How to Comment in Kotlin?

When you write code, it’s important to make sure that not only you understand it but also anyone who might read or maintain the code in the future. That’s where comments come in. Commenting is often overlooked by beginners, but it is one of the most essential practices in coding, especially in Kotlin.

In this article, we’ll explore how to write comments in Kotlin, why they are crucial for every developer, and tips to ensure you write effective and meaningful comments.

What Are Comments?

A comment is a piece of text in your code that is ignored by the Kotlin compiler. It is meant solely for human readers. The primary goal of a comment is to explain what the code is doing or why it was written in a certain way. Comments can also provide helpful notes for others (or yourself) when maintaining or debugging code in the future.

Kotlin, like most programming languages, supports two types of comments:

1. Single-line comments
2. Multi-line comments

Let’s break down each type and understand how to use them effectively.

Single-Line Comments

A single-line comment begins with // and is used to write a comment on a single line. Everything after // on that line is ignored by the Kotlin compiler.

Example:

fun main() {
    // This is a single-line comment
    println("Hello, Kotlin!")  // This prints a greeting message to the console
}

Single-line comments are perfect for adding short explanations or reminders in your code. They’re often used to clarify what a particular line or block of code is doing.

Tip: Use Single-Line Comments Sparingly

While single-line comments are useful, they should not be overused. If you find yourself commenting on every line of code, you may want to reconsider the clarity of your code itself. Clear, readable code often reduces the need for excessive commenting.

Multi-Line Comments

A multi-line comment starts with /* and ends with */. Everything between these markers is treated as a comment, making this type ideal for longer explanations or for temporarily commenting out blocks of code during debugging.

Example:

fun calculateSum(a: Int, b: Int): Int {
    /*
    This function takes two integers as input
    and returns their sum. It uses the addition operator.
    */
    return a + b
}

Commenting Out Code

Another great use of multi-line comments is for temporarily removing a block of code from execution while you’re debugging.

fun main() {
    println("Before debugging")

    /*
    println("This line is commented out for now")
    println("Another commented line")
    */

    println("After debugging")
}

In this example, the two println() lines within the multi-line comment block are ignored by the compiler. This can be helpful when you’re trying to isolate an issue in your code or testing new logic.

Why Comments Are So Important

Now that we’ve covered how to write comments, let’s discuss why comments are so essential in Kotlin (and any programming language, for that matter).

1. Improving Code Readability

One of the main purposes of comments is to improve the readability of your code. While code should always be as self-explanatory as possible, sometimes complex logic or calculations need a little extra explanation. This is where comments come in handy.

Imagine coming back to your own code six months later. Will you instantly remember what each line does? Probably not! A well-placed comment can make that process much easier, saving you (and your team) time and effort.

2. Helping Other Developers

Unless you’re working on a solo project, your code will likely be read and maintained by other developers. Writing comments helps your teammates understand your thought process, which is especially important if your code has to be modified or debugged in the future.

Even the best developers appreciate good comments that explain tricky parts of the code.

3. Clarifying the “Why”

While code itself usually explains the “what” (what is happening), comments often explain the “why”. For example, if you’ve made a design choice that might not be immediately obvious to other developers, leaving a comment to explain your reasoning can prevent confusion later on.

4. Documentation

Comments can serve as lightweight documentation for your code. They allow you to explain the purpose of classes, methods, and complex blocks of code without needing to create a separate document. This makes it easier for both you and others to navigate your codebase.

Best Practices for Writing Comments

Writing good comments is an art. Here are some best practices to help you write clear and effective comments in Kotlin.

1. Comment Why, Not What

Comments should explain why something is happening rather than what is happening. Code should be self-explanatory enough to explain the “what.” Here’s an example of a bad comment versus a good comment:

• Bad comment:

val total = price * quantity  // Multiply price by quantity

This comment isn’t necessary because the code itself is straightforward. It’s clear that the price is being multiplied by the quantity.

• Good comment:

val total = price * quantity  // Calculating total cost before applying discount

This comment explains why we’re multiplying the price and quantity—it’s to calculate the total cost before a discount is applied.

2. Keep Comments Up-to-Date

One of the worst practices is leaving outdated or inaccurate comments in your code. If you modify the code, always update the associated comments. An outdated comment can be more harmful than no comment at all because it can mislead developers trying to understand the logic.

3. Don’t Over-Comment

As tempting as it may be, don’t comment on every single line of code. Over-commenting can clutter your code, making it harder to read. Instead, focus on adding comments where they are needed most—like explaining complex logic or providing insight into non-obvious decisions.

4. Use Proper Grammar and Punctuation

Your comments should be easy to read. Just like writing an email or an article, comments should use proper grammar, punctuation, and capitalization. This might seem trivial, but clear and professional comments reflect the quality of your code.

5. Avoid Personal Notes

While writing comments, avoid adding personal notes or messages like “I’ll fix this later” or “This is a hacky solution.” Comments should be professional, concise, and relevant to the code itself. If you need to track future tasks, use proper project management tools like Jira or GitHub Issues.

Commenting for Documentation: Kotlin KDoc

Kotlin provides a special type of comment known as KDoc, which is used to generate documentation from your code. KDoc comments start with /** and are written above classes, functions, or properties.

Example:

/**
 * This class represents a basic user with a name and an age.
 */
class User(val name: String, val age: Int)

/**
 * This function prints the user's name and age.
 *
 * @param user the User object to be printed
 */
fun printUserInfo(user: User) {
    println("${user.name}, Age: ${user.age}")
}

KDoc comments are especially useful when you’re building libraries or APIs and want to automatically generate documentation that other developers can refer to.

Conclusion

Comments are a vital part of coding in Kotlin, allowing you to create more readable, maintainable, and user-friendly code. They help both you and other developers understand the “why” behind the code, prevent confusion, and provide documentation without needing external tools.

However, it’s important to strike a balance. Too few comments can make code hard to follow, while too many can clutter it. By following the best practices outlined in this article, you’ll be well on your way to writing clean, well-documented Kotlin code.

Ready for the next step? In the upcoming article, we’ll explore Kotlin’s powerful data types and how to use them effectively in your programs!