从 Map::getOrElse 返回的 Kotlin mutableList() 不公开 add()

Question

我有

data class Person(var name: String, var hobbies: List<String>)

val people = mutableListOf(
        Person("joe", asList("biking", "baking")),
        Person("jane", asList("baking")),
        Person("jack", asList("painting"))
)
val map = mutableMapOf<String, List<String>>()

for ((name, hobbies) in people) {

    for (hobby in hobbies) {

        var peopleWithThisHobby = map.getOrElse(hobby, { mutableListOf() })
        peopleWithThisHobby.plus(name)
        map[hobby] = peopleWithThisHobby
    }
}

println(map)

在 peopleWithThisHobby.plus(name) 行，IntelliJ 自动完成没有给我 add() 方法。

我必须改用:

var peopleWithThisHobby = map.getOrElse(hobby, { mutableListOf() })
peopleWithThisHobby = peopleWithThisHobby.plus(name)
map[hobby] = peopleWithThisHobby

什么给了？这是设计使然吗？从 getOrElse 调用中返回的 mutableList() 发生了什么？

我使用的是 Kotlin 插件版本:1.1.3-eap-85-IJ2017.2-1

Answer 1

那是因为你的 map 定义有一个 List 而不是 MutableList 作为值。将其更改为 MutableList 即可。

val map = mutableMapOf<String, List<String>>()

将上面的改成:

val map = mutableMapOf<String, MutableList<String>>()