Interfaces and implements

Dart has no separate interface keyword for defining one: every class is also an interface. implements promises to provide all of a class's members — without inheriting any code.

Real-world analogy: an interface is a job description ("must be able to drive, must have a license"). Anyone who meets it can do the job.

Example

abstract class Notifier {
  String get channel;
  String send(String message);
}

class EmailNotifier implements Notifier {
  final String address;
  EmailNotifier(this.address);

  @override
  String get channel => 'email';

  @override
  String send(String message) => 'Email to $address: $message';
}

class SmsNotifier implements Notifier {
  @override
  String get channel => 'sms';

  @override
  String send(String message) =>
      'SMS: ${message.length > 20 ? message.substring(0, 20) : message}';
}

void main() {
  final List<Notifier> notifiers = [EmailNotifier('ada@example.com'), SmsNotifier()];
  for (final n in notifiers) {
    print('[${n.channel}] ${n.send('Your order has shipped today')}');
  }
}

Expected output

[email] Email to ada@example.com: Your order has shipped today
[sms] SMS: Your order has shipp

• An abstract class with no implementation is the usual way to define an interface.
• A class can implement several interfaces: class Robot implements Walker, Talker.
• Code that uses a Notifier doesn't care which kind it has, so adding a new channel needs no change to that code.

extends vs implements

• extends — inherit the superclass's code; only one superclass.
• implements — promise the same members, but write them all yourself; as many as you like.

Interfaces make code testable

Depending on an interface lets tests pass a fake implementation — for example, a FakeNotifier that records messages instead of sending them. You'll use this in the Testing lesson.

Common mistakes

• Implementing an interface and forgetting a member: The non-abstract class 'SmsNotifier' is missing implementations for these members. Your editor's quick fix can add stubs.