mixin vs inheritance

Learn the difference between mixin and inheritance.

mixin vs inheritance

Inheritance sets up an is-a relation: Dog extends Animal says a Dog is an Animal. A mixin sets up a can-do relation: class Bird with Flyer says a Bird can fly.

Inheritance is taken from a single base class; the subclass inherits all of its behavior and extends it. A mixin composes behavior blocks; it does not disturb the class hierarchy. Inheritance fits type relations; mixins fit adding behavior. The two can sit side by side: class Bird extends Animal with Flyer.

Which one when?

Choosing a mixin or inheritance depends on the nature of the relation. For an is-a relation pick inheritance; for a can-do relation pick a mixin.

Example: Dog extends Animal is right because a dog is an animal. Bird with Flyer is right because a bird can fly; but a bird is not a Flyer type. Wrong choice breaks code design: instead of building a subclass chain for every flight capability, the Flyer mixin is shared and applied to different classes. This avoids duplication and keeps the hierarchy lean.

Exercise Combine inheritance and a mixin

Define a class named Animal; add a void breathe() { print('Breathing'); } method. Define a mixin named Flyer; add a void fly() { print('Flying'); } method. Write a class named Bird: class Bird extends Animal with Flyer { Bird(); }. In main, create an instance with var b = Bird(); and call b.breathe() and b.fly().

Show example solution

// Define the Animal class and the Flyer mixin
class Animal {
  void breathe() {
    print('Breathing');
  }
}

mixin Flyer {
  void fly() {
    print('Flying');
  }
}

class Bird extends Animal with Flyer {
  Bird();
}

void main() {
  var b = Bird();
  b.breathe();
  b.fly();
}

Test yourself

Combine inheritance and a mixin

class Bird extends Animal ____ Flyer {
  Bird();
}

Show answer

class Bird extends Animal with Flyer {
  Bird();
}

Which is true?
- Inheritance is an is-a relation, a mixin is a can-do relation
- Inheritance is a can-do relation, a mixin is an is-a relation
- A mixin is only used with abstract classes
- They are the same
Show answer

Inheritance is an is-a relation, a mixin is a can-do relation — Inheritance sets a type relation, a mixin sets a behavior relation; they serve different goals.
Match the relation types

extends

Is-a relation
with

Can-do relation
Animal

Base class

Next lesson: Summary - Mixins

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