Scala中Case Class使用詳細解析
簡介
Case Class是樣例類,能夠被優化以用於模式匹配。
- 構造器中的引數如果不被宣告為var的話,預設是val型別
- 自動建立伴生物件,實現apply方法,可以不直接顯示地new物件
- 伴生物件實現unapply方法,從而可以將case class應用於模式匹配
- 實現toString、hashCode、copy、equals方法
簡單例項
``` //抽象類Person abstract class Person
//case class Student case class Student(name:String,age:Int,studentNo:Int) extends Person //case class Teacher case class Teacher(name:String,age:Int,teacherNo:Int) extends Person //case class Nobody case class Nobody(name:String) extends Person
object CaseClassDemo{ def main(args: Array[String]): Unit = { //case class 會自動生成apply方法,從而省去new操作 val p:Person=Student("john",18,1024)
//match case 匹配語法
p match { case Student(name,age,studentNo)=>println(name+":"+age+":"+studentNo) case Teacher(name,age,teacherNo)=>println(name+":"+age+":"+teacherNo) case Nobody(name)=>println(name) } } } ```
常用方法
``` //toString方法演示 scala> val s=Teacher("john",38,1024) s: Teacher = Teacher(john,38,1024)
//無參copy方法演示 scala> val s1=s.copy() s1: Teacher = Teacher(john,38,1024)
//copy方法是深度拷貝 scala> println(s eq s1) false
//equal方法根據物件內容進行比較 scala> println(s equals s1) true
scala> println(s == s1) true
//hashcode方法 scala> s1.hashCode res45: Int = 567742485
//toString方法 scala> s1.toString res46: String = Teacher(john,38,1024)
//帶一個引數的copy方法 scala> s1.copy(name="stephen") res47: Teacher = Teacher(stephen,38,1024) //帶二個引數的copy方法 scala> s1.copy(name="stephen",age=58) res49: Teacher = Teacher(stephen,58,1024) //帶三個引數的copy方法 scala> s1.copy(name="stephen",age=58,teacherNo=2015) res50: Teacher = Teacher(stephen,58,2015) ```
多引數的Case Class
``` abstract class Person
case class Student( name:String, age:Int, studentNo:Int) extends Person
case class Teacher( name:String, age:Int, teacherNo:Int) extends Person
case class Nobody( name:String) extends Person
//SchoolClass為接受多個Person型別引數的類 case class SchoolClass(classDescription:String,persons:Person*)
//下列程式碼給出的是其模式匹配應用示例 object CaseClassDemo{ def main(args: Array[String]): Unit = { val sc=SchoolClass("學途無憂網Scala培訓班",Teacher("搖擺少年夢",27,2015),Student("搖擺少年夢",27,2015)) sc match{ case SchoolClass(,,Student(name,age,studetNo))=>println(name) case _ => println("Nobody") } } } ```
sealed case class
sealed關鍵字作用
- 其修飾的trait,class只能在當前檔案裡面被繼承
- 用sealed修飾的目的是告訴scala編譯器在檢查模式匹配時,讓scala能在編譯時檢查程式碼是否有漏掉什麼沒case到的程式碼,減少程式設計的錯誤。
簡單例項
``` //Person最前面加了個關鍵字sealed sealed abstract class Person
case class Student( name:String, age:Int, studentNo:Int) extends Person
case class Teacher( name:String, age:Int, teacherNo:Int) extends Person
case class Nobody( name:String) extends Person
case class SchoolClass(classDescription:String,persons:Person*)
object CaseClassDemo{ def main(args: Array[String]): Unit = { val s:Person=Student("john",18,1024) //這邊僅僅給出了匹配Student的情況,在編譯時 //編譯器會提示 //match may not be exhaustive. It would fail on the following inputs: Nobody(), Teacher(, _, _) s match{ case Student(name,age,studentNo)=>println("Student") } } } ```
文章來自:https://www.itjmd.com/news/show-4236.html