Sealed classes for restricted hierarchies in Kotlin - Mini Project: Build & Apply

Sealed Classes for Restricted Hierarchies

📖 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 way of showing a message.

🎯 Goal: You will create a sealed class hierarchy to represent these notification types. Then, you will write code to handle each notification type safely and print the correct message.

📋 What You'll Learn

Create a sealed class called Notification.

Create three subclasses: Email, SMS, and Push inside Notification.

Add a message property to each subclass.

Write a function showNotification that takes a Notification and prints a message based on its type using when.

Call showNotification for each notification type.

💡 Why This Matters

🌍 Real World

Sealed classes help you model fixed sets of types, like different notification kinds, so your app can handle them safely and clearly.

💼 Career

Understanding sealed classes is important for Kotlin developers to write safe and maintainable code, especially when working with restricted hierarchies or state machines.

1

Create the sealed class and subclasses

Create a sealed class called Notification. Inside it, create three subclasses: Email, SMS, and Push. Each subclass should have a message property of type String.

Kotlin

sealed class Notification {
    // Your code here
}

Need a hint?

Use sealed class Notification and inside it define data class Email, SMS, and Push each with a message property.

2

Create a function to show notifications

Create a function called showNotification that takes a parameter notification of type Notification. Inside the function, use a when expression to check the type of notification and print the message with a prefix: "Email: ", "SMS: ", or "Push: ".

Kotlin

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

fun showNotification(notification: Notification) {
    // Your code here
}

Need a hint?

Use when (notification) and check for each subclass with is Notification.Email, etc. Print the message with the correct prefix.

3

Create instances of each notification type

Create three variables: emailNotification, smsNotification, and pushNotification. Assign each one an instance of Notification.Email, Notification.SMS, and Notification.Push respectively, with messages: "Welcome to your inbox!", "You have a new text.", and "You have a new alert."

Kotlin

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

fun showNotification(notification: Notification) {
    when (notification) {
        is Notification.Email -> println("Email: ${notification.message}")
        is Notification.SMS -> println("SMS: ${notification.message}")
        is Notification.Push -> println("Push: ${notification.message}")
    }
}

val emailNotification = Notification.Email("")
val smsNotification = Notification.SMS("")
val pushNotification = Notification.Push("")
// Your code here

Need a hint?

Create variables with exact names and assign instances of each subclass with the given messages.

4

Print messages for all notifications

Call showNotification for each variable: emailNotification, smsNotification, and pushNotification. This will print the messages with their prefixes.

Kotlin

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

fun showNotification(notification: Notification) {
    when (notification) {
        is Notification.Email -> println("Email: ${notification.message}")
        is Notification.SMS -> println("SMS: ${notification.message}")
        is Notification.Push -> println("Push: ${notification.message}")
    }
}

val emailNotification = Notification.Email("Welcome to your inbox!")
val smsNotification = Notification.SMS("You have a new text.")
val pushNotification = Notification.Push("You have a new alert.")

showNotification(emailNotification)
// Your code here

Need a hint?

Call showNotification with each notification variable to print their messages.