Oppiskele Composition vs Inheritance | Composition and Polymorphism

Definition

Composition is an object-oriented design principle where a class contains other objects and uses them to perform its work instead of inheriting their behavior.

Composition helps you build complex features by combining simpler classes rather than extending a single parent class. In Dart, this means storing instances of other classes as fields and delegating responsibilities to them. Compared to inheritance, composition reduces tight coupling, improves flexibility, and makes your code easier to test and maintain.

'Has-a' vs 'Is-a' relationships

Inheritance models an "is-a" relationship: a Dog is an Animal.
Composition models a "has-a" relationship: a Car has an Engine.

main.dart


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class Engine {
  void start() {
    print('Engine started');
  }
}

class Car {
  final Engine engine;

  Car(this.engine);

  void start() {
    engine.start();
  }
}

void main() {
  Engine myEngine = Engine();
  Car myCar = Car(myEngine);
  myCar.start(); // Output: Engine started
}

In this example, the Car class does not inherit from Engine. Instead, it holds an instance of Engine and delegates the start() action to it. This is a classic use of composition: Car has-an Engine, and can use its functionality without forming a rigid inheritance relationship. You should prefer composition over inheritance when you want to use features from another class but do not want to expose your class to all behaviors of the parent, or when you want to change the implementation details without affecting a class hierarchy.

main.dart


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class Screen {
  void display() {
    print('Screen is displaying content');
  }
}

class Phone {
  final Screen screen;

  Phone(this.screen);

  void showHomeScreen() {
    screen.display();
  }
}

void main() {
  Screen phoneScreen = Screen();
  Phone myPhone = Phone(phoneScreen);
  myPhone.showHomeScreen(); // Output: Screen is displaying content
}

Note

Favoring composition over inheritance means designing your classes to include (compose) other objects to share functionality, rather than always extending classes. This leads to more flexible, maintainable, and reusable code.

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Pyyhkäise näyttääksesi valikon

Definition

Composition is an object-oriented design principle where a class contains other objects and uses them to perform its work instead of inheriting their behavior.

'Has-a' vs 'Is-a' relationships

Inheritance models an "is-a" relationship: a Dog is an Animal.
Composition models a "has-a" relationship: a Car has an Engine.

main.dart


              123456789101112131415161718192021
            
class Engine {
  void start() {
    print('Engine started');
  }
}

class Car {
  final Engine engine;

  Car(this.engine);

  void start() {
    engine.start();
  }
}

void main() {
  Engine myEngine = Engine();
  Car myCar = Car(myEngine);
  myCar.start(); // Output: Engine started
}

main.dart


              123456789101112131415161718192021
            
class Screen {
  void display() {
    print('Screen is displaying content');
  }
}

class Phone {
  final Screen screen;

  Phone(this.screen);

  void showHomeScreen() {
    screen.display();
  }
}

void main() {
  Screen phoneScreen = Screen();
  Phone myPhone = Phone(phoneScreen);
  myPhone.showHomeScreen(); // Output: Screen is displaying content
}

Note

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Kiitos palautteestasi!

Osio 3. Luku 1