Interfaces in Dart

main.dart

// Interfaces in Dart (as of v2.5)

// See live at https://dartpad.dartlang.org/19a8c3c203f27fd1820c004582480ff5

/*
 * TLDR;
 * There is no interface keyword in Dart. Instead we use a regular class with either the extends or
 * implements keyword. If a subclass uses the extends keyword, the child class inherits from the 
 * parent class normally. Else, if the subclass uses the implements keyword, the parent class is
 * treated as an interface: all inherited members must be overriden.
 * 
 * */

void main() {
  
  // We use the methods inheritted from the parent, with the OPTION to override, as normal.
  NeutralParent regularChild = RegularChild();
  regularChild.doSomethingCool();
  print('Favorite color is ${regularChild.favoriteColor}');
 
  
  print('---------------------------------------');
  
  // Inheriting from in
  NeutralParent interfaceBasedChild = InterfaceBasedChild();
  interfaceBasedChild.doSomethingCool();
  print('Favorite color is ${interfaceBasedChild.favoriteColor}');
}

class NeutralParent {
  String favoriteColor = 'Green';
  
  void doSomethingCool() {
    print('Wow! Very, very cool!');
  }
}

// Normal inheritance
class RegularChild extends NeutralParent {
  String favoriteColor = 'NOT Green';
}

// Using the interface keyword here, forces our NeutralParent to behave like an interface
// In this case, we MUST provide concrete implemntations of all class members.
class InterfaceBasedChild implements NeutralParent {
  String favoriteColor = 'Mandatorily Pink';
  
  void doSomethingCool() {
    print('It isn\'t as cool the second time :`(');
  }
}