Decouples an abstraction from its implementation so both can vary independently.

Avoids a combinatorial explosion of subclasses.

abstract class Color {
  String fill();
}

class Red implements Color {
  @override
  String fill() => 'red';
}

class Blue implements Color {
  @override
  String fill() => 'blue';
}

abstract class Shape {
  final Color color;
  Shape(this.color);
  String draw();
}

class Circle extends Shape {
  Circle(super.color);

  @override
  String draw() => 'Circle filled with ${color.fill()}';
}

// Usage
print(Circle(Red()).draw());  // Circle filled with red
print(Circle(Blue()).draw()); // Circle filled with blue