# Data Classes — Kotlin

Source: https://www.skillbyai.com/en/kotlin/c-data

> equals, hashCode, toString, copy and destructuring for free.

## Value-like classes

Marking a class `data` makes the compiler generate `equals()`, `hashCode()`, `toString()`, `copy()` and `componentN()` functions from the properties in the **primary constructor**. Two data objects with equal properties are equal, which suits values such as DTOs, events and state. `copy(field = ...)` creates a modified copy, which pairs well with read-only `val` properties for immutable updates. `componentN` functions enable **destructuring declarations** such as `val (id, name) = user`. Properties declared in the class body are not part of the generated functions.

## copy and destructuring

Immutable updates.

```kotlin
data class User(val id: Long, val name: String, val active: Boolean = true)

fun main() {
    val ada = User(1, "Ada")
    val inactive = ada.copy(active = false)      // new object, ada unchanged

    println(ada == User(1, "Ada"))               // structural equality via equals()
    println(inactive)                            // generated toString()

    val (id, name) = ada                         // destructuring via component1/2
    println("$id -> $name")

    val users = mapOf(1L to "Ada", 2L to "Linus")
    for ((key, value) in users) println("$key=$value")
}
```

## == versus ===

In Kotlin `==` calls `equals()` (structural equality) and `===` checks whether two references point to the same object (referential equality).

**Quiz:** Which function is NOT generated for a data class?

- [ ] `equals()`
- [ ] `copy()`
- [x] `save()`
- [ ] `component1()`

*Answer:* `save()`. Data classes generate equals, hashCode, toString, copy and componentN.