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when with enum

Learn branching on enums with when.

when with enum: exhaustive branches

When you list every constant of an enum in a when, no else is needed. If you add a new constant, the compiler demands a new branch; the error is caught early.

fun describe(c: Color) = when (c) {
    Color.RED -> "red"
    Color.GREEN -> "green"
    Color.BLUE -> "blue"
}

This is called an exhaustive when; every constant is written out. If you forget one, the compiler warns you. Adding a new constant is safe; every branch gets reviewed.

Exhaustion and new constants

If you add a new constant to an enum with an exhaustive when, the compiler warns you. This means adding a constant is safe.

enum class Color { RED, GREEN, BLUE }
// adding YELLOW breaks describe
// a new branch covering it is required

When you see the warning you add a branch; no constant slips through. This prevents future breakage. Writing every constant explicitly is safer than an else.

Practice Describe directions

Define an enum class called Direction with NORTH, SOUTH, EAST, WEST. Write fun describe(d: Direction): String giving each direction a word with when. In main print describe(Direction.NORTH).

Starter code

Main.kt
fun main() {
    // Define a direction type; print a direction name via describe
}

Sample solution

Main.kt
enum class Direction { NORTH, SOUTH, EAST, WEST }

fun describe(d: Direction) = when (d) {
    Direction.NORTH -> "north"
    Direction.SOUTH -> "south"
    Direction.EAST -> "east"
    Direction.WEST -> "west"
}

fun main() {
    println(describe(Direction.NORTH))
}

Expected output

kuzey

Questions and answers

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

1 If a when lists every enum constant, what is not needed?

  • No else branch needed Correct answer
  • It needs a loop
  • It only runs one branch
  • It changes constants

Explanation: Listing all constants means no else; the compiler enforces it.

2 Fill in the blank: add a when branch

Question 
when (c) {
    Color.RED -> "red"
    ____ -> "green"
}
Answer 
when (c) {
    Color.RED -> "red"
    Color.GREEN -> "green"
}

Explanation: you added a branch for Color.GREEN.

3 Match constant to text

Term Meaning
Color.RED red
Color.GREEN green
Color.BLUE blue

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