Secondary constructors in Kotlin

Introduction

Secondary constructors let you create extra ways to make objects with different starting information.

When you want to create an object but have different sets of information available.

When you need to provide default values for some properties but allow others to be set.

When you want to offer simple ways to create an object without repeating code.

When the main constructor is not enough to cover all ways to create an object.

Syntax

Kotlin

class ClassName {
    constructor(param1: Type1) {
        // initialization code
    }

    constructor(param1: Type1, param2: Type2) {
        // initialization code
    }
}

Secondary constructors are inside the class body.

Each secondary constructor starts with the constructor keyword.

Examples

This class has two ways to create a Person: with just a name or with a name and age.

Kotlin

class Person {
    var name: String
    var age: Int

    constructor(name: String) {
        this.name = name
        this.age = 0
    }

    constructor(name: String, age: Int) {
        this.name = name
        this.age = age
    }
}

Box can be created as a square or a rectangle using different constructors.

Kotlin

class Box {
    var width: Int
    var height: Int

    constructor(size: Int) {
        this.width = size
        this.height = size
    }

    constructor(width: Int, height: Int) {
        this.width = width
        this.height = height
    }
}

Sample Program

This program shows two ways to create a Car object using secondary constructors. One sets a default year, the other sets a specific year.

Kotlin

class Car {
    var brand: String
    var year: Int

    constructor(brand: String) {
        this.brand = brand
        this.year = 2020
    }

    constructor(brand: String, year: Int) {
        this.brand = brand
        this.year = year
    }
}

fun main() {
    val car1 = Car("Toyota")
    val car2 = Car("Honda", 2018)

    println("Car1: ${car1.brand}, Year: ${car1.year}")
    println("Car2: ${car2.brand}, Year: ${car2.year}")
}

Important Notes

Secondary constructors can call the primary constructor using this() to avoid repeating code.

If a class has a primary constructor, secondary constructors must delegate to it either directly or indirectly.

Summary

Secondary constructors provide multiple ways to create objects with different inputs.

They help keep code clean by avoiding repeated initialization.

Use them when one constructor is not enough for your class needs.