Classes, Constructors & Properties

What is a class?

A class is a blueprint for creating objects. It bundles together data (properties) and behaviour (functions). An object is one instance built from that blueprint.

class Car {
    var brand: String = ""
    var speed: Int = 0

    fun accelerate(by: Int) {
        speed += by
    }
}

val car = Car()          // create an instance
car.brand = "Tata"
car.accelerate(20)
println(car.speed)       // 20

The primary constructor

Kotlin lets you declare properties right in the constructor — this removes the boilerplate Java needs.

class Car(val brand: String, var speed: Int = 0) {
    fun accelerate(by: Int) { speed += by }
}

val car = Car("Tata")
val car2 = Car("BMW", 60)

Here val brand and var speed in the parentheses become properties automatically. speed even has a default value.

init blocks

Need to run setup code when an object is created? Use an init block:

class Account(val owner: String, balance: Double) {
    var balance = balance
        private set                 // others can read but not write

    init {
        require(balance >= 0) { "Balance cannot be negative" }
    }
}

Custom getters and setters

Properties can compute their value or validate changes:

class Rectangle(val width: Int, val height: Int) {
    val area: Int
        get() = width * height      // computed each time it's read
}

class Temperature {
    var celsius: Double = 0.0
        set(value) {
            field = value           // 'field' is the backing storage
        }
    val fahrenheit get() = celsius * 9 / 5 + 32
}

Secondary constructors

class Person(val name: String) {
    var age: Int = 0
    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }
}

Common mistakes

• Writing Java-style getters/setters by hand — Kotlin properties already do this.
• Exposing a mutable property when callers should only read it — use private set.
• Putting heavy logic in init — keep object creation cheap.

Summary: Declare properties in the primary constructor, use init for setup/validation, and custom getters for computed values. Kotlin removes nearly all getter/setter boilerplate.