Demystifying Kotlin Sealed Classes: Powerful Alternatives to Enums

Introduction:

In the world of Kotlin programming, sealed classes provide a versatile solution for representing restricted sets of classes or data types. This blog post aims to shed light on sealed classes, exploring their unique features and how they compare to enums. We will delve into the scenarios where sealed classes are most beneficial, and provide code examples as well as real-life use cases to illustrate their practical applications.

What is a Sealed Class?

A sealed class in Kotlin is a special type of class that restricts the inheritance of its subclasses within a defined scope. It acts as a superclass for a limited number of related classes, forming a closed hierarchy. Sealed classes are declared using the sealed keyword before the class declaration.

Sealed Classes vs. Enums:

While enums represent a fixed set of constant values, sealed classes go beyond that. Sealed classes allow not only a predefined set of values but also the ability to define complex class hierarchies. Unlike enums, sealed classes can have multiple instances with their own state, behavior, and properties. Sealed classes offer more flexibility and are an excellent choice when you need to represent a finite number of related classes or data types with variations.

When to Use Sealed Classes:

• Restricted Class Hierarchy: Sealed classes are particularly useful when you have a limited and defined set of classes or data types that need to be grouped together. If you find yourself needing to represent different states, types, or variations within a specific scope, sealed classes are a great fit. They help ensure that only the classes you explicitly define are used, preventing unexpected extensions.
• Pattern Matching and Exhaustiveness: Sealed classes work seamlessly with Kotlin's pattern matching capability, often referred to as "when expressions." When using a when expression to check the type of a sealed class, the compiler guarantees that all possible subclasses are covered. This ensures that you handle all cases exhaustively, improving code readability and reducing the risk of missing specific scenarios.

Code Example:

sealed class Vehicle {
    class Car(val model: String) : Vehicle()
    class Bike(val brand: String) : Vehicle()
    object Unknown : Vehicle()
}

fun processVehicle(vehicle: Vehicle) {
    when (vehicle) {
        is Vehicle.Car -> println("Car model: ${vehicle.model}")
        is Vehicle.Bike -> println("Bike brand: ${vehicle.brand}")
        Vehicle.Unknown -> println("Unknown vehicle")
    }
}

fun main() {
    val car = Vehicle.Car("Sedan")
    val bike = Vehicle.Bike("Mountain")
    val unknown = Vehicle.Unknown

    processVehicle(car)    // Output: Car model: Sedan
    processVehicle(bike)   // Output: Bike brand: Mountain
    processVehicle(unknown) // Output: Unknown vehicle
}

Real-Life Scenarios:

• Parsing API Responses: Sealed classes are commonly used when dealing with API responses that can have different states. For instance, an API response can be "Success" with data, "Error" with an error message, or "Loading" indicating a loading state. By using a sealed class, such as ApiResponse, with subclasses representing each state, you can handle and process various response scenarios in a structured manner.
• User Authentication: Sealed classes can help model different authentication states. For example, you may have states like "Authenticated" with user details, "Unauthenticated" indicating a user is not logged in, or "Error" when there is an issue with authentication. By defining a sealed class called AuthState with appropriate subclasses, you can manage and handle authentication flows effectively.

Conclusion:

Sealed classes in Kotlin offer a powerful alternative to enums, allowing you to define restricted class hierarchies with more flexibility and variations. They excel in scenarios where you need to group related classes or data types within a specific scope. By leveraging pattern matching and exhaustiveness checks, sealed classes enhance code readability and reliability. Whether you're parsing API responses, managing authentication states, or working with other situations that require a finite set of related classes, sealed classes prove to be a valuable tool in your Kotlin programming arsenal.