Data Classes

Jun 23, 2026Trushi Jasani

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Data Classes

What are Data Classes?

A data class is a class designed to hold data. Kotlin automatically generates:

• equals() — structural comparison
• hashCode() — hash based on properties
• toString() — readable string representation
• copy() — create a modified copy
• componentN() — destructuring support

Basic Syntax

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

fun main() {
    val alice = Person("Alice", 30)
    println(alice)   // Person(name=Alice, age=30)
}

Auto-Generated toString()

data class Point(val x: Int, val y: Int)

fun main() {
    val p = Point(3, 4)
    println(p)   // Point(x=3, y=4)
}

Auto-Generated equals()

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

fun main() {
    val u1 = User("Alice", "alice@example.com")
    val u2 = User("Alice", "alice@example.com")
    val u3 = User("Bob", "bob@example.com")

    println(u1 == u2)  // true — same content
    println(u1 == u3)  // false — different content
    println(u1 === u2) // false — different references
}

copy() Function

Create a copy with some fields changed:

data class Config(
    val host: String = "localhost",
    val port: Int = 8080,
    val debug: Boolean = false
)

fun main() {
    val dev  = Config()
    val prod = dev.copy(host = "example.com", debug = false)
    val test = dev.copy(port = 9090, debug = true)

    println(dev)   // Config(host=localhost, port=8080, debug=false)
    println(prod)  // Config(host=example.com, port=8080, debug=false)
    println(test)  // Config(host=localhost, port=9090, debug=true)
}

Destructuring

data class Product(val name: String, val price: Double, val stock: Int)

fun main() {
    val item = Product("Laptop", 999.99, 10)

    val (name, price, stock) = item
    println("$name costs $$price, $stock in stock")

    // In loops
    val products = listOf(
        Product("Phone", 499.0, 25),
        Product("Tablet", 299.0, 15)
    )

    for ((n, p, s) in products) {
        println("$n — $$p — qty: $s")
    }
}

Data Class in Collections

data class Student(val name: String, val gpa: Double)

fun main() {
    val students = listOf(
        Student("Alice", 3.9),
        Student("Bob", 3.5),
        Student("Charlie", 3.7)
    )

    val top = students.maxByOrNull { it.gpa }
    val sorted = students.sortedByDescending { it.gpa }

    println("Top student: $top")
    sorted.forEach { println(it) }
}

Nested Data Classes

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

data class Person(
    val name: String,
    val age: Int,
    val address: Address
)

fun main() {
    val person = Person(
        "Alice", 28,
        Address("123 Main St", "Bangalore", "560001")
    )

    println(person)
    println(person.address.city)  // Bangalore

    // Copy with nested update
    val moved = person.copy(address = person.address.copy(city = "Mumbai"))
    println(moved.address.city)  // Mumbai
}

Data Class Requirements

• Must have at least one parameter in the primary constructor.
• All primary constructor parameters must be val or var.
• Cannot be abstract, open, sealed, or inner.

Data Class vs Regular Class

// Regular class
class PointRegular(val x: Int, val y: Int)

// Data class
data class PointData(val x: Int, val y: Int)

fun main() {
    val r1 = PointRegular(1, 2)
    val r2 = PointRegular(1, 2)
    println(r1 == r2)   // false — reference equality

    val d1 = PointData(1, 2)
    val d2 = PointData(1, 2)
    println(d1 == d2)   // true — structural equality
}

Summary

Feature	Auto-generated by data class
toString()	Yes — shows all properties
equals()	Yes — structural comparison
hashCode()	Yes — based on properties
copy()	Yes — clone with overrides
componentN()	Yes — for destructuring