提取器 模式匹配
提取器效果与构造器相反：构造器从给定的参宿列表创建一个对象，而提取器却是从传递给它的对象中提取出构造该对象的参数
样例类
scala自动为样例类创建一个伴生对象：一个包含了apply和unappley方法的单例对象
apply方法用来创建样例类的实例，unapply需要被伴生对象实现，以使其成为提取器
第一个提取器
trait User {
def name:String
}
class FreeUser(val name:String) extends User
class PremiumUser(val name:String) extends Userobject FreeUser {
def unapply(user:FreeUser):Option[String] = Some(user, name)
}
object PremiumUser {
def unapply(user:PremiumUser):Option[String] = Some(user, name)
}//test code
val user:User = new PremiumUser("Daniel")
user match {
case FreeUser(name) => "Hello, " + name
case PremiumUser(name) => "Welcome back, " + name
}
提取多个值
trait User {
def name:String
def score:Int
}
class FreeUser(
val name:String,
val score:Int,
val upgradeProbability:Double
) extends User
class PremiumUser(
val name:String,
val score:Int
) extends Userobject FreeUser {
def unapply(user:FreeUser):Option[(String, Int, Double)] = Some((user.name, user.score, user.upgradeProbability))
}
object PremiumUser {
def unapply(user:PremiumUser):Option[(String, Int)] = Some((user.name, user.score))
}//test code
val user:User = new FreeUser("Daniel", 3000, 0.7d)
user match {
case FreeUser(name, _, p) =>
if (p > 0.75) "$name, what can we do for you today?"
else "Hello $name"
case PremiumUser(name, _) => "Welcome back, dear $name"
}
布尔提取器 用于检查是否匹配
object premiumCandidate {
def unapply(user:FreeUser):Boolean = user.upgradeProbability > 0.75
}//test code
val user:User = new FreeUser("Daniel", 2500, 0.8d)
user match {
case freeUser @ premiumCandidate() => initiateSpamProgram(freeUser)
case _ => sendRegularNewsletter(user)
}
-
@操作符
将提取器匹配成功的实例绑定到一个变量上，该变量有着与提取器所接受的对象相同的类型
val xs = 58 #:: 43 #:: 93 #:: Stream.empty
xs match {
case first #:: second #:: _ => first - second
case _ => -1
}
- 结构列表、流的方法与创建方法类似，都使用cons操作符
cons操作符：::,#:: - 列表和流的cons操作符一般使用中缀表达式
object #:: {
def unapply[A](xs: Stream[A]):Option[(A, Stream[A])] =
if (xs.isEmpty) None
else Some(xs.head, xs.tail)
}
使用提取器
- 使用样例类自动获得可用的提取器
- 只有当从无法掌控的类型中提取数据，或者是需要其他进行模式匹配的方法时，才需要实现自己的提取器
提取器的一种常见用法是从字符串中提取出有意义的值序列提取 【Scala笔记】scala提供了提取任意多个参数的模式匹配方法
接受某一类型的对象，将其解构成列表
提取给定的名字
object GivenNames {
def unapplySeq(name:String):Option[Seq[String]] = {
val names = name.trim.split(" ")
if (name.forall(_.isEmpty)) None
else Some(names)
}
}def greetWithFirstName(name:String) = name match {
case GivenNames(firstName, _*) => "Good morning, $firstName"
case _ => "Welcome! Please make sure to fill in your name"
}
固定和可变的参数提取
object Names {
def unapplySeq(name:String):Option[(String, String, Seq[String])] = {
val names = name.trim.split(" ")
if (names.size < 2) None
else Some((names.last, names.head, names.drop(1).dropRight(1)))
}
}def greet(fullName:String) = fullName match {
case Names(lastName, firstName, _*) =>
"Good morning, $firstName $lastName!"
case _ =>
"Welcome! Please make sure to fill in your name"
}
无处不在的模式 模式匹配表达式 模式匹配表达式：其返回值是由第一个匹配的模式中的代码块决定的
模式匹配允许解耦两个并不真正属于彼此的东西，使得代码易于测试
值定义中的模式
def gameResult():(String, Int) = ("Daniel", 3500)
val result = gameResult()
println(result._1 + ":" + result._2)val (name, score) = gameResult()
println()
