Interface Declaration in Kotlin

Introduction

Interface Declaration 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.

An interface defines a contract that classes can implement. 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

• An interface defines a contract that classes can implement.
• Interfaces can declare functions and properties.
• A class can implement multiple interfaces.

Syntax

interface Clickable {
    fun click()
}

Interface Declaration in Kotlin Example Program in Kotlin

interface Clickable {
    fun click()
}
class Button : Clickable {
    override fun click() = println("Button clicked")
}

fun main(args: Array<String>) {
    Button().click()
}

Sample Output

Button clicked

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

3. An interface defines a contract that classes can implement.

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: an interface defines a contract that classes can implement.
• 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

• An interface defines a contract that classes can implement.
• Interfaces can declare functions and properties.
• A class can implement multiple interfaces.
• 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.