Kotlin Comments & Variables (Lesson 3 PDF)

Kotlin is a statically typed programming language that runs on the Java Virtual Machine (JVM). It was developed by JetBrains and is now an official language for Android development. Kotlin is designed to be concise, expressive, and safe. It is also interoperable with Java, which means that you can use Kotlin and Java code together in the same project.

Kotlin Comments & Variables (Lesson 3 PDF)

Overview about the Lesson

In this lesson, you will learn about comments and variables in Kotlin. You will learn how to use comments to document your code and make it more readable. You will also learn how to declare variables and constants in Kotlin.

Kotlin Comments & Variables (Lesson 3 PDF) – page 2

Kotlin Comments & Variables (Lesson 3 PDF) – page 3

The Content of the Lesson

The lesson is divided into two parts. In the first part, you will learn about comments in Kotlin. You will learn how to use single-line and multi-line comments to document your code. You will also learn how to use comments to disable code temporarily. In the second part, you will learn how to declare variables and constants in Kotlin. You will learn about data types, type inference, and how to declare variables and constants.

Why This Lesson

This lesson is designed for programmers who are new to Kotlin and want to learn how to use comments and declare variables and constants. It is also suitable for programmers who are familiar with Java and want to learn Kotlin.

Kotlin Comments and Variables: val vs var Explained

This lesson’s PDF introduces comments and variables. Here are the key ideas with examples you can run in the Kotlin Playground.

Comments

// A single-line comment: ignored by the compiler

/*
   A multi-line comment
   for longer explanations
*/

/**
 * A KDoc comment documents functions and classes.
 * @param name the person to greet
 */
fun greet(name: String) = println("Hello, $name")

Good comments explain why code does something. Clear names usually explain what it does.

val and var

Keyword	Meaning	Can be reassigned?
val	Read-only value (like a constant reference)	No
var	Mutable variable	Yes

fun main() {
    val birthYear = 1998
    var score = 10
    score = score + 5      // OK: var can change
    // birthYear = 2000    // Error: val cannot be reassigned
    println("Born $birthYear, score $score")   // Born 1998, score 15
}

Rule of thumb: use val by default and switch to var only when the value really needs to change. Code with fewer changing values is easier to understand and has fewer bugs.

Type inference and explicit types

val city = "Cairo"           // Kotlin infers String
val count: Int = 5           // explicit type
var price: Double = 19.99
var nickname: String? = null // ? means the variable may hold null

Kotlin works out the type from the value, so you rarely need to write it. Add it when it makes the code clearer or when you declare a variable without a value yet.

Naming rules and conventions

Names can contain letters, digits and underscores, but cannot start with a digit.
Use camelCase for variables: totalPrice, userName.
Names are case-sensitive: total and Total are different.
Keywords such as val, fun or class cannot be used as names.
For true compile-time constants, use const val MAX_USERS = 100 at the top level, written in UPPER_SNAKE_CASE.

Common mistakes

Reassigning a val: the error “Val cannot be reassigned” means you should either use var or create a new value.
Changing the type: var x = 5 then x = "five" fails, because x is an Int.
Assigning null to a non-nullable type: add ? to the type if null is allowed.

Practice exercise

Create a read-only variable for a product name, a mutable variable for stock, reduce the stock by 2 and print the result.

fun main() {
    val product = "Keyboard"
    var stock = 10
    stock -= 2
    println("$product stock: $stock")   // Keyboard stock: 8
}

Frequently Asked Questions

Is val the same as final in Java?

Yes, a val behaves like a final variable: it can be assigned once. The object it points to can still be mutable (for example a MutableList).

When should I use const val?

For values known at compile time that never change, such as configuration limits or keys, declared at the top level or inside an object.

Continue the course

← Previous: Lesson 2: Syntax & Output | Next: Lesson 4: Data Types →

Conclusion

Comments and variables are essential parts of any programming language. In this lesson, you learned how to use comments to document your code and make it more readable. You also learned how to declare variables and constants in Kotlin. With these skills, you can write more efficient and maintainable code.