Inheritance and Overriding

Inheritance and Overriding

When several classes need to share the same starting point, rewriting the same code in each one gets tiring and error-prone. Dart solves this with inheritance. Inheritance lets one class build on another: the new class automatically gets the fields and methods of the class it builds on, and can add its own or replace some of them.

The class you start from is called the parent (or superclass). The class you create from it is the child (or subclass). The keyword is extends: a child extends its parent.

Here is a small example. A Vehicle class holds what every vehicle has. A Car class extends it and adds something extra.

class Vehicle {
  int wheels;
  Vehicle(this.wheels);
  String describe() => 'I have $wheels wheels';
}

class Car extends Vehicle {
  String horn;
  Car(this.horn) : super(4); // pass wheels up to the parent
  void honk() => print('$horn!');
}

void main() {
  final car = Car('beep');
  print(car.describe()); // inherited from Vehicle
  car.honk();            // added by Car
}

Notice two things. First, car.describe() works even though Car never defined it — the child inherits it. Second, Car's constructor calls super(4). The word super refers to the parent class; here it hands the value 4 to the parent's constructor.

Overriding: Replacing Inherited Behavior

Sometimes a child should behave differently from its parent. Overriding means writing a method in the child with the same name, replacing the parent's version. In Dart you must mark it with @override so the tool can check you actually matched a real parent method — this catches typos early.

class Animal {
  String speak() => '...';
}

class Dog extends Animal {
  @override
  String speak() => 'Woof!';
}

void main() {
  final Animal pet = Dog();
  print(pet.speak()); // Woof! — the child's version wins
}

Even though the variable is typed as Animal, Dart runs the Dog version at runtime. This is called polymorphism: the same call behaves differently depending on the actual object.

A Child Can Reuse the Parent's Version

An overriding method can also call the parent's version with super, then add to it:

class Truck extends Vehicle {
  Truck() : super(6);
  @override
  String describe() => '${super.describe()} and I haul heavy loads';
}

One Parent Only

Dart allows a class to extend exactly one parent — no multiple inheritance. If you need to share behavior from several sources, Dart offers mixins, which a later chapter covers.

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Show diagram source

classDiagram
    Animal <|-- Dog
    Animal <|-- Cat
    Animal : speak()
    Dog : speak()
    Cat : speak()