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.