Class Basics

A class is a blueprint that describes what kind of information and abilities an object should have, like a cookie cutter for making cookies.

In this page:

Declaring a Simple Class
Properties in the Primary Constructor
Creating Instances
Member Functions

Declaring a Simple Class

A class groups related data and behavior under one name, defined with the class keyword. Even a class with no properties or methods can be declared and instantiated.

Example: Declaring a Simple Class

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class Greeter {
    fun greet() {
        println("Hello from Greeter!")
    }
}

fun main() {
    val greeter = Greeter()
    greeter.greet()
}

Properties in the Primary Constructor

Declaring parameters with val or var directly in the class's parentheses creates properties automatically, avoiding the need to write separate field declarations and assignments.

Example: Properties in the Primary Constructor

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class Dog(val name: String, val breed: String)

fun main() {
    val dog = Dog("Rex", "Labrador")
    println("${dog.name} is a ${dog.breed}")
}

Creating Instances

An instance of a class is created by calling its name like a function, passing arguments that match its primary constructor's parameters.

Note: No new keyword is needed in Kotlin, unlike Java.

Example: Creating Instances

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class Point(val x: Int, val y: Int)

fun main() {
    val p1 = Point(1, 2)
    val p2 = Point(3, 4)
    println("p1=(${p1.x}, ${p1.y}), p2=(${p2.x}, ${p2.y})")
}

Member Functions

Functions declared inside a class body are member functions (methods) that operate on that instance's own properties, accessible via this implicitly.

Example: Member Functions

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

fun main() {
    val circle = Circle(3.0)
    println("Area: ${circle.area()}")
}

Common Mistakes

Forgetting the class keyword needs a following pair of parentheses () for a primary constructor even when it has parameters, or omitting them entirely if there are none.
Not realizing that properties declared in the primary constructor with val/var automatically become accessible class members.
Creating an object but forgetting the () when calling the constructor, e.g. writing Dog instead of Dog().

Chapter Summary

A class is declared with the class keyword followed by a name and, optionally, a primary constructor.
Properties declared with val/var inside the constructor parentheses become class members automatically.
An object of a class is created by calling its constructor like a function: ClassName(...).
Kotlin classes are final (cannot be subclassed) by default unless marked open.

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Class Basics

Declaring a Simple Class

Properties in the Primary Constructor

Creating Instances

Member Functions

Chapter Quiz — Complete all 7 topics to unlock

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