Kotlin Constructors

The Primary Constructor

Kotlin's primary constructor is part of the class header itself, right after the class name. Parameters declared there with val or var automatically become properties of the class, eliminating the need to manually assign fields inside a constructor body.

class Employee(val name: String, var salary: Double)

fun main() {
    val emp = Employee("Diego", 55000.0)
    println("${emp.name} earns ${emp.salary}")
    emp.salary += 5000.0
    println("New salary: ${emp.salary}")
}

The init Block

If you need to run logic during initialization - like validation or computing a derived value - use an init block. Init blocks execute in the order they appear, interleaved with property initializers, right after the primary constructor runs.

class Employee(val name: String, var salary: Double) {
    init {
        require(salary >= 0) { "Salary cannot be negative" }
        println("Created employee record for $name")
    }
}

fun main() {
    val emp = Employee("Priya", 62000.0)
}

Secondary Constructors

A class can also declare one or more secondary constructors using the constructor keyword. Secondary constructors must delegate to the primary constructor (directly or indirectly) using the this(...) syntax, which keeps a single source of truth for initialization.

class Employee(val name: String, var salary: Double) {
    constructor(name: String) : this(name, 40000.0)
}

fun main() {
    val defaultEmp = Employee("Sam")
    println("${defaultEmp.name} starts at ${defaultEmp.salary}")
}

• Primary constructor lives in the class header and can hold val/var parameters
• Secondary constructors use the constructor keyword inside the class body
• Every secondary constructor must call the primary constructor via this(...)
• init blocks run as part of the primary constructor's initialization sequence

With constructors in place, you're ready to explore class functions - the behavior that operates on an object's properties.