1. 模式匹配

1.1. 更好的 Switch

Scala 中类似 Java 的 switch 代码：

object PatternDemo {

  def main(args: Array[String]): Unit = {
    var sign = 0
    val ch: Char  = 'p'
    val valchar = 'p'
    var digit = 0

   //match 是表达式
    ch match {
      case '+' => sign = 1
      case '-' => sign = -1
      //使用|分割多个选项
      case '*' | 'x' => sign = 2
      //可以使用变量
      case valchar => sign = 3
      //case _ 类似Java中的default
      // 如果没有模式能匹配，会抛出MacthError
      //可以给模式添加守卫
      case _ if Character.isDigit(ch) => digit = Character.digit(ch, 10)
    }
    println("sign = "+ sign)
  }

}

1.2. 样例类匹配

//定义样例类
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

//基于样例类的模式匹配
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))  //结果：You got an SMS from 12345! Message: Are you there?
println(showNotification(someVoiceRecording))  //结果：you received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123

1.3. 带守卫的模式

增加布尔表达式使得匹配更具体。

def showImportantNotification(notification: Notification, importantPeopleInfo: Seq[String]): String = {
  notification match {
  //仅匹配email在importantPeople列表里的内容
    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", "[email protected]")

val someSms = SMS("867-5309", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")
val importantEmail = Email("[email protected]", "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))

1.4. 类型匹配

val arr = Array("hello", 1, 2.0, 'a')
val obj = arr(Random.nextInt(4))
obj match {
  case x: Int => x
  case s: String => Integer.parseInt(s)
  case _: BigInt => Int.MaxValue
  case _ => 0
}

1.5. 匹配数组、列表、元组

数组匹配

val arr1 = Array(1,1)
  val res = arr1 match {
  case Array(0) => "0"
  //匹配包含0的数组
  case Array(x, y) => s"$x $y"
  // 匹配任何带有两个元素的数组，并将元素绑定到x和y
  case Array(0, _*) => "0..."
  //匹配任何以0开始的数组
  case _ => "something else"
}

列表匹配

val lst = List(1,2)
val res2 =  list match {
   case 0 :: Nil => "0"
   case x :: y :: Nil => x + " " + y
   case 0 :: tail => "0 ..."
   case _ => "something else"
 }

元组匹配

var pair = (1,2)
val res3 =  pair match {
  case (0, _)  => "0 ..."
  case (y, 0) => s"$y 0"
  case _ => "neither is 0"
}

2. Sealed 类 (密封类)

Scala 中，Traits 和 class 可以被关键字 Sealed 修饰，被该关键字修饰后，它所有的子类都必须在同一文件中被定义。

sealed abstract class Furniture
case class Couch() extends Furniture
case class Chair() extends Furniture
//此时无需定义能匹配所有的类型了
def findPlaceToSit(piece: Furniture): String = piece match {
  case a: Couch => "Lie on the couch"
  case b: Chair => "Sit on the chair"
}

3. 样例类

在 Scala 中样例类是一中特殊的类，样例类是不可变的，可以通过值进行比较，可用于模式匹配。
定义一个样例类：

case class Point(x: Int, y: Int)

创建样例类对象：

val point = Point(1, 2)
val anotherPoint = Point(1, 2)
val yetAnotherPoint = Point(2, 2)

通过值对样例类对象进行比较：

if (point == anotherPoint) {
  println(point + " and " + anotherPoint + " are the same.")
} else {
  println(point + " and " + anotherPoint + " are different.")
}
// Point(1,2) 和 Point(1,2)一样的.

if (point == yetAnotherPoint) {
  println(point + " and " + yetAnotherPoint + " are the same.")
} else {
  println(point + " and " + yetAnotherPoint + " are different.")
}
// Point(1,2)和Point(2,2)是不同的.

在模式匹配中使用样例类：

abstract class Amount
// 继承了普通类的两个样例类
case class Dollar(value: Double) extends Amount
case class Currency(value: Double, unit: String) extends Amount
case object Nothing extends Amount
object CaseClassDemo {

  def main(args: Array[String]): Unit = {
    val amt = new Dollar(10);
    patternMatch(amt)
  }
  def patternMatch(amt: Amount) {
    amt match {
      case Dollar(v) => println("$" + v)
      case Currency(_, u) => println("Oh noes, I got " + u)
      case Nothing => println("nothing") //样例对象没有()
    }
  }
}

声明样例类，以下几件事会自动发生：

(1) 构造器中每一个参数都是 val，除非显示地声明为 var
(2) 伴生对象提供 apply ，让你不使用 new 关键字就能构造出相应的对象
(3) 提供 unapply 方法，让模式匹配可以工作
(4) 生成 toString，equals，hashCode，copy 方法，除非显示给出这些方法的定义。

4. Option 类型

在 Scala 中 Option 类型样例类用来表示可能存在或也可能不存在的值( Option 的子类有 Some 和 None )。Some 包装了某个值，None 表示没有值。

object OptionDemo {
  def main(args: Array[String]) {
    val map = Map("a" -> 1, "b" -> 2)
    val v = map.get("b") match {
      case Some(i) => i
      case None => 0
    }
    println(v)
    //更好的方式
    val v1 = map.getOrElse("c", 0)
    println(v1)
  }
}

5. 偏函数

被包在花括号内没有 match 的一组 case 语句是一个偏函数，它是 PartialFunction[A, B] 的一个实例，A 代表参数类型，B 代表返回类型，常用作输入模式匹配。

object PartialFunctionDemo {
  def f: PartialFunction[String, Int] = {
    case "one" => 1
    case "two" => 2
   // case _ => -1
  }

  def main(args: Array[String]) {
    //调用f.apply("one")
    println(f("one"))
    println(f.isDefinedAt("three"))
    //抛出MatchError
    println(f("three"))
  }
}

Scala之模式匹配和样例类