In variance (contravariance) in Kotlin - Mini Project: Build & Apply

In variance (contravariance) in Kotlin

📖 Scenario: Imagine you are building a simple system to handle different types of fruits and their baskets. You want to learn how Kotlin's in keyword helps with contravariance, allowing you to safely use a basket that can accept fruits or their supertypes.

🎯 Goal: You will create a generic interface with contravariance using in, then implement it and use it to add fruits to baskets safely.

📋 What You'll Learn

Create a generic interface called Basket with contravariance using in

Create classes Fruit and Apple where Apple inherits from Fruit

Implement the Basket interface in a class called FruitBasket

Use the contravariant Basket type to add an Apple to a basket of Fruit

Print confirmation messages showing the fruit added

💡 Why This Matters

🌍 Real World

Contravariance is useful when you want to write flexible code that can accept inputs of a more general type, such as event handlers or consumers in UI programming.

💼 Career

Understanding variance helps you work with Kotlin generics safely and effectively, a skill important for Android development and backend Kotlin projects.

1

Create the Fruit and Apple classes

Create a class called Fruit and a class called Apple that inherits from Fruit.

Kotlin

# Your code here

Need a hint?

Use open class Fruit to allow inheritance, then class Apple : Fruit() to inherit.

2

Create the contravariant Basket interface

Create a generic interface called Basket with a contravariant type parameter T using in. Add a function addItem that takes a parameter of type T.

Kotlin

open class Fruit

class Apple : Fruit()

# Your code here

Need a hint?

Use interface Basket to declare contravariance and define fun addItem(item: T).

3

Implement FruitBasket class

Create a class called FruitBasket that implements Basket<Fruit>. Inside, override addItem to print "Added a fruit to the basket".

Kotlin

open class Fruit

class Apple : Fruit()

interface Basket {
    fun addItem(item: T)
}

# Your code here

Need a hint?

Implement Basket<Fruit> and override addItem to print the message.

4

Use contravariance to add an Apple to FruitBasket

Create a variable basket of type Basket<Apple> and assign it a FruitBasket(). Then call basket.addItem(Apple()). Print the output.

Kotlin

open class Fruit

class Apple : Fruit()

interface Basket {
    fun addItem(item: T)
}

class FruitBasket : Basket {
    override fun addItem(item: Fruit) {
        println("Added a fruit to the basket")
    }
}

# Your code here

Need a hint?

Assign FruitBasket() to Basket<Apple> variable and call addItem(Apple()).