Classes and interfaces

Dart’s interfaces and classes provide you with a set of reusable and extensible building blocks. An interface defines a basic set of methods and constants, sometimes by inheriting from other interfaces. A class can implement multiple interfaces, but it only inherits from a single superclass.
The following example defines an interface, along with a class and subclass that implement it:

interface Shape {
  num perimeter();
}

class Rectangle implements Shape {
  final num height, width; 
  Rectangle(num this.height, num this.width);  // Compact constructor syntax.
  num perimeter() => 2*height + 2*width;       // Short function syntax.
}

class Square extends Rectangle {
  Square(num size) : super(size, size);
}