可变MultiMap到不可变地图 [英] Mutable MultiMap to immutable Map

问题描述

我创建一个 MultiMap

val ms =
  new collection.mutable.HashMap[String, collection.mutable.Set[String]]()
  with collection.mutable.MultiMap[String, String]

在填充条目之后，必须将其传递给一个需要 Map [String，Set [String]] 。通过 ms 直接不起作用，并尝试通过 toMap

which, after it has been populated with entries, must be passed to a function that expects a Map[String, Set[String]]. Passing ms directly doesn't work, and trying to convert it into a immutable map via toMap

ms.toMap[String, Set[String]]

Cannot prove that (String, scala.collection.mutable.Set[String]) <:< (String, Set[String]).

可以解决这个问题，而无需手动迭代 ms 并将所有条目插入到新的不可变地图中？

Can this be solved without manually iterating over ms and inserting all entries into a new immutable map?

推荐答案

似乎问题是可变的设置。所以变成不可变的集合起作用：

It seems that the problem is mutable set. So turning into immutable sets works:

scala> (ms map { x=> (x._1,x._2.toSet) }).toMap[String, Set[String]]
res5: scala.collection.immutable.Map[String,Set[String]] = Map()

或者更好的是遵循Daniel Sobral的建议：

Or even better by following Daniel Sobral suggestion:

scala> (ms mapValues { _.toSet }).toMap[String, Set[String]]
res7: scala.collection.immutable.Map[String,Set[String]] = Map()

这篇关于可变MultiMap到不可变地图的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！