文章目錄

• • mutable HashMap
  • immutable HashMap

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(yǔ)言的魅力。在scala中集合主要在三個(gè)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(fēng)險(xiǎn)。如果一個(gè)集合被定義為不可變的，那么我們?cè)谑褂玫倪^(guò)程中就可以指定該集合是不會(huì)變化的，可以放心使用。

我們看下這三個(gè)包的層次結(jié)構(gòu)：

scala.collection的層次結(jié)構(gòu)如下：

scala.collection.immutable的層次結(jié)構(gòu)如下：

scala.collection.mutable的層次結(jié)構(gòu)如下：

接下來(lái)我們通過(guò)兩個(gè)HashMap的例子來(lái)看一下immutable和mutable的使用。

mutable HashMap

我們看下怎么定義一個(gè)mutable hashMap :

import scala.collection.mutable.HashMapprintln("\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 ++= hashMap2println(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(gè)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(gè)immutable HashMap：

import scala.collection.immutable.HashMapprintln("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?cè)倏匆幌略趺磳?duì)集合進(jìn)行操作，注意因?yàn)槭莍mmutable HashMap所以所有的操作都會(huì)返回一個(gè)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 ++ hashMap2println(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")

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更多教程請(qǐng)參考 flydean的博客

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