Method Overriding in Kotlin

Introduction

Method Overriding 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.

Overriding replaces parent class function implementation in child class. 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

• Overriding replaces parent class function implementation in child class.
• Parent function must be marked open and child function must use override.
• super keyword calls the parent version of a function.

Syntax

override fun show() { super.show() }

Method Overriding in Kotlin Example Program in Kotlin

open class Shape {
    open fun draw() = println("Drawing shape")
}
class Circle : Shape() {
    override fun draw() {
        super.draw()
        println("Drawing circle")
    }
}

fun main(args: Array<String>) {
    Circle().draw()
}

Sample Output

Drawing shape
Drawing circle

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("Drawing shape") statement writes a line to the console — this produces part of the sample output below.

3. The println("Drawing circle") statement writes a line to the console — this produces part of the sample output below.

4. Overriding replaces parent class function implementation in child class.

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

Common Mistakes

Key Points

• Overriding replaces parent class function implementation in child class.
• Parent function must be marked open and child function must use override.
• super keyword calls the parent version of a function.
• 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.