映射中列表类型的Kotlin可空接收器

在Kotlin中，从map访问值会以不同的方式提供不同的接收器类型

映射的类型为LinkedHashMap类型

如果我不在此行中为空判断

println("Morning: ${daypartEvents[Daypart.MORNING].size}")个

编译器给出了错误

List类型的可空接收器上只允许安全(？.)或非空的断言(！！.)调用.

但是，如果try 通过使用forEach函数迭代映射来实现相同的结果，则如下所示

daypartEvents.forEach { (daypart, events) -> println("$daypart: ${events.size}") }个

它不想让我对它进行空判断.

即使我做了空判断，它也会说

对List类型的非null接收器进行不必要的安全调用

我知道这个问题表达得很糟糕，所以请原谅我

这里我想知道的是，为什么List类型的映射值一开始是可以为空的，之后是不可为空的.

为清晰起见，完整代码

enum class Daypart {
    MORNING, AFTERNOON, EVENING
}

data class Event (
    val title: String,
    val description: String? = null,
    val daypart: Daypart,
    val duration: Int
)

fun main() {
    
    val event1 = Event(title = "Wake up", description = "Time to get up", daypart = Daypart.MORNING, duration = 0)
    val event2 = Event(title = "Eat breakfast", daypart = Daypart.MORNING, duration = 15)
    val event3 = Event(title = "Learn about Kotlin", daypart = Daypart.AFTERNOON, duration = 30)
    val event4 = Event(title = "Practice Compose", daypart = Daypart.AFTERNOON, duration = 60)
    val event5 = Event(title = "Watch latest DevBytes video", daypart = Daypart.AFTERNOON, duration = 10)
    val event6 = Event(title = "Check out latest Android Jetpack library", daypart = Daypart.EVENING, duration = 45)
    
    val events = mutableListOf<Event>(event1, event2, event3, event4, event5, event6)
    
    
    val daypartEvents = events.groupBy {
        it.daypart
    }
    
    print(daypartEvents::class.simpleName)
    
    
    println("Morning: ${daypartEvents[Daypart.MORNING]?.size}")
    
    daypartEvents.forEach { (daypart, events) ->
        println("$daypart: ${events.size}")
    }
    
}

我对Kotlin还是个新手，这是Developer.android.com上Android Jetpack Compose for Beginners课程中的练习之一