What are data classes and sealed classes used for in Kotlin?

Data classes simplify the creation of classes intended solely for storing data. The compiler automatically generates for them:

• equals()
• hashCode()
• toString()
• copy()
• componentN() for destructuring

data class User(val name: String, val age: Int)

// Usage
val user = User("Alice", 30)
println(user) // Outputs User(name=Alice, age=30)
val user2 = user.copy(age = 31)
val (name, age) = user // Destructuring

Sealed classes represent a restricted class hierarchy where all possible subclasses are known at compile time. This allows their use in when expressions without an else block, guaranteeing exhaustive checking of all possible cases.

sealed class Result {
    data class Success(val data: String) : Result()
    data class Error(val message: String) : Result()
    object Loading : Result() // Object, as it has no parameters
}

fun handleResult(result: Result) {
    when (result) {
        is Result.Success -> println("Success: ${result.data}")
        is Result.Error -> println("Error: ${result.message}")
        Result.Loading -> println("Loading...")
    }
}

Comparison:

Attribute	Data class	Sealed class
Purpose	Data storage	Restricted class hierarchy
Capabilities	Automatic method generation	Exhaustive when checks
Subclasses	No specific restriction	Must be defined in the same file or module
Usage	Data models, DTOs	State, event, variant representation