scala 将可变映射转换为不可变映射
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Converting mutable to immutable map
提问by Jeriho
private[this]object MMMap extends HashMap[A, Set[B]] with MultiMap[A, B]
How convert it to immutable?
如何将其转换为不可变的？
采纳答案by Rex Kerr
The immutable hierarchy doesn't contain a MultiMap, so you won't be able to use the converted structure with the same convenient syntax. But if you're happy to deal with key/valueset pairs, then:
不可变层次结构不包含 MultiMap，因此您将无法使用具有相同方便语法的转换结构。但是，如果您乐于处理键/值集对，那么：
If you just want a mutable HashMap, you can just use x.toMapin 2.8 or collection.immutable.Map(x.toList: _*)in 2.7.
如果你只想要一个 mutable HashMap，你可以x.toMap在 2.8 或collection.immutable.Map(x.toList: _*)2.7 中使用。
But if you want the whole structure to be immutable--including the underlying set!--then you have to do more: you need to convert the sets along the way. In 2.8:
但是如果你希望整个结构是不可变的——包括底层的集合！——那么你必须做更多的事情：你需要一路转换集合。在 2.8 中：
x.map(kv => (kv._1,kv._2.toSet)).toMap
In 2.7:
在 2.7：
collection.immutable.Map(
x.map(kv => (kv._1,collection.immutable.Set(kv._2.toList: _*))).toList: _*
)
回答by michael.kebe
scala> val mutableMap = new HashMap[Int, String]
mutableMap: scala.collection.mutable.HashMap[Int,String] = Map()
scala> mutableMap += 1 -> "a"
res5: mutableMap.type = Map((1,a))
scala> mutableMap
res6: scala.collection.mutable.HashMap[Int,String] = Map((1,a))
scala> val immutableMap = mutableMap.toMap
immutableMap: scala.collection.immutable.Map[Int,String] = Map((1,a))
scala> immutableMap += 2 -> "b"
<console>:11: error: reassignment to val
immutableMap += 2 -> "b"
^
回答by Fahad Siddiqui
You can use myMap.toMapto convert an a mutable map into immutable in Scala 2.8 and later versions.
myMap.toMap在 Scala 2.8 及更高版本中，您可以使用将可变映射转换为不可变映射。
Looking at definition of toMapfrom documentation:
查看toMap文档中的定义：
def toMap[T, U](implicit ev: A <:< (T, U)): immutable.Map[T, U] = {
val b = immutable.Map.newBuilder[T, U]
for (x <- self) b += x
b.result
}
回答by Harish Gajapathy
You can just to the following
你可以只到以下
val imm_map = MMMap.toMap
