Dart Exercises: OOP and Async

Let's think about it this way for a sec

This lesson brings together the null safety, OOP (classes, constructors, inheritance, mixins), and Future/async/await concepts from the Intermediate/Advanced chapter into some tougher practice tasks. In real app development, you rarely use these concepts in isolation — they're almost always combined. So these exercises give you practice handling nullable fields within a class hierarchy and returning objects from async functions. Once you've written your solution, check where null safety errors (`!`, `?`, `??`) could still crop up.

Exercises

Task 1: Build an `Animal` base class with a name (String) field and a nullable `sound` (String?) field. Write a `makeSound()` method that prints "..." when sound is null, and uses the null-aware operator (`??`) to print the sound when it's not null. Task 2: Create a `Dog` class that inherits from `Animal`, calling `super` in its constructor and defaulting sound to "Woof". Task 3: Build a `Flyable` mixin with a `fly()` method, then have a `Bird` class extend `Animal` and use `with Flyable`. Task 4: Write an async function `Future<Animal> fetchRandomAnimal()` that waits 1 second via `Future.delayed` and then returns a `Dog` object — call it with `await` inside `main()` and then call `makeSound()`.

• Can safely handle nullable fields with `??` or `?.`
• Can combine inheritance and mixins in a single class
• Can correctly call an async function that returns an object, using await

Code Example

class Animal {
  String name;
  String? sound;

  Animal(this.name, [this.sound]);

  void makeSound() {
    // TODO: use ?? to handle null sound
    print('$name says ${sound ?? "..."}');
  }
}

class Dog extends Animal {
  Dog(String name) : super(name, 'Woof');
}

mixin Flyable {
  void fly() => print('Flying...');
}

class Bird extends Animal with Flyable {
  Bird(String name) : super(name, 'Tweet');
}

Future<Animal> fetchRandomAnimal() async {
  await Future.delayed(Duration(seconds: 1));
  return Dog('Rex');
}

void main() async {
  final cat = Animal('Cat');
  cat.makeSound();

  final dog = Dog('Buddy');
  dog.makeSound();

  final bird = Bird('Sky');
  bird.makeSound();
  bird.fly();

  print('Fetching...');
  final randomAnimal = await fetchRandomAnimal();
  randomAnimal.makeSound();
}

Your console should print, in order: Cat ('...'), Buddy ('Woof'), Sky ('Tweet' + 'Flying...'), and then — after a 1-second wait — Rex ('Woof').