将元素添加到scala.collection.mutable.Map的语法是什么？

Question

将元素添加到scala.collection.mutable.Map的语法是什么？

以下是一些失败的尝试:

val map = scala.collection.mutable.Map

map("mykey") = "myval"

map += "mykey" -> "myval"

map.put("mykey","myval")

Answer 1

关键是您的代码的第一行不是您所期望的.你应该用

val map = scala.collection.mutable.Map[A,B]()

要么

scala> val map = scala.collection.mutable.Map[String,String]()
map: scala.collection.mutable.Map[String,String] = Map()


scala> map("k1") = "v1"

scala> map
res1: scala.collection.mutable.Map[String,String] = Map((k1,v1))


scala> map += "k2" -> "v2"
res2: map.type = Map((k1,v1), (k2,v2))


scala> map.put("k3", "v3")
res3: Option[String] = None

scala> map
res4: scala.collection.mutable.Map[String,String] = Map((k3,v3), (k1,v1), (k2,v2))

代替.

scala> map.addOne("k4" -> "v4")
res5: map.type = HashMap(k1 -> v1, k2 -> v2, k3 -> v3, k4 -> v4)

Answer 2

一如既往,你应该质疑你是否真的需要一个可变的地图.

构建不可变的地图是微不足道的:

val map = Map(
  "mykey" -> "myval",
  "myotherkey" -> "otherval"
)

首次构建时,可变映射没有区别:

val map = collection.mutable.Map(
  "mykey" -> "myval",
  "myotherkey" -> "otherval"
)

map += "nextkey" -> "nextval"

在这两种情况下,推理都将用于确定Map实例的正确类型参数.

您还可以在a中保存不可变映射,var然后每次执行"更新"时,都会使用新的不可变映射实例更新变量

var map = Map(
  "mykey" -> "myval",
  "myotherkey" -> "otherval"
)

map += "nextkey" -> "nextval"

如果您没有任何初始值,则可以使用Map.empty:

val map : Map[String, String] = Map.empty //immutable
val map = Map.empty[String,String] //immutable
val map = collection.mutable.Map.empty[String,String] //mutable

Answer 3

当你说

val map = scala.collection.mutable.Map

您不是在创建地图实例,而是使用Map类型别名.

map: collection.mutable.Map.type = scala.collection.mutable.Map$@fae93e

请尝试以下方法:

scala> val map = scala.collection.mutable.Map[String, Int]()
map: scala.collection.mutable.Map[String,Int] = Map()

scala> map("asdf") = 9

scala> map
res6: scala.collection.mutable.Map[String,Int] = Map((asdf,9))