Final Keyword for Classes in Dart 3.10

16 May 2023 - 2 mins read time
Tags: Dart Flutter Switch Dart 3.10

Source: Nagaraj Alagusundaram

Dart 3.10 introduces a significant keyword for classes - final. This keyword is used to close the type hierarchy and prevent subtyping from a class outside of the current library. It disallows both inheritance and implementation entirely. This is a fundamental change to how Dart handles class hierarchies and opens up a number of possibilities for safer and more controlled object-oriented programming. Consider the following example:

// audio_player.dart
final class AudioPlayer {
  void play() {
    // ...
  }
  void stop() {
    // ...
  }
}

In this case, AudioPlayer is a final class, meaning it cannot be extended or implemented outside of its own library (audio_player.dart).

Attempting to do so will result in a compile-time error:

// audio_player_state.dart
import 'audio_player.dart';

// Can be constructed
AudioPlayer audioPlayer = AudioPlayer();

// ERROR: Cannot be inherited
// The type 'Radio' must be 'base', 'final' or 'sealed' because the supertype 'AudioPlayer' is 'final'.
class Radio extends AudioPlayer {
  // ...
}

//The type 'MockPlayer' must be 'base', 'final' or 'sealed' because the supertype 'AudioPlayer' is 'final'.
class MockPlayer implements AudioPlayer {
  // ERROR: Cannot be implemented
}

The final modifier for classes provides several advantages:

  1. Safer API Changes: Since a final class cannot be extended or implemented outside its own library, you can safely add incremental changes to the API without worrying about breaking subclasses in other libraries.
  2. Guaranteed Behavior: When you call an instance method on a final class, you can be sure that the method hasn’t been overridden in a subclass, because no subclasses can exist outside the class’s own library.
  3. Controlled Inheritance: Final classes can still be extended or implemented within their own library. This allows for controlled, localized inheritance where it’s needed, while preventing uncontrolled subclassing elsewhere.

In summary, the final keyword for classes in Dart 3.10 provides a powerful tool for controlling and managing class hierarchies. It helps create safer, more predictable code and can make APIs easier to maintain and evolve over time.