Scala学习 -- 集合 Map

　　Scala的集合类Map，跟集(Set) 类似，Scala提供了Map的可变和不可变的版本，用类的继承关系来区分。在 scala.collection包里面有一个基础的 Map 特质，还有两个子特质，都叫Map，可变的那个位于 scala.collection.mutable，而不可变的那个位于 scala.collection.immutable。

　　创建一个 Map：

1、scala> var D: Map[Char, List[String]] = Map()
D: Map[Char,List[String]] = Map()

2、scala> import scala.collection.mutable
import scala.collection.mutable
scala> val treasureMap = mutable.Map[Int, List[String]]()
treasureMap: scala.collection.mutable.Map[Int,List[String]] = Map()

　　通过 -> 和 += 的方法向 Map 里面添加键值对(key/value pair)：

scala> list
res108: List[String] = List(Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday)

scala> treasureMap += (1 -> list)
res107: treasureMap.type = Map(1 -> List(Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday))

　　通过 -= 的方法将 Map 中对应的元素删除：

scala> D('a')
res114: List[String] = List(a, b, c)

scala> D -= ('a')

scala> D
res116: Map[Char,List[String]] = Map()

　　获得 Map 中 key 对应的value：

scala> treasureMap
res109: scala.collection.mutable.Map[Int,List[String]] = Map(1 -> List(Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday))

scala> treasureMap(1)
res110: List[String] = List(Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday)

　　如果你更倾向于使用不可变的映射，则不需要任何引入，因为默认的映射就是不可变的

scala> val romanNumeral = Map(1 -> "A", 2 -> "B", 3 -> "C", 4 -> "D", 5 -> "E")
romanNumeral: scala.collection.immutable.Map[Int,String] = Map(5 -> E, 1 -> A, 2 -> B, 3 -> C, 4 -> D)

scala> romanNumeral(2)
res0: String = B

scala> romanNumeral += (6 -> "F")
<console>:26: error: value += is not a member of scala.collection.immutable.Map[Int,String]
  Expression does not convert to assignment because receiver is not assignable.
       romanNumeral += (6 -> "F")
                    ^

　　Scala的 Map中 如果 key 相同的话，在做 += 操作的时候 key 所对应的 value 会被覆盖成新的元素

scala> D
res3: Map[Char,List[String]] = Map()

scala> D += ('a' -> a);

scala> D
res5: Map[Char,List[String]] = Map(a -> List(a, b, c))

scala> D += ('a' -> list);

scala> D
res7: Map[Char,List[String]] = Map(a -> List(Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday))