Object-Oriented Programming

Classes and Objects

Class is blueprint
Object is instance of a class

class Car(var brand: String, var speed: Int) {
    fun drive() {
        println("$brand is driving at $speed km/h")
    }
}

fun main() {
    val myCar = Car("Toyota", 120)
    myCar.drive()  // Toyota is driving at 120 km/h
}

Constructors

1. Primary Constructor (concise):

class Person(val name: String, var age: Int)
val p = Person("Alice", 25)
println(p.name)  // Alice

2. Secondary Constructor:

class Person {
    var name: String
    constructor(name: String) {
        this.name = name
    }
}
val p = Person("Bob")
println(p.name)  // Bob

Inheritance

• By default, classes are final. Use open to allow inheritance.

open class Animal {
    open fun sound() = println("Animal sound")
}

class Dog : Animal() {
    override fun sound() = println("Woof")
}

fun main() {
    val dog = Dog()
    dog.sound()  // Woof
}

Polymorphism

• Overloading → same function name, different parameters
• Overriding → child class changes parent function behavior

open class Shape {
    open fun area(): Double = 0.0
}

class Circle(val radius: Double) : Shape() {
    override fun area() = Math.PI * radius * radius
}

fun main() {
    val shape: Shape = Circle(5.0)
    println(shape.area())  // 78.5398...
}

Abstraction

Use abstract classes or interfaces.

abstract class Vehicle {
    abstract fun start()
}

class Bike : Vehicle() {
    override fun start() = println("Bike started")
}

interface Drivable {
    fun drive()
}

class Car : Drivable {
    override fun drive() = println("Car driving")
}

Data Classes

Special class for storing data, auto-generates toString, equals, hashCode, copy.

data class User(val name: String, val age: Int)

fun main() {
    val u1 = User("Alice", 25)
    val u2 = u1.copy(age = 26)
    println(u1)  // User(name=Alice, age=25)
    println(u2)  // User(name=Alice, age=26)
}

Singleton (Object Declaration)

Kotlin has object keyword for singletons.

object Database {
    fun connect() = println("Connected")
}

fun main() {
    Database.connect()  // Connected
}

Any

In Kotlin, every class implicitly inherits from Any, not Object.

Any is the root of the Kotlin class hierarchy.

It’s smaller than Java’s Object (only 3 methods):

open class Any {
    open operator fun equals(other: Any?): Boolean
    open fun hashCode(): Int
    open fun toString(): String
}