Out variance (covariance) in Kotlin - Mini Project: Build & Apply

Out Variance (Covariance) in Kotlin

📖 Scenario: Imagine you are creating a simple program to manage a list of animals and their sounds. You want to understand how Kotlin's out variance (covariance) works when dealing with lists of different animal types.

🎯 Goal: You will build a Kotlin program that demonstrates out variance by creating a producer interface and using it with a list of animals and their subclasses.

📋 What You'll Learn

Create a base class Animal with a function makeSound() that prints a generic sound.

Create two subclasses Dog and Cat that override makeSound() with specific sounds.

Create an interface AnimalProducer with an out variance type parameter and a function produce() that returns an animal.

Create a function that accepts an AnimalProducer<Animal> and calls makeSound() on the produced animal.

Demonstrate covariance by passing an AnimalProducer<Dog> to the function that expects AnimalProducer<Animal>.

💡 Why This Matters

🌍 Real World

Understanding covariance helps when working with collections or producers that return objects of different but related types, such as UI components, data models, or event handlers.

💼 Career

Many Kotlin and Java jobs require knowledge of generics and variance to write safe and reusable code, especially in Android development and backend services.

1

Create Animal classes

Create a base class called Animal with a function makeSound() that prints "Some generic animal sound". Then create two subclasses called Dog and Cat that override makeSound() to print "Bark" and "Meow" respectively.

Kotlin

# Create class Animal with makeSound() printing "Some generic animal sound"
# Create subclasses Dog and Cat overriding makeSound() to print "Bark" and "Meow"

Need a hint?

Remember to use open keyword for the base class and override for subclass methods.

2

Create AnimalProducer interface with out variance

Create an interface called AnimalProducer with an out variance type parameter T. Add a function produce() that returns T.

Kotlin

interface AnimalProducer {
    // Add produce() function that returns T
    // Your code here
}

Need a hint?

Use out before the type parameter T to make it covariant.

3

Implement DogProducer and function to use AnimalProducer

Create a class DogProducer that implements AnimalProducer<Dog>. Implement produce() to return a new Dog. Then create a function called makeAnimalSound that accepts an AnimalProducer<Animal> and calls makeSound() on the produced animal.

Kotlin

class DogProducer : AnimalProducer {
    // Implement produce() to return Dog
    // Your code here
}

fun makeAnimalSound(producer: AnimalProducer) {
    // Call makeSound() on the produced animal
    // Your code here
}

Need a hint?

Remember to override produce() in DogProducer and call makeSound() on the produced animal in makeAnimalSound.

4

Call makeAnimalSound with DogProducer

Create an instance of DogProducer called dogProducer. Then call makeAnimalSound(dogProducer) to demonstrate covariance. This should print "Bark".

Kotlin

// Create dogProducer instance
// Call makeAnimalSound(dogProducer)
// Your code here

Need a hint?

Create the DogProducer instance and pass it to makeAnimalSound to see the output.