Enum with when exhaustive check in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an enum in Kotlin?

An enum in Kotlin is a special class that represents a fixed set of constants. It helps group related values under one type, like days of the week or directions.

beginner

What does it mean for a when expression to be exhaustive in Kotlin?

A when expression is exhaustive when it covers all possible cases of the input, so Kotlin knows no other case can happen. This means you don't need an else branch.

intermediate

How does Kotlin ensure when is exhaustive when used with enums?

When you use a when expression with an enum, Kotlin requires you to handle all enum constants. If you miss one, the code won't compile unless you add an else branch.

intermediate

Why is using an exhaustive when with enums helpful?

It helps catch errors early by making sure you handle every possible enum value. This avoids bugs where some cases are forgotten and improves code safety.

beginner

Show a simple Kotlin enum and an exhaustive when expression example.

Example:<br><pre>enum class Direction { NORTH, SOUTH, EAST, WEST }
fun describe(dir: Direction) = when(dir) {
  Direction.NORTH -> "Up"
  Direction.SOUTH -> "Down"
  Direction.EAST -> "Right"
  Direction.WEST -> "Left"
}</pre>

What happens if you miss an enum case in a when expression without an else branch?

A The code will not compile.

B The code will compile but throw an error at runtime.

C The missing case will be ignored silently.

D Kotlin will automatically add the missing case.

Kotlin requires all enum cases to be handled in a when expression or an else branch must be present. Missing cases cause a compile error.

Which keyword is used to define an enum in Kotlin?

A class

B enum

C object

D sealed

The enum keyword defines an enum class in Kotlin.

Why might you prefer an exhaustive when over using else?

A To allow missing cases without errors.

B To make the code shorter.

C To ensure all cases are explicitly handled.

D To avoid using enums.

Exhaustive when forces you to handle every enum case explicitly, reducing bugs.

What type of Kotlin construct is best suited for representing a fixed set of constants?

A fun

B data class

C interface

D enum class

An enum class is designed to represent a fixed set of constants.

If you add a new value to an enum, what must you do to keep your when expressions exhaustive?

A Add a new case for the new enum value in all <code>when</code> expressions.

B Nothing, Kotlin updates them automatically.

C Remove the <code>when</code> expressions.

D Add an <code>else</code> branch to all <code>when</code> expressions.

You must update all when expressions to handle the new enum value to keep them exhaustive.

Explain what an exhaustive when expression is and why it is useful when working with enums in Kotlin.

Think about how Kotlin checks all enum values are handled.

Describe how Kotlin enforces handling all enum cases in a when expression and what happens if a case is missing.

Focus on Kotlin's compile-time checks.

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