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scala_case.md

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Case

在Scala中不存在Switch语句，但这里存在比Switch更强大的Case

Match Case

匹配值相等

import scala.util.Random

val x: Int = Random.nextInt(10)

x match {
  case 0 => "zero"
  case 1 => "one"
  case 2 => "two"
  case _ => "many"
}

_是可选的。类似于 Switch 中的 default。当上方的场景都没有相应的匹配项的时候，就执行该语句。

match case其执行顺序是从上往下执行，直到找到匹配项之后，执行完全，退出。

def matchTest(x: Int): String = x match {
  case 1 => "one"
  case 2 => "two"
  case _ => "many"
}
matchTest(3)  // many
matchTest(1)  // one

可以像if语句那样结果赋值给变量或者构成方法。

Case Class

abstract class Notification

case class Email(sender: String, title: String, body: String) extends Notification

case class SMS(caller: String, message: String) extends Notification

case class VoiceRecording(contactName: String, link: String) extends Notification

def showNotification(notification: Notification): String = {
  notification match {
    case Email(email, title, _) =>
      s"You got an email from $email with title: $title"
    case SMS(number, message) =>
      s"You got an SMS from $number! Message: $message"
    case VoiceRecording(name, link) =>
      s"you received a Voice Recording from $name! Click the link to hear it: $link"
  }
}
val someSms = SMS("12345", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")

println(showNotification(someSms))  // prints You got an SMS from 12345! Message: Are you there?

println(showNotification(someVoiceRecording))  // you received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123

匹配Class类型，这里Email、SMS、VoiceRecording都继承于Notification，当case匹配到对象实例相匹配的类型时，就执行与之相匹配的代码，同时会将构造参数进行填充。

还可以在类匹配之后，还可以使用boolean表达式

def showImportantNotification(notification: Notification, importantPeopleInfo: Seq[String]): String = {
  notification match {
    case Email(email, _, _) if importantPeopleInfo.contains(email) =>
      "You got an email from special someone!"
    case SMS(number, _) if importantPeopleInfo.contains(number) =>
      "You got an SMS from special someone!"
    case other =>
      showNotification(other) // nothing special, delegate to our original showNotification function
  }
}

val importantPeopleInfo = Seq("867-5309", "jenny@gmail.com")

val someSms = SMS("867-5309", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")
val importantEmail = Email("jenny@gmail.com", "Drinks tonight?", "I'm free after 5!")
val importantSms = SMS("867-5309", "I'm here! Where are you?")

println(showImportantNotification(someSms, importantPeopleInfo))
println(showImportantNotification(someVoiceRecording, importantPeopleInfo))
println(showImportantNotification(importantEmail, importantPeopleInfo))
println(showImportantNotification(importantSms, importantPeopleInfo))

类型匹配

abstract class Device
case class Phone(model: String) extends Device{
  def screenOff = "Turning screen off"
}
case class Computer(model: String) extends Device {
  def screenSaverOn = "Turning screen saver on..."
}

def goIdle(device: Device) = device match {
  case p: Phone => p.screenOff
  case c: Computer => c.screenSaverOn
}

被Sealed标记的类

sealed abstract class Furniture
case class Couch() extends Furniture
case class Chair() extends Furniture

def findPlaceToSit(piece: Furniture): String = piece match {
  case a: Couch => "Lie on the couch"
  case b: Chair => "Sit on the chair"
}

sealed 关键字主要有2个作用：

  1. 其修饰的trait，class只能在当前文件里面被继承；
  2. 在检查模式匹配的时候，用sealed修饰目的是让scala知道这些case的所有情况，scala就能够在编译的时候进行检查，看你写的代码是否有没有漏掉什么没case到，减少编程的错误。