Kotlin by lazy throws NullPointerException - kotlin

Question

I am currently trying to learn Kotlin with the help of the book "Kotlin Programming The Big Nerd Ranch Guide" and so far everything worked.
But now I am struggling with the "lazy" initialization which throws a NullPointerException which says
Cannot invoke "kotlin.Lazy.getValue()" because "< local1>" is null
The corresponding lines are:
val hometown by lazy { selectHometown() }
private fun selectHometown(): String = File("data/towns.txt").readText().split("\n").shuffled().first()
In case you want to compile it yourself or need more code for a better understanding I provide the Game.kt and Player.kt down below. If "lazy" is dropped for a "normal" initialization the hometown gets assigned as intended.
Any tips for solving the problem and understanding the cause of it is welcome.
// Game.kt
package com.bignerdranch.nyethack
fun main(args: Array<String>) {
val player = Player("Madrigal")
player.castFireball()
}
private fun printPlayerStatus(player: Player) {
println("(Aura: ${player.auraColor()}) " + "(Blessed: ${if (player.isBlessed) "YES" else "NO"})")
println("${player.name} ${player.formatHealthStatus()}")
}
// Player.kt
package com.bignerdranch.nyethack
import java.io.File
class Player(_name: String, var healthPoints: Int = 100, val isBlessed: Boolean, private val isImmortal: Boolean) {
var name = _name
get() = "${field.capitalize()} of $hometown"
private set(value) {
field = value.trim()
}
constructor(name: String) : this(name, isBlessed = true, isImmortal = false) {
if (name.toLowerCase() == "kar") healthPoints = 40
}
init {
require(healthPoints > 0, { "healthPoints must be greater than zero." })
require(name.isNotBlank(), { "Player must have a name" })
}
val hometown by lazy { selectHometown() }
private fun selectHometown(): String = File("data/towns.txt").readText().split("\n").shuffled().first()
fun castFireball(numFireballs: Int = 2) =
println("A glass of Fireball springs into existence. (x$numFireballs)")
fun auraColor(): String {
val auraVisible = isBlessed && healthPoints > 60 || isImmortal
return if (auraVisible) "GREEN" else "NONE"
}
fun formatHealthStatus() =
when (healthPoints) {
100 -> "is an excellent condition!"
in 90..99 -> "has a few scratches."
in 75..89 -> if (isBlessed) {
"has some minor wounds but is healing quite quickly"
} else {
"has some minor wounds"
}
in 15..74 -> "looks pretty hurt"
else -> "is in awful condition!"
}
}
I forgot the towns.txt so here it is (not that it matters much)
Neversummer
Abelhaven
Phandoril
Tampa
Sanorith
Trell
Zan'tro
Hermi Hermi
Curlthistle Forest

Answer 1

When something like this happens, it's usually due to bad ordering of initialization.
The initialization of the Player class goes this way:
the name property has its backing field initialized with the _name value
the init block is run, and tries to access name
the getter of name tries to read the hometown property, but fails because hometown is still not initialized
...if things had gone right, the hometown property would be initialized now with the lazy delegate
So basically you're trying to access hometown before the lazy delegate is configured.
If you move hometown's declaration above the init block, you should be fine.
You can see the fix in action on the playground