Sealed classes with when exhaustive check in Kotlin - Mini Project: Build & Apply

Sealed classes with when exhaustive check

📖 Scenario: Imagine you are building a simple app that handles different types of notifications: Email, SMS, and Push notifications. Each notification type has its own data.

🎯 Goal: You will create sealed classes to represent these notification types and use a when expression to handle each type with an exhaustive check.

📋 What You'll Learn

Create a sealed class called Notification with three subclasses: Email, SMS, and Push

Add properties to each subclass as specified

Create a variable called notification with a specific subclass instance

Use a when expression with exhaustive check to handle all notification types

Print a message describing the notification

💡 Why This Matters

🌍 Real World

Sealed classes help represent fixed sets of related types, like different notification types in an app, making code safer and easier to maintain.

💼 Career

Understanding sealed classes and exhaustive <code>when</code> expressions is important for Kotlin developers building robust and clear applications.

1

Create sealed class and subclasses

Create a sealed class called Notification with three subclasses: Email with a subject property of type String, SMS with a phoneNumber property of type String, and Push with a message property of type String.

Kotlin

# Create sealed class Notification and subclasses Email, SMS, Push
// Your code here

Hint

Use sealed class Notification and inside it define data class Email, SMS, and Push each inheriting from Notification.

2

Create a notification instance

Create a variable called notification and assign it an instance of Notification.Email with the subject set to "Welcome to Kotlin".

Kotlin

sealed class Notification {
    data class Email(val subject: String) : Notification()
    data class SMS(val phoneNumber: String) : Notification()
    data class Push(val message: String) : Notification()
}

// Create variable notification as Email with subject "Welcome to Kotlin"
// Your code here

Hint

Use val notification: Notification = Notification.Email("Welcome to Kotlin") to create the instance.

3

Use when expression with exhaustive check

Write a when expression using the variable notification to handle all subclasses: Email, SMS, and Push. For each case, create a string message describing the notification. Assign the result to a variable called message. Make sure the when expression is exhaustive.

Kotlin

sealed class Notification {
    data class Email(val subject: String) : Notification()
    data class SMS(val phoneNumber: String) : Notification()
    data class Push(val message: String) : Notification()
}

val notification: Notification = Notification.Email("Welcome to Kotlin")

// Use when expression with exhaustive check to assign message
// Your code here

Hint

Use val message = when (notification) { ... } with all subclasses handled.

4

Print the message

Write a println statement to display the value of the variable message.

Kotlin

sealed class Notification {
    data class Email(val subject: String) : Notification()
    data class SMS(val phoneNumber: String) : Notification()
    data class Push(val message: String) : Notification()
}

val notification: Notification = Notification.Email("Welcome to Kotlin")

val message = when (notification) {
    is Notification.Email -> "Email with subject: ${notification.subject}"
    is Notification.SMS -> "SMS to phone number: ${notification.phoneNumber}"
    is Notification.Push -> "Push notification: ${notification.message}"
}

// Print the message
println()

Hint

Use println(message) to display the message.

Practice

1. What is the main benefit of using sealed classes in Kotlin when combined with a when expression?

easy

A. The when expression becomes exhaustive, ensuring all cases are handled.

B. It allows subclassing from any file in the project.

C. It automatically generates getter and setter methods.

D. It makes the class open for inheritance everywhere.

Solution

1. Step 1: Understand sealed classes restriction

   Sealed classes restrict subclassing to the same file, so all subclasses are known at compile time.

2. Step 2: Effect on when expression

   This allows the when expression to be exhaustive because the compiler knows all possible subclasses.

3. Final Answer:

   The when expression becomes exhaustive, ensuring all cases are handled. -> Option A

4. Quick Check:

   Sealed classes + when = exhaustive check [OK]

Hint: Sealed classes limit subclasses, making when exhaustive [OK]

Common Mistakes:

• Thinking sealed classes allow subclassing anywhere
• Confusing sealed classes with data classes
• Believing sealed classes auto-generate methods

2. Which of the following is the correct syntax to declare a sealed class in Kotlin?

easy

A. sealed class Shape {}

B. open sealed class Shape {}

C. sealed interface Shape {}

D. final sealed class Shape {}

Solution

1. Step 1: Recall sealed class declaration syntax

   In Kotlin, sealed classes are declared with the keyword sealed followed by class and the class name.

2. Step 2: Check options for correctness

   sealed class Shape {} uses correct syntax: sealed class Shape {}. open sealed class Shape {} incorrectly adds open, which is not allowed. sealed interface Shape {} uses sealed interface, which is invalid. final sealed class Shape {} uses final sealed class, which is not allowed.

3. Final Answer:

   sealed class Shape {} -> Option A

4. Quick Check:

   sealed class declaration = sealed class Shape {} [OK]

Hint: Use only 'sealed class ClassName' to declare sealed classes [OK]

Common Mistakes:

• Adding 'open' modifier to sealed class
• Trying to declare sealed interfaces (not allowed)
• Using 'final' modifier with sealed class

3. Given the sealed class hierarchy below, what will be the output of the describe function when called with Shape.Circle(5.0)?

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}"
}

medium

A. Rectangle with width 5.0 and height 5.0

B. Compilation error due to missing else branch

C. Shape.Circle

D. Circle with radius 5.0

Solution

1. Step 1: Identify the input and matching when branch

   The input is Shape.Circle(5.0). The when expression matches is Shape.Circle branch.

2. Step 2: Evaluate the returned string

   The branch returns the string "Circle with radius 5.0" using the radius property.

3. Final Answer:

   Circle with radius 5.0 -> Option D

4. Quick Check:

   Input Circle(5.0) returns "Circle with radius 5.0" [OK]

Hint: Match input type in when to get correct output string [OK]

Common Mistakes:

• Confusing Circle with Rectangle output
• Expecting a compilation error due to else missing
• Returning class name instead of formatted string

4. Consider the following code snippet. What is the error and how can it be fixed?

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

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

medium

A. Runtime error: Result.Failure is not handled and causes crash.

B. Compilation error: when expression is not exhaustive; add branch for Result.Failure.

C. No error; code runs fine and returns null for Result.Failure.

D. Syntax error: sealed classes cannot have object subclasses.

Solution

1. Step 1: Identify the sealed class subclasses

   Result has two subclasses: Success and Failure.

2. Step 2: Check when expression branches

   The when expression only handles Result.Success, missing Result.Failure branch.

3. Step 3: Understand Kotlin's exhaustiveness check

   Since Result is sealed, when must handle all subclasses or have an else branch. Missing Failure causes compilation error.

4. Final Answer:

   Compilation error: when expression is not exhaustive; add branch for Result.Failure. -> Option B

5. Quick Check:

   Sealed class when must cover all subclasses [OK]

Hint: Add all sealed subclasses in when to avoid errors [OK]

Common Mistakes:

• Assuming runtime error instead of compile error
• Ignoring missing branches in when
• Thinking sealed classes can't have object subclasses

5. You have a sealed class Operation with subclasses Add, Subtract, and Multiply. Write a Kotlin function calculate that takes an Operation and two integers, and returns the result of applying the operation. Which of the following implementations correctly uses an exhaustive when expression?

hard

A. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Add -> a + b
else -> 0
}

B. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Add -> a + b
is Operation.Subtract -> a - b
}

C. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Add -> a + b
is Operation.Subtract -> a - b
is Operation.Multiply -> a * b
}

D. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) {
is Operation.Multiply -> a * b
else -> a + b
}

Solution

1. Step 1: Check all subclasses handled in when

   Operation has Add, Subtract, Multiply. fun calculate(op: Operation, a: Int, b: Int): Int = when(op) { is Operation.Add -> a + b is Operation.Subtract -> a - b is Operation.Multiply -> a * b } handles all three explicitly.

2. Step 2: Verify exhaustiveness and correctness

   fun calculate(op: Operation, a: Int, b: Int): Int = when(op) { is Operation.Add -> a + b is Operation.Subtract -> a - b is Operation.Multiply -> a * b } covers all subclasses, so when is exhaustive. The other options miss some subclasses or rely on else, which is less clear.

3. Final Answer:

   fun calculate(op: Operation, a: Int, b: Int): Int = when(op) { is Operation.Add -> a + b is Operation.Subtract -> a - b is Operation.Multiply -> a * b } -> Option C

4. Quick Check:

   All subclasses covered in when = exhaustive [OK]

Hint: Cover all sealed subclasses explicitly for exhaustive when [OK]

Common Mistakes:

• Missing some subclasses in when branches
• Using else instead of explicit branches
• Returning wrong default values