Kotlin Data Classes Cheat Sheet

Covers declaring data classes, the auto-generated equals/hashCode/toString/copy methods, destructuring, and best practices for immutable models in Kotlin.

1 PageBeginnerMar 25, 2026

Declaring Data Classes

A one-line declaration generates equals, hashCode, and toString.

kotlin

data class User(val id: Int, val name: String, val email: String)val user1 = User(1, "Ana", "ana@example.com")val user2 = User(1, "Ana", "ana@example.com")println(user1 == user2)       // true (structural equality via equals())println(user1)                // User(id=1, name=Ana, email=ana@example.com)println(user1.hashCode() == user2.hashCode()) // true

The copy() Function

Creating a modified copy while keeping the original immutable.

kotlin

data class User(val id: Int, val name: String, val email: String)val user = User(1, "Ana", "ana@example.com")// copy() creates a new instance, overriding only the named parametersval renamed = user.copy(name = "Ana Maria")println(renamed) // User(id=1, name=Ana Maria, email=ana@example.com)println(user)     // original is unchanged (immutability preserved with val)

Destructuring Declarations

Unpacking a data class into individual variables.

kotlin

data class Point(val x: Int, val y: Int)val point = Point(10, 20)val (x, y) = point // uses auto-generated component1()/component2()println("x=$x, y=$y")// Common in loops over mapsval scores = mapOf("Ana" to 90, "Bob" to 85)for ((name, score) in scores) {    println("$name scored $score")}// Underscore to skip a component you don't need (Kotlin 1.1+)val (_, onlyY) = point

Auto-Generated Members

What the compiler generates for every data class.

equals()/hashCode()- Structural equality based on all properties declared in the primary constructor
toString()- Readable format: ClassName(prop1=value1, prop2=value2, ...)
copy()- Creates a shallow copy with optionally overridden properties
componentN()- One function per constructor property, enabling destructuring
Only primary constructor properties- Properties declared in the class body (not the constructor) are excluded from these

Data Class Rules

Constraints the compiler enforces on data class declarations.

Primary constructor required- Must have at least one parameter, all val or var
Cannot be abstract/open/sealed/inner- Data classes cannot be extended in these ways
val for immutability- Prefer val properties so equals/hashCode stay consistent over the object's lifetime
Avoid mutable var as a map/set key- Mutating it after insertion breaks hashCode-based lookups
Can implement interfaces- Data classes can implement interfaces and have extra members beyond the constructor

Pro Tip

Never use a `var` property in a data class as a HashMap/HashSet key — mutating it after insertion changes the hashCode, and the entry becomes unfindable even though `contains()` on the original reference still works.

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