Why Enum with when exhaustive check in Kotlin? - Purpose & Use Cases

The Big Idea

What if your code could warn you every time you forget to handle a case?

The Scenario

Imagine you have a list of tasks, each with a status like "To Do", "In Progress", or "Done". You want to write code that does something different for each status. Without enums and exhaustive checks, you might write many if-else statements, and it's easy to forget one status or add a new one and miss updating your code.

The Problem

Manually checking each status with if-else is slow and error-prone. You might forget a case, causing bugs that are hard to find. When you add a new status, you have to hunt through your code to update every check. This makes your code fragile and hard to maintain.

The Solution

Using enums with a when expression that is exhaustive means Kotlin forces you to handle every possible status. If you miss one, the compiler will warn you. This makes your code safer, easier to read, and easier to update when you add new statuses.

Before vs After

✗ Before

if (status == "To Do") { /*...*/ } else if (status == "In Progress") { /*...*/ } // forgot "Done" case

✓ After

when (status) {
  Status.TODO -> /*...*/
  Status.IN_PROGRESS -> /*...*/
  Status.DONE -> /*...*/
}

What It Enables

This lets you write clear, safe code that automatically reminds you to handle every possible case, preventing bugs before they happen.

Real Life Example

Think of a traffic light system with states like RED, YELLOW, and GREEN. Using enums with exhaustive when ensures your program reacts correctly to every light color, and if a new color is added, you won't forget to update your logic.

Key Takeaways

Manual checks can miss cases and cause bugs.

Enums with exhaustive when force you to handle all cases.

This leads to safer, clearer, and easier-to-maintain code.

Practice

1. What does the Kotlin when expression require when used with an enum class?

easy

A. All enum cases must be handled to ensure exhaustive checking.

B. Only one enum case needs to be handled.

C. The when expression cannot be used with enums.

D. You must use else even if all enum cases are handled.

Solution

1. Step 1: Understand enum and when usage

   Kotlin enums define fixed values. The when expression can check these values.

2. Step 2: Check compiler requirements for exhaustive when

   Kotlin requires all enum cases to be handled in when to be exhaustive, avoiding missing cases.

3. Final Answer:

   All enum cases must be handled to ensure exhaustive checking. -> Option A

4. Quick Check:

   Exhaustive when = all enum cases handled [OK]

Hint: Remember: when with enum must cover all cases [OK]

Common Mistakes:

• Forgetting to handle all enum values
• Using else unnecessarily when all cases are covered
• Assuming when can't be exhaustive with enums

2. Which of the following is the correct syntax for a when expression that exhaustively checks all values of this enum?

enum class Direction { NORTH, SOUTH, EAST, WEST }

easy

A. when(dir) { Direction.NORTH -> "N"; Direction.SOUTH -> "S"; Direction.EAST -> "E"; Direction.WEST -> "W" }

B. when(dir) { Direction.NORTH, Direction.SOUTH -> "NS" }

C. when(dir) { "N" -> Direction.NORTH; "S" -> Direction.SOUTH }

D. when(dir) { Direction.NORTH -> "N"; Direction.SOUTH -> "S" }

Solution

1. Step 1: Check all enum values handled

   when(dir) { Direction.NORTH -> "N"; Direction.SOUTH -> "S"; Direction.EAST -> "E"; Direction.WEST -> "W" } lists all four enum values with their results, no else needed.

2. Step 2: Verify syntax correctness

   when(dir) { Direction.NORTH -> "N"; Direction.SOUTH -> "S"; Direction.EAST -> "E"; Direction.WEST -> "W" } uses correct when syntax with enum constants and returns strings.

3. Final Answer:

   when(dir) { Direction.NORTH -> "N"; Direction.SOUTH -> "S"; Direction.EAST -> "E"; Direction.WEST -> "W" } -> Option A

4. Quick Check:

   All enum cases handled without else = when(dir) { Direction.NORTH -> "N"; Direction.SOUTH -> "S"; Direction.EAST -> "E"; Direction.WEST -> "W" } [OK]

Hint: List all enum values explicitly for exhaustive when [OK]

Common Mistakes:

• Using else when all enum cases are handled
• Missing some enum values in when
• Using wrong types in when branches

3. What will be the output of this Kotlin code?

enum class Light { RED, GREEN, YELLOW }

fun signalMessage(light: Light) = when(light) {
    Light.RED -> "Stop"
    Light.GREEN -> "Go"
    Light.YELLOW -> "Caution"
}

fun main() {
    println(signalMessage(Light.YELLOW))
}

medium

A. Stop

B. Go

C. Caution

D. Compilation error

Solution

1. Step 1: Identify the enum value passed

   The function is called with Light.YELLOW.

2. Step 2: Match enum value in when expression

   The when expression returns "Caution" for Light.YELLOW.

3. Final Answer:

   Caution -> Option C

4. Quick Check:

   Light.YELLOW maps to "Caution" [OK]

Hint: Match enum value to when branch to find output [OK]

Common Mistakes:

• Confusing enum values with strings
• Assuming else is needed for exhaustive when
• Expecting compilation error due to missing else

4. This Kotlin code does not compile. What is the error?

enum class Status { SUCCESS, ERROR, LOADING }

fun getStatusMessage(status: Status) = when(status) {
    Status.SUCCESS -> "Done"
    Status.ERROR -> "Failed"
}

medium

A. Missing else branch in when expression

B. Not all enum cases handled in when expression

C. Incorrect enum class declaration

D. Function return type missing

Solution

1. Step 1: Check enum cases handled in when

   Only Status.SUCCESS and Status.ERROR are handled; Status.LOADING is missing.

2. Step 2: Understand Kotlin's exhaustive when requirement

   When used as expression, all enum cases must be handled or else an else branch is needed.

3. Final Answer:

   Not all enum cases handled in when expression -> Option B

4. Quick Check:

   Missing enum case causes compilation error [OK]

Hint: Cover all enum values or add else to fix compilation [OK]

Common Mistakes:

• Assuming else is optional when missing enum cases
• Ignoring missing enum values in when
• Thinking function return type causes error

5. Given this enum and function, how can you modify getDayType to use an exhaustive when expression without an else branch?

enum class Day { MON, TUE, WED, THU, FRI, SAT, SUN }

fun getDayType(day: Day): String = when(day) {
    Day.SAT, Day.SUN -> "Weekend"
    Day.MON, Day.TUE, Day.WED, Day.THU, Day.FRI -> "Weekday"
}

medium

A. Add an else branch to cover any missing cases.

B. Add all enum values explicitly in separate branches without combining.

C. Add a default return statement after when.

D. Keep combined branches as is; Kotlin treats it as exhaustive.

Solution

1. Step 1: Analyze current when branches

   The branches cover Day.SAT, Day.SUN and Day.MON, Day.TUE, Day.WED, Day.THU, Day.FRI, which includes all seven enum values.

2. Step 2: Verify Kotlin's exhaustive when rules

   Kotlin considers the when exhaustive if all enum cases are explicitly covered across branches, even when grouped with commas in the same branch. No else branch is needed.

3. Final Answer:

   Keep combined branches as is; Kotlin treats it as exhaustive. -> Option D

4. Quick Check:

   Grouped coverage of all enum cases = exhaustive without else [OK]

Hint: Comma-grouped enum values in branches make when exhaustive [OK]

Common Mistakes:

• Thinking combined branches don't count as exhaustive
• Believing you must use separate branches for each enum value
• Unnecessarily adding an else branch