Case Classes
Scala 支持 case classes 記法。Case Class 就是普通的類, 除了:
- 默認不可變
- 可以通過模式匹配拆分
- 通過結(jié)構(gòu)相等比較而非通過引用比較
- 易于實例化和操作
下面是一個 Notification 類型等級的例子, 它由一個抽象的超類 Notification 和三個具體的用 case classes Email, SMS, VoiceRecording 實現(xiàn)的 Notification 類型。
abstract class Notification
case class Email(sourceEmail: String, title: String, body: String) extends Notification
case class SMS(sourceNumber: String, message: String) extends Notification
case class VoiceRecording(contactName: String, link: String) extends Notification
實例化一個 case class 很容易:(注意我們不需要使用 new 關(guān)鍵字)
val emailFromJohn = Email("john.doe@mail.com", "Greetings From John!", "Hello World!")
case classes 的構(gòu)造函數(shù)參數(shù)被當作公共值, 可以被直接訪問。
val title = emailFromJohn.title
println(title) // 打印 "Greetings From John!"
使用 case classes, 你不能直接改變它們的字段(field)。(除非你在字段前插入 var, 但是并不鼓勵這樣做)。
emailFromJohn.title = "Goodbye From John!" // 這會導致編譯器錯誤,我們不能將另一個值賦值給一個不可變的字段, 其中所有的 case classes 字段默認都是不可變的。
相反, 你使用 copy 方法來做一份拷貝。如下所示, 你可以替換某些字段:
val editedEmail = emailFromJohn.copy(title = "I am learning Scala!", body = "It's so cool!")
println(emailFromJohn) // prints "Email(john.doe@mail.com,Greetings From John!,Hello World!)"
println(editedEmail) // prints "Email(john.doe@mail.com,I am learning Scala,It's so cool!)"
對于每個 case classes, Scala 編譯器生成一個實現(xiàn)了結(jié)構(gòu)性相等的 equals 方法和 toString 方法。例如:
val firstSms = SMS("12345", "Hello!")
val secondSms = SMS("12345", "Hello!")
if (firstSms == secondSms) {
println("They are equal!")
}
println("SMS is: " + firstSms)
它會打印:
They are equal!
SMS is: SMS(12345, Hello!)
使用 case classes, 你可以使用 模式匹配來操作你的數(shù)據(jù)。這兒有一個函數(shù), 它根據(jù)所檢索的 Notification 的類型打印不同的信息:
def showNotification(notification: Notification): String = {
notification match {
case Email(email, title, _) => "You got an email from " + email + " with title: " + title
case SMS(number, message) => "You got an SMS from " + number + "! Message: " + message
case VoiceRecording(name, link) => "你收到一段來自" + name + "的錄音!點擊鏈接 " + link + " 收聽它"
}
}
val someSms = SMS("12345", "Are you there?")
val someVoiceRecoding = VoiceRecoding("Tom", "voicerecoding.org/id/123")
println(showNotification(someSms))
println(showNotification(someVoiceRecoding))
// 打印
/ You got an SMS from 12345! Message: Are you there?
// 你收到一段來自 Tom的語音!點擊鏈接 voicerecording.org/id/123 收聽它
下面有一個更復(fù)雜的使用 if guards 的例子。使用 if guard, 如果 guard 中的條件返回 false, 那么模式匹配分支會失敗。
def showNotificationSpecial(
notification: Notification,
specialEmail: String,
specialNumber: String): String = {
notification match {
case Email(email, _, _) if email == specialEmail => "你有一封來自特別關(guān)注人的郵件!"
case SMS(number, _) if number == specialNumber => "你有一條來自特別關(guān)注人的短信!"
case other => showNotification(other) // 沒有特殊, 代理給我們原來的 showNotification 函數(shù)
}
}
val SPECIAL_NUMBER = "55555"
val SPECIAL_EMAIL = "jane@mail.com"
val someSms = SMS("12345", "Are you there?")
val someVoiceRecoding = VoiceRecoding("Tom", "voicerecording.org/id/123")
val specialEmail = Email("jane@mail.com", "Drinks tonight?", "I'm free after 5!")
val specialSms = SMS("55555", "I'm here! Where are you?")
println(showNotificationSpecial(someSms, SPECIAL_EMAIL, SPECIAL_NUMBER))
println(showNotificationSpecial(someVoiceRecording, SPECIAL_EMAIL, SPECIAL_NUMBER))
println(showNotificationSpecial(specialEmail, SPECIAL_EMAIL, SPECIAL_NUMBER))
println(showNotificationSpecial(specialSms, SPECIAL_EMAIL, SPECIAL_NUMBER))
// 打印:
// You got an SMS from 12345! Message: Are you there?
// you received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123
// You got an email from special someone!
// You got an SMS from special someone!
當使用 Scala 編程的時候, 推薦你使用 case classes 來對數(shù)據(jù)進行建模/分組, 因為它們有助于編更具表達性和可維護性的代碼:
- 不可變性使你無需跟蹤變化的時間和地點
- 按值比較可以讓你比較實例, 就像它們是原始值一樣, 沒有更多的關(guān)于類的實例是按值還是按引用進行比較的不確定性
- 模式匹配簡化了分支的邏輯, 這使代碼不易出錯,代碼更可讀。
