case classes

case classe 其實跟一般的 class 差不多，只是在建立物件時不用使用 new，然後建構子的參數都是val，不可修改但可以直接存取．所以當有很多資訊要存時可以用 case classe 封裝．還有一個優點就是比較方便．定義 case classe 只要在 class 前面加上 case 關鍵字即可 :

scala> case class Student(id:String,name:String,tel:String) {def echoMsg = println("Hi")}
defined class Student

scala> val s1 = Student("1","Daniel","123")
s1: Student = Student(1,Daniel,123)

scala> s1.name
res10: String = Daniel

scala> s1.echoMsg
Hi

case classe 的 copy :

可以 copy 一份新的物件出來 :

scala> val s1 = Student("1","Sam","456")
s1: Student = Student(1,Sam,456)

scala> val s2 = s1.copy()
s2: Student = Student(1,Sam,456)

copy 時還可以更新指定的值(更新id)，其他的值會自動複製 :

scala> val s3 = s1.copy("2")
s3: Student = Student(2,Sam,456)

scala 的 eq , equals , ==

對 copy 的物件與原來的物件作比較 :
eq
是用來比對 reference，由於兩個物件不一樣所以會是 false :

scala> println(s1 eq s2)
false

equals
用來比對物件的值，要比對物件的值是否相等要實作 equals 和 hashCode 兩個方法，
但如果使用 case classe 預設已經實作好了，所以可以直接比較，這邊內容一樣所以為 true :

scala> println(s1 equals s2)
true

==
一般是用 equals 但如果是比對 null 的話是用 eq :

scala> println(s1 == s2)
true

case classe 搭配 match expression

比對值 :

scala> case class Student(id:String,name:String,tel:String) {
     |   def echoMsg = println("Hi")
     | }
defined class Student

scala> def chekStudent(s: Student) = s match {
     |     case Student("1","Daniel","123") => "student1"
     |     case Student("2","Sam","456") => "student2"
     |     case Student("3","Jack","789") => "student3"
     |     case _ => ""
     |   }
chekStudent: (s: Student)String

scala> chekStudent(Student("2","Sam","456"))
res12: String = student2

比對型態 :

scala> class People
defined class People

scala> case class Student() extends People
defined class Student

scala> case class Teacher() extends People
defined class Teacher

scala> case class Employee() extends People
defined class Employee

scala> def checkPeople(s: People) = s match {
     |   case Student() => "this is Student"
     |   case Teacher() => "this is Teacher"
     |   case Employee() => "this is Employee"
     |   case _ => ""
     | }
checkPeople: (s: People)String

scala> def checkPeople(s: People) = s match {
     |   case Student() => "this is Student"
     |   case Teacher() => "this is Teacher"
     |   case Employee() => "this is Employee"
     |   case _ => ""
     | }
checkPeople: (s: People)String

scala> println(checkPeople(Teacher()))
this is Teacher

scala> println(checkPeople(Employee()))
this is Employee

總結

• scala 的 case classes 使用上感覺很像 java 存資料時都會封裝成一個 bean 物件．
  而且又實作了 equals 和 hashCode 兩個方法，比對也很方便，再搭配 Match Expression 使用起來真得很方便．
• compare :

1. 比對值時用 equals
2. 比對 reference 時用 eq
3. == 一般是用 equals ，但如果是比對 null 的話是用 eq