open Classes in Kotlin

Introduction

open Classes is a fundamental concept every Kotlin developer should understand. Inheritance models is-a relationships and enables code reuse. Kotlin classes are final by default, so you opt in to subclassing deliberately.

By default, Kotlin classes cannot be inherited. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• By default, Kotlin classes cannot be inherited.
• The open keyword allows a class to be extended.
• This supports safe design and controlled inheritance.

Syntax

open class Base

open Classes in Kotlin Example Program in Kotlin

open class Vehicle(val brand: String)
class Car(brand: String, val model: String) : Vehicle(brand)

fun main(args: Array<String>) {
    val car = Car("Toyota", "Innova")
    println("${car.brand} ${car.model}")
}

Sample Output

Toyota Innova

When to use

Use inheritance when a subclass truly is-a parent type and shares an interface, not just to reuse a few helper methods.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. val car = Car("Toyota", "Innova") assigns or updates a value used later in the program.

3. The println("${car.brand} ${car.model}") statement writes a line to the console — this produces part of the sample output below.

4. By default, Kotlin classes cannot be inherited.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Understand the core idea: by default, Kotlin classes cannot be inherited.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• By default, Kotlin classes cannot be inherited.
• The open keyword allows a class to be extended.
• This supports safe design and controlled inheritance.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.