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

Recall & Review

beginner

What is a sealed class in Kotlin?

A sealed class is a special kind of class that restricts which classes can inherit from it. All subclasses must be declared in the same file, making the type hierarchy fixed and known at compile time.

beginner

Why does Kotlin require exhaustive checks with when expressions on sealed classes?

Because the compiler knows all possible subclasses of a sealed class, it can check if a when expression covers all cases. This helps avoid missing cases and makes the code safer.

beginner

How do you make a when expression exhaustive in Kotlin?

You include all possible subclasses of the sealed class as branches in the <code>when</code> expression. If all cases are covered, the compiler treats it as exhaustive without needing an <code>else</code> branch.

intermediate

What happens if a <code>when</code> expression on a sealed class is not exhaustive?

The Kotlin compiler will show an error or warning if the <code>when</code> expression does not cover all subclasses of the sealed class and lacks an <code>else</code> branch, forcing you to handle all cases.

beginner

Example: Given a sealed class Shape with subclasses Circle and Square, write an exhaustive when expression to calculate area.

sealed class Shape
class Circle(val radius: Double) : Shape()
class Square(val side: Double) : Shape()

fun area(shape: Shape): Double = when(shape) {
  is Circle -> Math.PI * shape.radius * shape.radius
  is Square -> shape.side * shape.side
}

What is the main benefit of using sealed classes with when in Kotlin?

A Sealed classes are faster than regular classes.

B You can inherit from sealed classes anywhere.

C The compiler can check all cases are handled.

D You don't need to import subclasses.

Sealed classes let the compiler know all subclasses, so it can check that when expressions cover all cases.

If a when expression on a sealed class misses a subclass, what happens?

A The code runs but returns null.

B The compiler shows an error or warning.

C The program crashes at runtime.

D Nothing, it works fine.

The Kotlin compiler forces you to handle all subclasses or provide an else branch.

Where must subclasses of a sealed class be declared?

A In the same file as the sealed class.

B Anywhere in the project.

C In a different package.

D In a separate module.

All subclasses of a sealed class must be declared in the same file to keep the hierarchy fixed.

Which keyword is used to declare a sealed class in Kotlin?

A final

B abstract

C open

D sealed

The sealed keyword declares a sealed class.

Can a sealed class have non-sealed subclasses?

A Yes, but they must be in the same file.

B No, all subclasses must be sealed.

C Yes, anywhere in the project.

D No, subclasses must be abstract.

Subclasses can be regular classes but must be declared in the same file as the sealed class.

Explain what a sealed class is and why it helps with when expressions in Kotlin.

Think about how knowing all options helps the compiler.

Describe how to write an exhaustive when expression for a sealed class and what happens if you miss a case.

Consider the compiler's role in checking completeness.