Subtyping

Learn subtyping and polymorphism.

Subtyping

Subtyping is when one type is a subtype of another. In class Dog extends Animal, Dog is a subtype of Animal. A Dog can be assigned to an Animal variable (polymorphism).

The subtype must preserve the supertype's contract: it must support its methods and behave consistently. In Dart, extends gives inheritance, implements interface implementation, and with mixin usage.

Polymorphism and override

Polymorphism is when a supertype variable can hold subtype instances. Animal a = Dog(); is valid. The @override annotation marks that a subtype redefines a supertype method: @override String speak() => 'Woof';.

Dart recommends @override; if the signature is wrong, a compile error is raised. Polymorphism lets you write code against the supertype and specialize in subtypes.

Exercise Animal and Dog subtype

Define an Animal class with a String speak() => 'Sound'; method. Make Dog extend Animal and include an @override String speak() => 'Woof'; method. In main create final Animal a = Dog();; print the result with print(a.speak());.

Show example solution

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

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

void main() {
  final Animal a = Dog();
  print(a.speak());
}

Test yourself

What must a subtype satisfy?
- Must preserve the supertype's contract
- Must have the same field names only
- Must contain fewer methods
- Must use the same constructor parameters
Show answer

Must preserve the supertype's contract — a subtype must preserve the supertype's contract (Liskov substitution).
Match the subtyping relations

extends

Inheritance
implements

Interface implementation
with

Mixin usage
Dog <: Animal

Subtype example

The @override annotation

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

Show answer

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

Next lesson: Records and patterns

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