Classes and Objects

What is a Class?

A class is a blueprint. An object is a real thing built from that blueprint. A Car class describes what every car has (brand, speed) and can do (drive). Each object made from it holds its own values.

Declare a class with the class keyword.
Create an object by calling the class like a function: Car(). Kotlin has no new keyword.
Variables inside a class are properties; functions inside a class are member functions (methods).

Create a Class and Objects

Example

Two objects from the same class keep separate values.

class Car {
    var brand = ""
    var speed = 0

    fun drive() {
        println("$brand is driving at $speed km/h")
    }
}

fun main() {
    val car1 = Car()
    car1.brand = "Tata Nexon"
    car1.speed = 80

    val car2 = Car()
    car2.brand = "Maruti Swift"
    car2.speed = 60

    car1.drive()
    car2.drive()
}

Output

Tata Nexon is driving at 80 km/h
Maruti Swift is driving at 60 km/h

Properties in the Constructor

Setting every property after creating the object is long and error-prone. Usually you pass values in the class header. Writing val or var in front of a parameter turns it into a property.

Example

class Student(val name: String, var marks: Int)

fun main() {
    val s = Student("Priya", 82)
    println(s.name + " scored " + s.marks)
    s.marks = 91          // var: can change
    // s.name = "Riya"    // val: compile error
    println("Updated marks: " + s.marks)
}

Output

Priya scored 82
Updated marks: 91

Exercise

Exercise: Create a class Book with title (val) and price (var). Create one book, raise its price by 50 and print both values.

Show answer

class Book(val title: String, var price: Int)

fun main() {
    val b = Book("Kotlin in Action", 450)
    b.price += 50
    println("${b.title} costs ${b.price}")
}