Kotlin – Comments: Writing Clear and Maintainable Code

Introduction – Why Use Comments in Kotlin?

Comments are a fundamental part of writing clean, readable, and maintainable code. In Kotlin, comments help explain logic, disable code temporarily, or annotate sections for collaboration. Kotlin supports both single-line and multi-line comments, just like Java, making it easy for developers to document and debug efficiently.

In this guide, you’ll learn:

• Types of comments in Kotlin and their syntax
• How to use comments effectively for code documentation
• Best practices for writing helpful comments
• Common mistakes and when not to use comments

Types of Comments in Kotlin

Kotlin supports two types of comments:

Single-Line Comments

Use // to write brief notes on the same or separate line.

fun main() {
    val score = 95 // Declares a score variable
    println("Score: $score") // Output the score
}

Best for:

• Documenting logic inline
• Temporarily disabling one line
• Quick reminders

Multi-Line Comments

Use /* ... */ to span comments across multiple lines.

fun main() {
    /* This function prints
       a welcome message
       using Kotlin syntax */
    println("Welcome to Kotlin!")
}

Best for:

• Block-level documentation
• Temporarily disabling multiple lines
• Team annotations or TODOs

Nested Comments in Kotlin

Unlike Java, Kotlin supports nested block comments:

/* Outer comment
    /* Nested comment */
   End of outer comment */

This is helpful when you need to comment out large code sections that may already contain comments.

Commenting Out Code

You can disable code during testing using comments:

// val isActive = true
// println("User is active")

Or:

/*
val user = "Admin"
val role = "Editor"
println("$user is a $role")
*/

Tip: Avoid leaving commented-out code in production unless it’s for future reference or debugging.

Best Practices for Writing Comments

Common Mistakes with Comments

Summary – Recap & Next Steps

Kotlin supports both single-line and multi-line comments, including nested ones, to help you write better-documented and maintainable code. Use them wisely to enhance collaboration and future-proof your projects.

Key Takeaways:

• Use // for single-line and /* */ for multi-line comments.
• Kotlin allows nested multi-line comments.
• Comments should clarify logic, not restate code.
• Use TODO and FIXME for tracking issues and improvements.

Practical Use:
Use comments during development, code reviews, debugging, or while collaborating on projects—especially for explaining why something is done.

FAQs – Kotlin Comments

Does Kotlin support nested comments?
Yes. Kotlin allows nesting of multi-line comments using /* /* ... */ */, unlike Java.

What are TODO comments used for in Kotlin?
TODO() is a special function in Kotlin that throws NotImplementedError.

fun futureFeature() = TODO("Not implemented yet")

Should I comment every line of code?
No. Comment only complex logic or decisions, not trivial assignments or obvious code.

Can comments affect program output?
No. Comments are ignored by the Kotlin compiler and have no effect on output or performance.

How to quickly comment/uncomment in IDE?
In IntelliJ/Android Studio, use Ctrl + / (Windows/Linux) or Cmd + / (Mac) for toggling single-line comments.