集合在程序中是非常有用的，只有用好集合才能真正感受到該語言的魅力。在scala中集合主要在三個包里面：scala.collection， scala.collection.immutable和scala.collection.mutable。

scala中引入不可變集合是為了方便程序的使用并減少在程序中的未知風險。如果一個集合被定義為不可變的，那么我們在使用的過程中就可以指定該集合是不會變化的，可以放心使用。

我們看下這三個包的層次結構：

scala.collection的層次結構如下：

image.png

scala.collection.immutable的層次結構如下：

image.png

scala.collection.mutable的層次結構如下：

image.png

接下來我們通過兩個HashMap的例子來看一下immutable和mutable的使用。

mutable HashMap

我們看下怎么定義一個mutable hashMap :

  import scala.collection.mutable.HashMap
  println("\nStep 1: How to initialize a HashMap with 3 elements")
  val hashMap1: HashMap[String, String] = HashMap(("PD","Plain Donut"),("SD","Strawberry Donut"),("CD","Chocolate Donut"))
  println(s"Elements of hashMap1 = $hashMap1")

  println("\nStep 2: How to initialize HashMap using key -> value notation")
  val hashMap2: HashMap[String, String] = HashMap("VD"-> "Vanilla Donut", "GD" -> "Glazed Donut")
  println(s"Elements of hashMap2 = $hashMap2")

怎么取出HashMap中的值：

  println("\nStep 3: How to access elements of HashMap by specific key")
  println(s"Element by key VD = ${hashMap2("VD")}")
  println(s"Element by key GD = ${hashMap2("GD")}")

怎么改變hashMap：

  println("\nStep 4: How to add elements to HashMap using +=")
  hashMap1 += ("KD" -> "Krispy Kreme Donut")
  println(s"Element in hashMap1 = $hashMap1")



  println("\nStep 5: How to add elements from a HashMap to an existing HashMap using ++=")
  hashMap1 ++= hashMap2
  println(s"Elements in hashMap1 = $hashMap1")



  println("\nStep 6: How to remove key and its value from HashMap using -=")
  hashMap1 -= "CD"
  println(s"HashMap without the key CD and its value = $hashMap1")

怎么定義一個空的HashMap：

  println("\nStep 7: How to initialize an empty HashMap")
  val emptyMap: HashMap[String,String] = HashMap.empty[String,String]
  println(s"Empty HashMap = $emptyMap")

immutable HashMap

看一下怎么定義一個immutable HashMap：

  import scala.collection.immutable.HashMap
  println("Step 1: How to initialize a HashMap with 3 elements using Tuples of key and value")
  val hashMap1: HashMap[String, String] = HashMap(("PD","Plain Donut"),("SD","Strawberry Donut"),("CD","Chocolate Donut"))
  println(s"Elements of hashMap1 = $hashMap1")



  println("\nStep 2: How to initialize HashMap using key -> value notation")
  val hashMap2: HashMap[String, String] = HashMap("VD"-> "Vanilla Donut", "GD" -> "Glazed Donut")
  println(s"Elements of hashMap2 = $hashMap2")

獲取HashMap中的值：

  println("\nStep 3: How to access elements in HashMap by specific key")
  println(s"Element by key VD = ${hashMap2("VD")}")
  println(s"Element by key GD = ${hashMap2("GD")}")

我們再看一下怎么對集合進行操作，注意因為是immutable HashMap所以所有的操作都會返回一個新的HashMap：

  println("\nStep 4: How to add elements to HashMap using +")
  val hashMap3: HashMap[String, String] = hashMap1 + ("KD" -> "Krispy Kreme Donut")
  println(s"Element in hashMap3 = $hashMap3")



  println("\nStep 5: How to add two HashMaps together using ++")
  val hashMap4: Map[String, String] = hashMap1 ++ hashMap2
  println(s"Elements in hashMap4 = $hashMap4")



  println("\nStep 6: How to remove key and its value from HashMap using -")
  val hashMap5: Map[String, String] = hashMap4 - ("CD")
  println(s"HashMap without the key CD and its value = $hashMap5")

更多教程請參考 flydean的博客