Sealed classes for restricted hierarchies in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of sealed class when matching with 'when'

What is the output of this Kotlin code using a sealed class and a when expression?

Kotlin

sealed class Shape {
    data class Circle(val radius: Double) : Shape()
    data class Rectangle(val width: Double, val height: Double) : Shape()
}

fun describe(shape: Shape): String = when(shape) {
    is Shape.Circle -> "Circle with radius ${shape.radius}"
    is Shape.Rectangle -> "Rectangle with width ${shape.width} and height ${shape.height}"
}

fun main() {
    val shape: Shape = Shape.Circle(5.0)
    println(describe(shape))
}

A Circle with radius 5.0

B Rectangle with width 5.0 and height 5.0

C Compilation error: 'when' expression is not exhaustive

D Runtime error: Unhandled type

💡 Hint

Think about how sealed classes allow the compiler to know all subclasses and how 'when' handles them.

The sealed class Shape has two subclasses: Circle and Rectangle. The 'when' expression covers both cases, so it is exhaustive. The instance is a Circle with radius 5.0, so the output is 'Circle with radius 5.0'.

❓ Predict Output

intermediate

Exhaustiveness check with sealed classes

What happens if you add a new subclass to a sealed class but forget to update the when expression that matches on it?

Kotlin

sealed class Result {
    object Success : Result()
    object Failure : Result()
    object Loading : Result()
}

fun handle(result: Result): String = when(result) {
    Result.Success -> "Success"
    Result.Failure -> "Failure"
}

fun main() {
    println(handle(Result.Loading))
}

A Prints 'Failure'

B Compilation error: 'when' expression is not exhaustive

C Prints 'Success'

D Runtime error: Unhandled type

💡 Hint

Sealed classes let the compiler check if all subclasses are handled in 'when'.

Since Result.Loading is a new subclass and the 'when' expression does not handle it, the compiler will report an error that the 'when' is not exhaustive.

🔧 Debug

advanced

Why does this sealed class code fail to compile?

Identify the reason why this Kotlin code with sealed classes does not compile.

Kotlin

sealed class Animal
class Dog : Animal()
class Cat : Animal()

fun sound(animal: Animal) = when(animal) {
    is Dog -> "Bark"
    is Cat -> "Meow"
}

A Sealed class subclasses must be declared in the same file

B Missing 'else' branch in 'when' expression

C Cannot use 'is' checks with sealed classes

D Sealed class cannot have subclasses

💡 Hint

Think about Kotlin's rules for sealed class subclass declarations.

In Kotlin, all subclasses of a sealed class must be declared in the same file as the sealed class. Here, Dog and Cat are declared outside the sealed class file, causing a compilation error.

📝 Syntax

advanced

Correct syntax for sealed interface and its implementations

Which option shows the correct Kotlin syntax for a sealed interface and two implementations?

A

sealed interface Vehicle
sealed interface Car : Vehicle
sealed interface Bike : Vehicle

B

sealed interface Vehicle {
    class Car : Vehicle
    class Bike : Vehicle
}

C

sealed interface Vehicle
sealed class Car : Vehicle()
sealed class Bike : Vehicle()

D

sealed interface Vehicle
class Car : Vehicle
class Bike : Vehicle

💡 Hint

Sealed interfaces can be implemented by classes declared in the same file.

Option D correctly declares a sealed interface Vehicle and two classes Car and Bike implementing it. The classes are not sealed themselves but implement the sealed interface. Option D is invalid syntax because classes cannot be declared inside an interface body like that. Options A and D misuse sealed class/interface declarations.

🚀 Application

expert

Number of subclasses in sealed class hierarchy

Given this sealed class hierarchy, how many direct subclasses does the sealed class Operation have?

Kotlin

sealed class Operation {
    data class Add(val x: Int, val y: Int) : Operation()
    data class Subtract(val x: Int, val y: Int) : Operation()
    sealed class Multiply : Operation() {
        data class Simple(val x: Int, val y: Int) : Multiply()
        data class Complex(val x: Int, val y: Int, val z: Int) : Multiply()
    }
}

A 4

B 5

C 3

D 2

💡 Hint

Count only the immediate subclasses declared directly under Operation.

Operation has three direct subclasses: Add, Subtract, and Multiply. Although Multiply has its own subclasses, they are not direct subclasses of Operation.