Abstract Classes in Kotlin

Introduction

Abstract 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.

Abstract classes cannot be instantiated directly. 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

• Abstract classes cannot be instantiated directly.
• They may contain abstract members without implementation.
• Subclasses must implement all abstract members.

Syntax

abstract class Base {
    abstract fun execute()
}

Abstract Classes in Kotlin Example Program in Kotlin

abstract class Payment {
    abstract fun pay(amount: Double)
}
class CardPayment : Payment() {
    override fun pay(amount: Double) = println("Paid $amount by card")
}

fun main(args: Array<String>) {
    CardPayment().pay(499.0)
}

Sample Output

Paid 499.0 by card

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. The println("Paid $amount by card") statement writes a line to the console — this produces part of the sample output below.

3. Abstract classes cannot be instantiated directly.

4. 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

Common Mistakes

Key Points

• Abstract classes cannot be instantiated directly.
• They may contain abstract members without implementation.
• Subclasses must implement all abstract members.
• 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.