Enum with when exhaustive check in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an enum class named Direction.

Kotlin

enum class [1] {
    NORTH, SOUTH, EAST, WEST
}

Drag options to blanks, or click blank then click option'

AMove

Bdirection

CDirections

DDirection

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or plural names for enum class.

The enum class must be named Direction with capital D as per Kotlin naming conventions.

2fill in blank

medium

Complete the when expression to check the direction variable.

Kotlin

fun describeDirection(direction: Direction): String {
    return when ([1]) {
        Direction.NORTH -> "Going north"
        Direction.SOUTH -> "Going south"
        Direction.EAST -> "Going east"
        Direction.WEST -> "Going west"
    }
}

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Adirection

Bdir

Cd

Dmove

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that does not exist in the function.

The parameter name is direction, so the when expression must check direction.

3fill in blank

hard

Fix the error by making the when expression exhaustive.

Kotlin

fun describeDirection(direction: Direction): String {
    return when (direction) {
        Direction.NORTH -> "North"
        Direction.SOUTH -> "South"
        Direction.EAST -> "East"
        [1] -> "West"
        // Missing WEST case causes error
    }
}

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ADirection.WEST

BDirection.UP

CDirection.DOWN

DDirection.LEFT

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent enum values or missing cases.

The enum has a WEST value, so the when must handle Direction.WEST to be exhaustive.

4fill in blank

hard

Fill both blanks to create an exhaustive when expression without else.

Kotlin

fun describeDirection(direction: Direction): String {
    return when ([1]) {
        Direction.NORTH -> "North"
        Direction.SOUTH -> "South"
        Direction.EAST -> "East"
        [2] -> "West"
    }
}

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Adirection

Bdir

CDirection.WEST

DDirection.UP

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names or missing enum cases.

The when expression must check the variable direction and include all enum values, including Direction.WEST, to be exhaustive without else.

5fill in blank

hard

Fill all three blanks to return a message using when with exhaustive check and smart casting.

Kotlin

sealed class Result {
    data class Success(val data: String): Result()
    object Failure: Result()
}

fun handleResult(result: Result): String {
    return when ([1]) {
        is Result.Success -> "Data: [2]"
        [3] -> "Failed"
    }
}

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Aresult

Bresult.data

CResult.Failure

Dresult.failure

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names or missing the object case.

The when expression checks the result variable. For Success, it accesses result.data. For failure, it matches the singleton object Result.Failure.

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