Primary Constructors

Part of the Object Oriented Programming section of Coddy's Kotlin journey. Lesson 3 of 57.

Most Kotlin classes take their starting values in a primary constructor, written in parentheses after the class name. val or var in front of a parameter makes it a property at the same time:

class Book(val title: String, var pages: Int)

Inside main:

val book = Book("Dune", 412)
println(book.title)
book.pages += 8
println(book.pages)

Output:

Dune
420

A parameter without val or var is only available while the instance is being created, for example to compute a property. val properties cannot be reassigned after creation; var properties can:

class Temperature(celsius: Double) {
    val fahrenheit = celsius * 9 / 5 + 32
}

Inside main:

val t = Temperature(100.0)
println(t.fahrenheit)

Output:

212.0

Constructor parameters can have default values and can be passed by name, so one constructor covers many ways of creating an instance:

class Timer(val seconds: Int, val label: String = "timer")

Inside main:

val egg = Timer(300, "egg")
val quick = Timer(30)
val named = Timer(label = "tea", seconds = 180)
println("${egg.label} ${egg.seconds}")
println("${quick.label} ${quick.seconds}")
println("${named.label} ${named.seconds}")

Output:

egg 300
timer 30
tea 180

Inside the class body, this refers to the current instance. It is needed when a method parameter has the same name as a property:

class Player(var name: String) {
    fun rename(name: String) {
        this.name = name
    }
}

Inside main:

val p = Player("anon")
p.rename("Ada")
println(p.name)

Output:

Ada

Challenge

Easy

Declare the class Account with a primary constructor: a read-only owner and a changeable balance that defaults to 0. Add summary(), which returns Ada has 150.

The supplied code reads an owner and, when a second line is present, a starting balance. It creates the account with or without the balance and prints summary().

Your code goes in Account.kt. Main.kt holds the supplied input/output code and cannot be edited.

Try it yourself

fun main() {
    // Supplied input/output code: keep it as it is
    val input = generateSequence(::readLine).toList()
    val account = if (input.size > 1) Account(input[0], input[1].toInt()) else Account(input[0])
    println(account.summary())
}