Functional Interfaces in Kotlin

Introduction

Functional Interfaces 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.

A functional interface (fun interface) has exactly one abstract method. 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

• A functional interface (fun interface) has exactly one abstract method.
• It enables SAM conversion from lambda expressions.
• Useful for Java interop and callback APIs.

Syntax

fun interface Listener {
    fun onEvent()
}

Functional Interfaces in Kotlin Example Program in Kotlin

fun interface Listener {
    fun onEvent()
}

fun main(args: Array<String>) {
    val listener = Listener { println("Event received") }
    listener.onEvent()
}

Sample Output

Event received

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

3. A functional interface (fun interface) has exactly one abstract method.

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

• Understand the core idea: a functional interface (fun interface) has exactly one abstract method.
• 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

• A functional interface (fun interface) has exactly one abstract method.
• It enables SAM conversion from lambda expressions.
• Useful for Java interop and callback APIs.
• 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.