Kotlin Classes

A class is a blueprint for creating objects. It defines the properties (data) and functions (behavior) that all objects of that type will have. Think of a class as a cookie cutter and objects as the individual cookies you make from it.

Defining a Class

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

    fun accelerate(amount: Int) {
        speed += amount
        println("$brand accelerates to $speed km/h")
    }

    fun brake() {
        speed = 0
        println("$brand stops.")
    }
}

Creating Objects (Instances)

fun main() {
    val car1 = Car()
    car1.brand = "Toyota"
    car1.accelerate(60)
    car1.accelerate(30)
    car1.brake()

    val car2 = Car()
    car2.brand = "Honda"
    car2.accelerate(80)
}

Output:

Toyota accelerates to 60 km/h
Toyota accelerates to 90 km/h
Toyota stops.
Honda accelerates to 80 km/h

Class vs Object: Blueprint vs Product

Class (the blueprint):
  class Car { brand, speed, accelerate(), brake() }
  ↓ defined once

Objects (the products):
  val car1 = Car()   → brand="Toyota", speed=90
  val car2 = Car()   → brand="Honda",  speed=80
  val car3 = Car()   → brand="BMW",    speed=0

Each object has its own data, but shares the same structure.

Primary Constructor

class Person(val name: String, val age: Int) {
    fun greet() {
        println("Hi, I'm $name and I'm $age years old.")
    }
}

fun main() {
    val p1 = Person("Alice", 30)
    val p2 = Person("Bob", 25)

    p1.greet()   // Hi, I'm Alice and I'm 30 years old.
    p2.greet()   // Hi, I'm Bob and I'm 25 years old.
}

Init Block

The init block runs automatically when an object is created:

class BankAccount(val owner: String, initialBalance: Double) {
    var balance: Double = initialBalance

    init {
        println("Account created for $owner with ₹$balance")
        require(initialBalance >= 0) { "Balance cannot be negative" }
    }

    fun deposit(amount: Double) {
        balance += amount
        println("Deposited ₹$amount. New balance: ₹$balance")
    }
}

fun main() {
    val acc = BankAccount("Priya", 5000.0)
    acc.deposit(2000.0)
}

Output:

Account created for Priya with ₹5000.0
Deposited ₹2000.0. New balance: ₹7000.0

Member Functions

class Rectangle(val width: Double, val height: Double) {
    fun area(): Double = width * height
    fun perimeter(): Double = 2 * (width + height)
    fun isSquare(): Boolean = width == height
    override fun toString(): String = "Rectangle(${width}×${height})"
}

fun main() {
    val r = Rectangle(6.0, 4.0)
    println(r)                  // Rectangle(6.0×4.0)
    println("Area: ${r.area()}")         // Area: 24.0
    println("Perimeter: ${r.perimeter()}") // Perimeter: 20.0
    println("Is square: ${r.isSquare()}")  // Is square: false
}

Visibility Modifiers

Modifier   │ Visible from
───────────┼───────────────────────────────────
public     │ everywhere (default)
private    │ only inside this class
protected  │ this class and subclasses
internal   │ same module (same project/library)

class UserAccount(val username: String) {
    private var password: String = ""   // hidden from outside

    fun setPassword(newPass: String) {
        if (newPass.length >= 8) {
            password = newPass
            println("Password updated")
        } else {
            println("Password too short")
        }
    }

    fun login(attempt: String): Boolean = attempt == password
}