How is a Kotlin class different from a Java class?

Kotlin classes are final by default (cannot be inherited without open). Properties and getters/setters are auto-generated. No need for new keyword. Primary constructor is part of the class header.

Do I need the new keyword to create an object?

No. Instantiate with ClassName() — Kotlin does not use new. Example: val car = Car().

Beginner 8 min readKotlin 2.0

Kotlin Classes — Creating and Using Classes

A class is a blueprint for objects. Kotlin classes are concise — properties and constructors can be declared in one line.

What You Will Learn

Declare a class with a primary constructor
Define properties and methods
Create instances
Use init blocks
Understand val vs var in a class

Basic Class Declaration

Use the class keyword. Properties in the primary constructor are declared with val (read-only) or var (mutable).

Basic Class

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

fun main() {
    val person = Person("Juned", 25)
    person.introduce()
    person.age = 26  // mutable property
    println(person.age)
}

Output

Hi, I am Juned and I am 25 years old. 26

Person has two properties in the primary constructor. val name cannot be changed. var age can be reassigned. Methods are declared inside the class body.

Beginner Tip: Unlike Java, there is no need for explicit getters and setters — Kotlin generates them automatically for val and var properties.

init Block

The init block runs when an instance is created, after the constructor.

init Block

kotlin
class Circle(val radius: Double) {
    val area: Double

    init {
        require(radius > 0) { "Radius must be positive" }
        area = Math.PI * radius * radius
    }

    fun describe() = "Circle(r=$radius, area=${"%.2f".format(area)})"
}

fun main() {
    val c = Circle(5.0)
    println(c.describe())
}

Output

Circle(r=5.0, area=78.54)

init runs when Circle is created. area is computed inside init. require validates the input and throws if false.

Secondary Constructor

Use constructor keyword for additional constructors.

Secondary Constructor

kotlin
class Book(val title: String, val author: String) {
    var pages: Int = 0

    constructor(title: String, author: String, pages: Int) : this(title, author) {
        this.pages = pages
    }

    override fun toString() = ""$title" by $author ($pages pages)"
}

fun main() {
    val b1 = Book("Kotlin Guide", "KG Team")
    val b2 = Book("Effective Kotlin", "M. Moskala", 350)
    println(b1)
    println(b2)
}

Output

"Kotlin Guide" by KG Team (0 pages) "Effective Kotlin" by M. Moskala (350 pages)

The secondary constructor delegates to the primary with : this(). It sets the pages property afterward.

Best Practice: Prefer default arguments over secondary constructors — they are more concise and readable.

Practice Exercise

Exercisepredict output

What prints? class Dog(val name: String) { fun bark() = println("$name says: Woof!") } fun main() { val d = Dog("Rex") d.bark() }

Quick Quiz

In Kotlin, how do you declare a class property in the primary constructor?

Kotlin Classes — Creating and Using Classes

What You Will Learn

Basic Class Declaration

Basic Class

init Block

init Block

Secondary Constructor

Secondary Constructor

Practice Exercise

Quick Quiz

Frequently Asked Questions

Related Tutorials