Companion Objects

class Circle(val radius: Double) {
    companion object {
        const val PI_APPROX = 3.14159
    }
}

fun main() {
    println("Pi approximation: ${Circle.PI_APPROX}")
}

class User private constructor(val name: String) {
    companion object {
        fun createGuest(): User = User("Guest")
    }
}

fun main() {
    val guest = User.createGuest()
    println("User: ${guest.name}")
}

class Circle(val radius: Double) {
    companion object {
        const val UNIT = "cm"
    }

    fun describe() = "Radius: $radius $UNIT"
}

fun main() {
    val circle = Circle(5.0)
    println(circle.describe())
}

class Widget private constructor(val id: Int) {
    companion object Factory {
        fun create(id: Int): Widget = Widget(id)
    }
}

fun main() {
    val widget = Widget.Factory.create(7)
    println("Widget id: ${widget.id}")
}

1. Confusing a companion object's members with static members in Java; Kotlin has no static keyword, companion objects are the closest equivalent.
2. Forgetting that a companion object is a real singleton object, and it can implement interfaces or have its own properties beyond simple factory functions.
3. Trying to declare more than one companion object per class; only one companion object is allowed per class.

• companion object { ... } inside a class declares members that belong to the class itself, not to individual instances.
• Companion object members are accessed via ClassName.member, similar to static members in other languages.
• A companion object can be named, but if unnamed it defaults to the name Companion.
• Companion objects commonly hold factory functions, constants, or shared configuration related to the class.