Kotlin Programming: The Big Nerd Ranch Guide, First Edition

For the More Curious: Guarding Against Race Conditions

When a class property is both nullable and mutable, you must ensure that it is non-null before referencing it. For example, consider the following code that checks whether a player is wielding a weapon (since the player may have been disarmed or dropped their weapon) and, if so, prints its name:

    class Weapon(val name: String)
    class Player {
        var weapon: Weapon? = Weapon("Ebony Kris")

        fun printWeaponName() {
            if (weapon != null) {
                println(weapon.name)
            }
        }
    }

    fun main(args: Array<String>) {
       Player().printWeaponName()
    }

You may be surprised to learn that this code does not compile. Check out the error to see why (Figure 12.7):

Figure 12.7  Smart cast to ‘Weapon’ is impossible ...