Classes, Enums, Extensions

Inheritance

extends, super, override

extends makes one class inherit from another. A subclass constructor uses : super(...) to initialize inherited fields, and @override plus a same-named method replaces the parent's version. super.method() still reaches the parent body.

Program

Play the program to build a Car that extends Vehicle and overrides describe.

inheritance_extends.dart
Replay: real traced execution (multi-file project)
class Vehicle {
  final String name;
  final int wheels;
  Vehicle(this.name, this.wheels);
  String describe() => '$name has $wheels wheels';
}

class Car extends Vehicle {
  final int doors;
  Car(String name, this.doors) : super(name, 4);
  @override
  String describe() => '${super.describe()} and $doors doors';
}

void main() {
  var c = Car('Sedan', 4);
  print(c.describe());
}

  1. call ← Car('Sedan', 4)

    15void main() {16  var c = Car('Sedan', 4);17  print(c.describe());
    values this stepCar('Sedan', 4)call

  2. super call ← Vehicle('Sedan', 4)

    9final int doors;10Car(String name, this.doors) : super(name, 4);11@override
    values this stepVehicle('Sedan', 4)super callSedanname4this.doors

  3. Vehicle init ← name=Sedan, wheels=4

    3final int wheels;4Vehicle(this.name, this.wheels);5String describe() => '$name has $wheels wheels';
    values this stepname=Sedan, wheels=4Vehicle initSedanthis.name4this.wheels

  4. c ← Car(name: Sedan, wheels: 4, doors: 4)

    15void main() {16  var c = Car('Sedan', 4);17  print(c.describe());
    values this stepCar(name: Sedan, wheels: 4, doors: 4)c

  5. call ← c.describe()

    16  var c = Car('Sedan', 4);17  print(c.describe());18}
    values this stepc.describe()callCar(...)c

  6. calls ← super.describe()

    11  @override12  String describe() => '${super.describe()} and $doors doors';13}
    values this stepsuper.describe()callsCar instancethis

  7. return value ← Sedan has 4 wheels

    4  Vehicle(this.name, this.wheels);5  String describe() => '$name has $wheels wheels';6}
    values this stepSedan has 4 wheelsreturn valueSedanthis.name4this.wheels

  8. override return ← Sedan has 4 wheels and 4 doors

    11  @override12  String describe() => '${super.describe()} and $doors doors';13}
    values this stepSedan has 4 wheels and 4 doorsoverride return

  9. print(c.describe());

    16  var c = Car('Sedan', 4);17  print(c.describe());18}
    outputSedan has 4 wheels and 4 doors
extends `class Car extends Vehicle` makes `Car` inherit from `Vehicle`.
super constructor `: super(name, 4)` forwards to the parent constructor so inherited fields are initialized.
override `@override` plus a same-named method replaces the parent's; `super.describe()` still reaches the parent body.