How do I use data classes in Kotlin to store structured data?

In Kotlin, data classes are designed to store structured data with minimal boilerplate.

A data class automatically provides useful functions such as:

• toString()
• equals()
• hashCode()
• copy()
• component functions for destructuring, like component1(), component2()

Basic example

data class User(
    val id: Int,
    val name: String,
    val email: String
)

You can create and use it like this:

fun main() {
    val user = User(
        id = 1,
        name = "Alice",
        email = "alice@example.com"
    )

    println(user)
}

Output:

User(id=1, name=Alice, email=alice@example.com)

Accessing properties

println(user.name)
println(user.email)

Because the properties are declared in the primary constructor, they are available directly.

Comparing data objects

Data classes compare values, not object references:

val user1 = User(1, "Alice", "alice@example.com")
val user2 = User(1, "Alice", "alice@example.com")

println(user1 == user2) // true

Copying with changes

Use copy() to create a modified copy:

val updatedUser = user.copy(email = "newalice@example.com")

println(updatedUser)

The original object is unchanged.

Destructuring

Data classes support destructuring declarations:

val (id, name, email) = user

println(id)
println(name)
println(email)

Mutable vs immutable properties

Prefer val for immutable data:

data class Product(
    val id: Long,
    val name: String,
    val price: Double
)

Use var only if the property needs to change:

data class MutableUser(
    var name: String,
    var age: Int
)

Example with nested structured data

data class Address(
    val street: String,
    val city: String,
    val postalCode: String
)

data class Customer(
    val id: Int,
    val name: String,
    val address: Address
)

fun main() {
    val customer = Customer(
        id = 100,
        name = "Maria",
        address = Address(
            street = "Main Street",
            city = "Berlin",
            postalCode = "10115"
        )
    )

    println(customer.address.city)
}

Important rules

A Kotlin data class must:

• Have at least one parameter in the primary constructor
• Mark primary constructor parameters with val or var
• Not be abstract, open, sealed, or inner

Example:

data class Book(
    val title: String,
    val author: String,
    val year: Int
)

Use data classes when you mainly need a class to hold data rather than define complex behavior.