# Classes, Constructors and Properties — Kotlin

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

> Primary constructors, init blocks and accessors.

## Properties instead of fields

A class can declare a **primary constructor** in its header; prefixing parameters with `val` or `var` turns them into **properties**. `init` blocks run during construction and are a good place for validation with `require`. Properties have optional custom **getters** and **setters**; `field` refers to the backing field inside an accessor. **Secondary constructors** (`constructor(...)`) must delegate to the primary one. Classes are **final by default**, there is no `new` keyword, and `lateinit var` allows non-null properties that are initialised after construction (for example by dependency injection).

## Model data with little boilerplate

Kotlin classes declare properties in the constructor; data, sealed and enum classes cover common modelling needs.

![Three ideas: classes and properties, data classes, sealed hierarchies and enums.](assets/figures/kotlin/section-3-map.svg) — Figure 3.1 — Class, data class and sealed hierarchy.

## A class with validation and accessors

Properties live in the constructor.

```kotlin
class Account(val owner: String, initialBalance: Long = 0) {
    var balance: Long = initialBalance
        private set                         // readable outside, writable only inside

    val isOverdrawn: Boolean
        get() = balance < 0                 // computed on every access

    init {
        require(owner.isNotBlank()) { "owner must not be blank" }
    }

    fun deposit(amount: Long) {
        require(amount > 0) { "amount must be positive" }
        balance += amount
    }
}

fun main() {
    val acc = Account("Ada", initialBalance = 100)
    acc.deposit(50)
}
```

## A form with built-in rules

The primary constructor is the form you fill in, `init` is the clerk who checks it, and properties with private setters are fields you can read but only the office can change.

**Quiz:** What does `class Point(val x: Int, val y: Int)` declare?

- [x] A class with a primary constructor and two read-only properties
- [ ] A function named Point
- [ ] A data class with copy()
- [ ] An open class that can be subclassed

*Answer:* A class with a primary constructor and two read-only properties. val in the constructor creates properties; classes are final by default.