Step 1 - One list, many types

abstract class Notifier {
  void send(String text);
}

class EmailNotifier implements Notifier {
  @override
  void send(String text) => print('email: $text');
}

class SmsNotifier implements Notifier {
  @override
  void send(String text) => print('sms: $text');
}

void notifyAll(List<Notifier> channels, String msg) {
  for (final c in channels) {
    c.send(msg);
  }
}

void main() {
  notifyAll([EmailNotifier(), SmsNotifier()], 'hello');
}

• channels is typed as List<Notifier>; each element uses its own send.

Step 2 - Subtype substitution

• Where a Notifier is expected, any class that satisfies Notifier is allowed.
• The caller depends on the abstraction, not concrete classes.

Good habit

• Push polymorphism to boundaries (services, repositories); keep leaf types small.

Practice tasks

• Add PushNotifier and include it in notifyAll without changing notifyAll’s body.
• Replace the loop with for (final c in channels) c.send(msg) using a void Function(String) if you want to contrast object polymorphism with function values (optional).