Inheritance vs Composition

Inheritance

• 'is a' relationship
• "A Window is a Rectangle"

class Rectangle {
  height: number;
  width: number;
  area(): number;
}
class Circle {
  radius: number;
  area(): number;
}

class Wall extends Rectangle {
  color: string;
}
class RectangleWindow extends Rectangle {
  open: boolean;
  toggleOpen(): void;
}
class CircleWindow extends Circle {
  open: boolean;
  toggleOpen(): void;
}

Composition

• 'has a' relationship
• "A Window has a Rectangle"
• Composition is not adequate when there is bidirectional relationship between the classes

class Wall {
  constructor(public shape: Shape) {}
  color: string;
  area(): number {
    shape.area();
  } // A Reference to the area of the shape Shape!
  dimensions: Shape; // The dimensions references a Shape! (Rectangle or Circle)
}
class Window {
  constructor(public shape: Shape) {}
  open: boolean;
  toggleOpen(): void;
  area(): number {
    shape.area();
  } // A Reference to the area of the shape Shape!
  dimensions: Shape; // The dimensions references a Shape! (Rectangle or Circle)
}

// The interface forces the Shape object to have a area method
interface Shape {
  area(): number;
}

class Rectangle {
  height: number;
  width: number;
  area(): number;
}
class Circle {
  radius: number;
  area(): number;
}