What are sealed classes and what is their purpose…

Sealed classes represent restricted class hierarchies. They are used to represent a limited set of possible states or values.

Main features and purpose:

Restricted hierarchy: In Kotlin versions before 1.5, subclasses of a sealed class must be declared in the same file or nested within the sealed class. From version 1.5 onwards, this restriction is relaxed: subclasses can be in any file within the same package.
Enum classes with state: Unlike enum classes, each subclass of a sealed class can have its own state and behavior.
Exhaustive when: The compiler guarantees that a when block for an instance of a sealed class is exhaustive (all cases are covered) if no else branch is specified. This reduces the likelihood of runtime errors.

Example:

sealed class Result<out T> {
    data class Success<out T>(val data: T) : Result<T>()
    data class Error(val exception: Exception) : Result<Nothing>()
    object Loading : Result<Nothing>() // Singleton instance
}

fun processResult(result: Result<String>) {
    when (result) {
        is Result.Success -> {
            // Handle success
            println("Data: ${result.data}")
        }
        is Result.Error -> {
            // Handle error
            println("Error: ${result.exception.message}")
        }
        Result.Loading -> {
            // Handle loading state
            println("Loading data...")
        }
        // No else branch needed, as all subclasses are handled by the compiler
    }
}

Purpose:

Safe representation of states: Reliably represent a finite number of states, such as the result of a network request (Success, Error, Loading).
Improved code readability: Make code more explicit when handling different cases.
Increased reliability of when expressions: The compiler helps avoid unhandled cases.
Replacement for enum classes with state: When it is necessary to associate data with specific "variants" of an enumeration.

Thus, sealed classes provide a flexible way to model finite sets of values with the ability to store specific data for each value, while ensuring safety and clarity when processing these values in when expressions.

What are sealed classes and what is their purpose?