Constructors

A constructor is the setup pattern that lets you feed initial data into a class when you create an object. In Kotlin, you can write the primary constructor right after the class name, which keeps the code short and clear.

kotlin

class Person(val firstName: String, var age: Int)

fun main() {
    val person = Person("John", 35)
    println("${person.firstName} is ${person.age} years old.")
}

You should see

John is 35 years old.

Summary

A constructor is essential for feeding correct data into an object right when it's created.

Easy traps

If you get the constructor parameter order wrong, the data can end up meaning something different. Using named arguments makes it clearer.

Practical example — named arguments

kotlin

class Course(val title: String, val durationHours: Int)

fun main() {
    val course = Course(
        title = "Kotlin Foundation",
        durationHours = 12
    )

    println("${course.title}: ${course.durationHours} hours")
}

You'll know it worked when: Kotlin Foundation: 12 hours