Implementing Interfaces in Kotlin

Introduction

Implementing 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.

Use colon syntax to implement one or more 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

• Use colon syntax to implement one or more interfaces.
• All interface members must be implemented unless they have default body.
• Interfaces support multiple inheritance of behavior.

Syntax

class A : Interface1, Interface2

Implementing Interfaces in Kotlin Example Program in Kotlin

interface Printable { fun printData() }
interface Saveable { fun save() }

class Document : Printable, Saveable {
    override fun printData() = println("Printing document")
    override fun save() = println("Document saved")
}

fun main(args: Array<String>) {
    val doc = Document()
    doc.printData()
    doc.save()
}

Sample Output

Printing document
Document saved

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

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

4. val doc = Document() assigns or updates a value used later in the program.

5. Use colon syntax to implement one or more interfaces.

6. 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: use colon syntax to implement one or more 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

• Use colon syntax to implement one or more interfaces.
• All interface members must be implemented unless they have default body.
• Interfaces support multiple inheritance of behavior.
• 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.