LESSON 287 OF 677 · FUNCTIONS & CLASSES

Constructors in Kotlin

Lesson

TL ; DR:

A constructor is a special method that runs automatically when you create an object. It's used to set up the object's initial state.

In Kotlin, a constructor sets up the object's fields when it is created:

class Dog {
    var name: String
    var age: Int

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

Then create objects with arguments: Dog("Rex", 4) — the constructor fills in the fields for you.

Full lesson:

In the previous lesson you set fields after creating the object:

val dog = Dog()
dog.name = "Rex"
dog.age = 4

That works, but it's noisy and easy to forget. Constructors let you initialize the fields the moment Dog(...) is called.

Defining a constructor

A Kotlin constructor has two distinguishing features:

It is declared with the constructor keyword inside the class.
It has no return type — not even Unit.

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

The this keyword

The parameter name and the field name have the same identifier. Without this, both sides of name = name would refer to the parameter (the local one wins). this is a built-in reference to "the current object", so this.name is the field, and bare name is the parameter.

Calling the constructor

val rex  = Dog("Rex", 4)
val luna = Dog("Luna", 7)

By the time these lines finish, rex.name is already "Rex", rex.age is 4, and the same for Luna.

The default constructor

If you don't write any constructor, Kotlin provides a hidden no-arg one for free — that's why Dog() worked in earlier lessons. The moment you define your own constructor, that freebie disappears: from now on you must call Dog with the arguments you specified.

Primary constructors:

The form above is a secondary constructor. In everyday Kotlin you will more often see the primary constructor, declared right in the class header. Marking a parameter val (or var) there also creates the property — it declares, stores, and exposes the value in one line:

class Dog(val name: String, var age: Int)

val rex = Dog("Rex", 4)
println(rex.name) // Rex

That single line does the same work as the field declarations plus the constructor block above.

Your task

Declare the Dog class with a primary constructor that takes var name: String and var age: Int, so no this assignments are needed. Then in mainFunction, create two dogs:

val rex  = Dog("Rex", 4)
val luna = Dog("Luna", 7)

Expected output:

Rex is 4
Luna is 7

STEP 1 OF 2

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