Classes, Data Classes, and Objects

Classes with a primary constructor

class Student(val name: String, var xp: Int = 0) {
    fun gainXp(amount: Int) {
        xp += amount
    }
}

val ada = Student("Ada")
ada.gainXp(50)
println(ada.xp) // 50

Declaring val name: String directly inside the class header does three things at once: declares a constructor parameter, creates a property, and generates its getter (and a setter too, for var properties), all without writing a single line of boilerplate. Compare that to Java's Methods and Classes lesson, where the constructor, field, and getter each had to be written out by hand.

Data classes

When a class exists mainly to hold data, data class generates equals(), hashCode(), toString(), and a handy copy() for you:

data class Point(val x: Int, val y: Int)

val p1 = Point(1, 2)
val p2 = Point(1, 2)

println(p1 == p2)        // true, structural equality, not reference equality
println(p1)                // Point(x=1, y=2)
val p3 = p1.copy(y = 5)  // Point(x=1, y=5), everything else copied as-is

p1 == p2 is true even though they're two separate objects, because data class compares by value, not by memory reference, exactly what you want for simple data-holding types.

object and companion object

object declares a class with exactly one instance, a singleton, created lazily the first time it's used:

object Config {
    val maxRetries = 3
}

println(Config.maxRetries)

A companion object attaches shared, class-level members to a regular class, similar to static in Java:

class User(val id: Int) {
    companion object {
        fun createGuest() = User(0)
    }
}

val guest = User.createGuest()