Scala实践10

1、模式匹配

模式匹配是一种根据模式检查值的机制。它是switch（Java中语句）的更强大版本，它同样可以用来代替一系列if / else语句。

• 句法

匹配表达式具有值，match关键字和至少一个case子句。

import scala.util.Random

val x: Int = Random.nextInt(10)

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

  val x上面是0和10之间的随机整数，x成为的左操作数match运算符和右边是与4箱子的表达式。最后一种情况（_）是任何其他可能Int值的“全部捕获”情况。案件也被称为替代品。

• 匹配案例类

案例类对模式匹配特别有用。

abstract class Notification//抽象超类

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

　　Notification是具有与壳体的类实现的三个具体的通知类型的抽象超类Email，SMS和VoiceRecording。现在我们可以对这些案例类进行模式匹配：

def showNotification(notification: Notification): String = {
  notification match {
    case Email(sender, title, _) =>
      s"You got an email from $sender 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(someSmps))  

println(showNotification(someVoiceRecording))

　　该函数showNotification作为参数的抽象类型Notification和类型相匹配Notification（即，它计算出它是否是一个Email，SMS或VoiceRecording）。在case Email(sender, title, _)字段中sender并且title在返回值中使用但是body忽略该字段_。

• 模式守护

模式保护只是布尔表达式，用于使案例更具体。只需if <boolean expression>在模式后添加。

def showImportantNotification(notification: Notification, importantPeopleInfo: Seq[String]): String = {
  notification match {
    case Email(sender, _, _) if importantPeopleInfo.contains(sender) =>
      "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)
  }
}

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))

　　在图中case Email(sender, _, _) if importantPeopleInfo.contains(sender)，只有sender在重要人物列表中才匹配模式。

• 仅在类型上匹配

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
}

　　def goIdle具有不同的行为取决于类型Device。当案例需要在模式上调用方法时，这很有用。它是使用类型的情况下标识（第一个字母的公约p和c在这种情况下）。

• 密封课程

可以标记特征和类sealed，这意味着必须在同一文件中声明所有子类型。这确保了所有亚型都是已知的。

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"
}

　　

2、案例类

• 定义案例类

最小的案例类需要关键字case class，标识符和参数列表（可能为空）：

case class Book(isbn: String)

val frankenstein = Book("912-0111182114")

　　注意：实例化Book案例时没有new关键字。这是因为case类apply默认有一个方法来处理对象构造。

使用参数创建案例类时，参数是公共val的。

case class Message(sender: String, recipient: String, body: String)
val message1 = Message("guillaume@quebec.ca", "jorge@catalonia.es", "Ça va ?")

println(message1.sender)  //合法的
message1.sender = "travis@washington.us"  // 不合法的

　　不能重新分配，message1.sender因为它是一个val（即不可变的）。