懒惰的 Kotlin 抛出 NullPointerException

Question

我目前正在尝试借助“Kotlin Programming The Big Nerd Ranch Guide”一书来学习 Kotlin，到目前为止一切正常。但是现在我正在为“懒惰”的初始化而苦苦挣扎，它会抛出一个 NullPointerException，它说

Cannot invoke "kotlin.Lazy.getValue()" because "< local1>" is null

对应的行是:

val hometown by lazy { selectHometown() } 
private fun selectHometown(): String = File("data/towns.txt").readText().split("\n").shuffled().first()

如果您想自己编译或需要更多代码以便更好地理解，我会在下面提供 Game.kt 和 Player.kt。如果为“正常”初始化而放弃“懒惰”，则家乡将按预期分配。欢迎提供任何解决问题和了解问题原因的提示。

// 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!"
        }

}

我忘记了 towns.txt 所以就在这里(不是很重要)

Neversummer
Abelhaven
Phandoril
Tampa
Sanorith
Trell
Zan'tro
Hermi Hermi
Curlthistle Forest

Answer 1

当发生这种情况时，通常是由于初始化顺序错误。

Player类的初始化是这样进行的:

1. name 属性的支持字段使用 _name 值初始化
2. 运行 init block ，并尝试访问 name
3. name 的 getter 尝试读取 hometown 属性，但由于 hometown 仍未初始化而失败
4. ...如果一切顺利，hometown 属性现在将使用惰性委托(delegate)进行初始化

所以基本上你是在配置惰性委托(delegate)之前尝试访问 hometown 。如果你把 hometown 的声明移到 init block 上面，应该没问题。

您可以看到修复的实际效果 on the playground