Interface Inheritance in Kotlin

Introduction

Interface Inheritance is a fundamental concept every Kotlin developer should understand. Interfaces define contracts without implementation details. Kotlin interfaces can include default method bodies and property getters.

Interfaces can extend other interfaces. 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

• Interfaces can extend other interfaces.
• A class implementing child interface must implement all inherited members.
• This helps build layered contracts.

Syntax

interface B : A

Interface Inheritance in Kotlin Example Program in Kotlin

interface Readable { fun read() }
interface ReadWrite : Readable { fun write() }
class FileHandler : ReadWrite {
    override fun read() = println("Reading file")
    override fun write() = println("Writing file")
}

fun main(args: Array<String>) {
    FileHandler().read()
    FileHandler().write()
}

Sample Output

Reading file
Writing file

When to use

Use interfaces to define capabilities (Drawable, Comparable) that unrelated classes can implement.

How it works

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

2. The println("Reading file") statement writes a line to the console — this produces part of the sample output below.

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

4. Interfaces can extend other interfaces.

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: interfaces can extend other interfaces.
• 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

• Interfaces can extend other interfaces.
• A class implementing child interface must implement all inherited members.
• This helps build layered contracts.
• 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.