case class介紹
樣例類（case class）適合用于不可變的數(shù)據(jù)。它是一種特殊的類，能夠被優(yōu)化以用于模式匹配。
case class定義
case class Book(name: String) {
def printBookName(): Unit = {
println(name)
}
}
object BookTest {
def main(args: Array[String]): Unit = {
val book = Book("Java入門到放棄")
book.printBookName()
}
}
在實(shí)例化case class類時(shí)，不需要使用關(guān)鍵字New，case class類編譯成class文件之后會(huì)自動(dòng)生成apply方法，這個(gè)方法負(fù)責(zé)對(duì)象的創(chuàng)建。
通過(guò)JD-GUI工具可以查看編譯后的.class文件（有興趣的可以自己看下）。
Scala自動(dòng)為Book生成了apply靜態(tài)方法，里面調(diào)用了Book$類的apply方法用來(lái)生成Book對(duì)象。
Book$類的截圖
case class類的參數(shù)都是可以直接訪問(wèn)的val（不能被修改），但是實(shí)際上編譯成的class字節(jié)碼會(huì)對(duì)book.name轉(zhuǎn)成book.name()方法調(diào)用。如下圖所示，name聲明的時(shí)候是加了final關(guān)鍵字，并且生成了對(duì)應(yīng)的name()方法。
printBookName()方法中使用到的book.name實(shí)際上是調(diào)用的name()方法。
模式匹配
模式匹配是檢查某個(gè)值（value）是否匹配某一個(gè)模式的機(jī)制，一個(gè)成功的匹配同時(shí)會(huì)將匹配值解構(gòu)為其組成部分。它是Java中的switch語(yǔ)句的升級(jí)版。
語(yǔ)法
一個(gè)模式匹配語(yǔ)句包括一個(gè)待匹配的值，match關(guān)鍵字，以及至少一個(gè)case語(yǔ)句。示例如下：
def matchTest(x: Int): String = x match {
case 1 => "one"
case 2 => "two"
case _ => "many"
}
case class的匹配
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
Notification 是一個(gè)虛基類，它有三個(gè)具體的子類Email, SMS和VoiceRecording，我們可以在這些Case Class類上使用模式匹配：
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! Link: $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
showNotification函數(shù)接受一個(gè)抽象類Notification對(duì)象作為輸入?yún)?shù)，然后匹配其具體類型。（也就是判斷它是一個(gè)Email，SMS，還是VoiceRecording）。在case Email(email, title, _ )中，對(duì)象的email和title屬性在返回值中被使用，而body屬性則被忽略，故使用_代替。
另外需要注意的一點(diǎn)是case Email(email, title, _)語(yǔ)句實(shí)際上是使用了Email提取器對(duì)象的unApply方法，這個(gè)方法也是Scala編譯字節(jié)碼的時(shí)候自動(dòng)生成的，它會(huì)去提取匹配到的Email對(duì)象的sender和title屬性填充到email和title屬性上。
showNotification方法還可以等價(jià)寫成下面這種形式，只匹配類型，而不使用Scala的提取器方式。
def showNotification(notification: Notification): String = {
notification match {
case e: Email =>
s"You got an email from ${e.sender} with title: ${e.title}"
case s: SMS =>
s"You got an SMS from ${s.caller}! Message: ${s.message}"
case vr: VoiceRecording =>
s"you received a Voice Recording from ${vr.contactName}! Link: ${vr.link}"
}
}
模式守衛(wèi)
為了讓匹配更加具體，可以使用模式守衛(wèi)，也就是在模式后面加上if表達(dá)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", "vr.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)) // You got an SMS from special someone!
println(showImportantNotification(someVoiceRecording, importantPeopleInfo)) //you received a Voice Recording from Tom! Link: vr.org/id/123
println(showImportantNotification(importantEmail, importantPeopleInfo)) // You got an email from special someone!
println(showImportantNotification(importantSms, importantPeopleInfo)) //You got an SMS from special someone!
在case Email(email, _ , _ ) if importantPeopleInfo.contains(email)中，除了要求notification是Email類型外，還需要email在重要人物列表importantPeopleInfo中，才會(huì)匹配到該模式。
密封類
特質(zhì)（trait）和類（class）可以用sealed標(biāo)記為密封的，這意味著其所有子類都必須與之定義在相同文件中，從而保證所有子類型都是已知的。
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"
}
