Inner classes and nested classes in Kotlin - Mini Project: Build & Apply

Inner classes and nested classes

📖 Scenario: You are creating a simple Kotlin program to understand how inner classes and nested classes work. Imagine you have a Car that has an Engine. The Engine can be represented as an inner class because it needs to access the Car details. You also want to have a CarInfo nested class that just holds some static information about cars.

🎯 Goal: Build a Kotlin program that defines a Car class with an Engine inner class and a CarInfo nested class. Then create objects and print details showing how inner and nested classes work.

📋 What You'll Learn

Create a class called Car with a property brand of type String set to "Toyota"

Inside Car, create an inner class called Engine with a function start() that returns a string "Engine of [brand] started"

Inside Car, create a nested class called CarInfo with a function info() that returns the string "Cars are vehicles"

Create an instance of Car, then create an instance of Engine from that car, and call start()

Call the info() function from the CarInfo nested class

Print the results of both function calls

💡 Why This Matters

🌍 Real World

Inner and nested classes help organize code that logically belongs together, like parts of a car. This keeps code clean and easier to maintain.

💼 Career

Understanding inner and nested classes is important for Kotlin developers working on Android apps or backend services, where encapsulation and code organization matter.

1

Create the Car class with brand property

Create a class called Car with a property brand of type String set to "Toyota".

Kotlin

class Car {
    // Your code here
}

Need a hint?

Use val brand: String = "Toyota" inside the Car class.

2

Add inner Engine class with start() function

Inside the Car class, create an inner class called Engine with a function start() that returns the string "Engine of [brand] started" using the brand property.

Kotlin

class Car {
    val brand: String = "Toyota"

    // Your code here
}

Need a hint?

Remember to use the inner keyword so Engine can access brand.

3

Add nested CarInfo class with info() function

Inside the Car class, create a nested class called CarInfo with a function info() that returns the string "Cars are vehicles".

Kotlin

class Car {
    val brand: String = "Toyota"

    inner class Engine {
        fun start(): String {
            return "Engine of $brand started"
        }
    }

    // Your code here
}

Need a hint?

Use a normal nested class (no inner) for CarInfo.

4

Create instances and print results

Create an instance of Car called myCar. Then create an instance of Engine from myCar called myEngine and call start(). Also call the info() function from the CarInfo nested class. Finally, print both results.

Kotlin

class Car {
    val brand: String = "Toyota"

    inner class Engine {
        fun start(): String {
            return "Engine of $brand started"
        }
    }

    class CarInfo {
        fun info(): String {
            return "Cars are vehicles"
        }
    }
}

// Your code here

Need a hint?

Remember to create Engine instance from myCar and call start(). Call info() from Car.CarInfo().