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Learn to write // and /* */ comments in Kotlin.

Comments: // and /* */

Comments are notes the Kotlin compiler ignores. Single-line comments start with // and run to the end of the line. Block comments start with /* and end with */; they can span multiple lines.

Comments explain code; they do not affect the compiled output. Clear names and short comments keep code readable.

// a single-line comment
/* this is
   a block
   comment */

KDoc /** */

Kotlin offers a special comment form called KDoc for documentation. KDoc starts with /** and ends with */; it can hold tags like @param and @return.

Functions with KDoc show up as tooltips in the IDE. So when you use a library, you easily see what a function does.

/**
 * Sums two numbers.
 * @param a the first number
 * @param b the second number
 * @return the sum
 */
fun add(a: Int, b: Int): Int = a + b

Practice Add a comment

Complete this code: a fun main() with a println("Hello Kotlin") inside. Add a single-line comment before println; the comment must start with // and contain a short note.

Sample solution

Main.kt
fun main() {
    // start
    println("Hello Kotlin")
}

Expected output

Hello Kotlin

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which is a single-line comment?

  • # a comment
  • <!-- a comment -->
  • -- a comment
  • // a comment Correct answer

Explanation: A line starting with // is a single-line comment.

2 Complete the comment

Question 
println("Hello") ____ a comment
Answer 
println("Hello") // a comment

Explanation: // starts a single-line comment.

3 Comments: match the terms

Term Meaning
// Single line
/* */ Block comment
/** */ KDoc
# Invalid

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together