Kotlin Classes and Objects

A class is a blueprint. An object is the actual thing built from that blueprint. Think of a class as the architectural plan for a house — the plan itself is not a house. When you build using the plan, you get an actual house (an object). You can build many houses from one plan.

Defining a Class

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

    fun accelerate(amount: Int) {
        speed += amount
        println("$brand is now going $speed km/h")
    }

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

The variables inside a class are called properties. The functions inside a class are called member functions or methods.

Creating Objects (Instances)

val car1 = Car()
car1.brand = "Toyota"
car1.speed = 0
car1.accelerate(60)   // Toyota is now going 60 km/h

val car2 = Car()
car2.brand = "Honda"
car2.accelerate(80)   // Honda is now going 80 km/h

Diagram — Class and Objects

CLASS Car (Blueprint)
┌──────────────────────────┐
│ brand: String            │
│ speed: Int               │
│ accelerate(amount: Int)  │
│ brake()                  │
└──────────────────────────┘
          │
     ─────┴──────────────
    │                    │
OBJECT car1         OBJECT car2
brand = "Toyota"    brand = "Honda"
speed = 60          speed = 80

Constructors

A constructor is a special function that runs when you create an object. It sets up the object's initial state. Kotlin's primary constructor lives right in the class header.

Primary Constructor

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

val p1 = Person("Ananya", 25)
p1.introduce()   // Hi, I'm Ananya and I'm 25 years old.

init Block

The init block runs immediately after the primary constructor. Use it for validation or setup logic.

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

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

val acc = BankAccount("Rohan", 5000.0)
// Account created for Rohan with ₹5000.0

Secondary Constructor

class Product {
    var name: String
    var price: Double

    constructor(name: String, price: Double) {
        this.name = name
        this.price = price
    }

    constructor(name: String) : this(name, 0.0) {
        println("$name added with no price set.")
    }
}

Properties with Getters and Setters

Kotlin properties can have custom logic for reading (getter) and writing (setter) their values.

class Temperature(private var celsius: Double) {
    val fahrenheit: Double
        get() = celsius * 9 / 5 + 32

    var kelvin: Double
        get() = celsius + 273.15
        set(value) { celsius = value - 273.15 }
}

val temp = Temperature(100.0)
println(temp.fahrenheit)   // 212.0
println(temp.kelvin)       // 373.15

Visibility Modifiers

Visibility modifiers control who can access a class member.

Modifier      | Accessible From
--------------|----------------------------------
public        | Everywhere (default)
private       | Only inside the same class
protected     | Inside the class and subclasses
internal      | Anywhere in the same module

class Wallet {
    private var balance: Double = 0.0   // hidden from outside

    fun deposit(amount: Double) {
        if (amount > 0) balance += amount
    }

    fun getBalance(): Double = balance
}

val w = Wallet()
w.deposit(500.0)
println(w.getBalance())   // 500.0
// println(w.balance)     // ERROR: balance is private

Companion Objects — Class-Level Members

A companion object holds properties and functions that belong to the class itself, not to any single instance. It is similar to static members in Java.

class MathUtils {
    companion object {
        const val PI = 3.14159

        fun circleArea(radius: Double): Double {
            return PI * radius * radius
        }
    }
}

println(MathUtils.PI)                  // 3.14159
println(MathUtils.circleArea(5.0))     // 78.53975
// No object creation needed to call these.