like I should be able to avoid this. Right now, part of my design requires use of the instanceof operator as well as casting.
Let's say you have a common notifier interface and two abstract classes which extend from this interface:
com.example.Notifier Interface:
public void doNotify(Notification notif, List notifiables);
public boolean canSend(Notification notif, Notifiable person);
com.example.SerialNotifier (abstract)
public void doNotify(Notification notif, List people)
    for (Iterator itList = people.iterator(); itList.hasNext();)
         Notifiable person = (Notifiable)itList.next();
         doNotify(person, notif);
public abstract doNotify(Notification notif, Notifiable person);
com.example.MulticastNotifier (abstract)
public abstract void doNotify(Notification notif, List notifiables);MulticastNotifier purposefully doesn't define a way to send a message to a single person, so that developers don't implement a non-multicast solution for a multicast protocol.
Here's the part that seems to require an "instanceof" clause. Now let's say you have a list of Notifiers. The list of notifiers is mixed, containing both Serial and Multicast Notifiers. Before you send out the notification, you need to filter the list (because for example, someone doesn't want to be notified at all):
    for (Iterator itNotifiers = notifiers.iterator(); itNotifiers.hasNext();)
         Notifier notifier= (Notifier)itNotifiers.next();
         List buildPeopleList = new ArrayList();
         for (Iterator itPeople = people.iterator(); itPeople.hasNext())
               Notifiable person= (Notifiable)itPeople.next();
               if (notifier.canSend(person, notification))
                       if (notifier instanceof MulticastNotifier)
                              buildPeopleList.add(person);
                       } else
                               SerialNotifier serialNotif = (SerialNotifier)notifier;
                               serialNotif.doNotify(notification, person);
         if (notifier instanceof MulticastNotifier)
                  notifier.doNotify(notification, buildPeopleList)
    }Is this bad design? Another option is to just iterate through the list essentially twice, but this is of course, less efficient for the serial notifier case:
    for (Iterator itNotifiers = notifiers.iterator(); itNotifiers.hasNext();)
         Notifier notifier= (Notifier)itNotifiers.next();
         List buildPeopleList = new ArrayList();
         for (Iterator itPeople = people.iterator(); itPeople.hasNext())
               Notifiable person= (Notifiable)itPeople.next();
               if (notifier.canSend(person, notification))
                       buildPeopleList.add(person);
         notifier.doNotify(notification, buildPeopleList);
    }Any suggestions?

Answer 1

I'm not sure what you mean. I moved a loop from your
code to a different class. AFAICT, it's a
one-for-one. My solution has the same number of
iterations as yours.Yeah, your code is actually missing a loop, because, as I explained earlier, the design supports filtering at both a parent notifier and a child notifier. To filter, it needs to loop through the list of people
So, your code actually will look like: (forgive me, doing code in a textarea is kinda a pain)
public void ParentNotifier implements Notifier
// member var for simplicity
protected Filter myFilter = new ExampleFilter();
protected List notifiers = // list of notifiers
public void doNotify(Notification n, List people)
  List intermediateList= new ArrayList();
  for (Iterator itPeople = people.iterator(); itPeople.hasNext();)
        Notifiable person = (Notifiable); itPeople.next()
        if (myFilter.canSend(n, person) // parent's filter
             intermediateList.add(person);
   for (Iterator itNotifiers = notifiers.iterator(); itNotifiers.hasNext();)
                Notifier notif = (Notifier)itNotifiers.next();
                notif.doNotify(n, intermediateList); // 2nd loop is in here where additional filtering takes place
}If you assume a serial notifier, you don't need this additional iteration:
public void ParentNotifier implements Notifier
// member var for simplicity
protected Filter myFilter = new ExampleFilter();
protected List notifiers = // list of notifiers
public void doNotify(Notification n, List people)
  for (Iterator itPeople = people.iterator(); itPeople.hasNext();)
        Notifiable person = (Notifiable); itPeople.next()
        if (myFilter.canSend(n, person) // parent's filter
             for (Iterator itNotifiers = notifiers.iterator(); itNotifiers.hasNext();)
                     Notifier notif = (Notifier)itNotifiers.next();
                     // PROBLEM HERE: assumes serial notifier
                     notif.doNotify(n, person); // child will do own filtering
}Edit: Corrected logic error in code