[Kotlin]學習筆記 - Types

繼續學習 Kotin，本篇筆記會紀錄 Types

這裡的 Types 就是常聽到的 Class 和 interface

筆記

Interface

1. 預設皆為 public

2. interface 有 basic definitions、default methods、properties

   interface Time {
       fun setTime(hours: Int, mins: Int = 0, secs: Int =0)
       fun setTime(time: KevinTime) = setTime(time.hours, time.mins, time.secs)
   }

3. 要 implement 一個 interface 是使用 : 來表示 implements

   class YetiTime: Time {}

4. 實做一個 interface 的 method 要使用 override keyword

   class YetiTime: Time {
   override fun setTime(hours: Int, mins: Int, secs: Int) {}
   }

5. multiple implements

   interface Time {
       fun setTime(hours: Int, mins: Int = 0, secs: Int = 0)
       fun setTime(time: KevinTime) = setTime(time.hours, time.mins,  time.secs)
   }
   
   interface EndOfTheWorld {
       fun setTime(time: KevinTime) {}
   }
   
   class YetiTime : Time, EndOfTheWorld {
       override fun setTime(time: KevinTime) {
           super<Time>.setTime(time)
       }
   
       override fun setTime(hours: Int, mins: Int, secs: Int) {
       }
   }

   • 使用 , 來實現多 interface implements
   • 可使用 super<T>.xxx 來決定使用哪一個 interface 的 default methods

Classes

1. final 、public by default - apply to class and functions

2. 使用 open keyword 讓 function 可以被 override，讓 Class 可以被繼承

   open class Person {
       var firstName: String = ""
       var lastName: String = ""
       open fun getName(): String = "$firstName $lastName"
   }
   
   class Student: Person() {
       override fun getName(): String {
           return "Hello"
       }
   }

3. with abstract keyword，該 class 或 function 就必須被 implement

   fun main() {
       var a = Person() // 出現 Error: Cannot create an instance of an abstract class
   }
   
   abstract class Person {
       var firstName: String = ""
       var lastName: String = ""
   
       open fun getName(): String = "$firstName $lastName"
       abstract fun getAddress(): String
   }
   
   class Student: Person() {    
       override fun getAddress(): String {
           return ""
       }
       
       override fun getName(): String {
           return "Hello"
       }
   
   }

4. sealed Class: represent restricted class hierarchies that provide more control over inheritance

   sealed class Operation {
       class Add(val value: Int) : Operation()
       class Subtract(val value: Int) : Operation()
       class Multiply(val value: Int) : Operation()
       class Divide(val value: Int) : Operation()
   }
   
   fun execute(x: Int, op: Operation) = when (op) {
       is Operation.Add ->      x + op.value
       is Operation.Subtract -> x - op.value
       is Operation.Multiply -> x * op.value
       is Operation.Divide ->   x / op.value
   }

   • 很像 Enum
   • 可以搭配 when 做 pattern match

5. 可以透過 construction parameters 定義 class property (Primary Constructor)

   open class Person(val name: String)
   val kevin = Person("Kevin")
   println(kevin.name)

6. 可以設定第二組 constructor

   open class Person(val name: String) {
       constructor(name: String, age: Int): this(name)
   }
   // 使用預設值
   open class Person(val name: String, var age: Int = 0)

7. Data Classes: 會自動實做 equals, hashCode, toString methods，且又是 immutable class 再加上 copy method

data class Meeting(val name: String, val location:String)
val aMeeting = Meeting("A Meeting", "Taiwan")
val bMeeting = Meeting("A Meeting", "Taiwan")
val anotherMeeting = aMeeting.copy(location = "Japan")

// equal: 
aMeeting == bMeeting // true
// ToString
println(aMeeting.ToString()) // Meeting(name=A Meeting, location=Taiwan)

• equal: 會比對 data class 內得資料是否相同
• ToString() 會印出文字而非 hashCode
• 可以使用 copy 來建立另外一個 data class

Objects

1. object keyword 用來建立 singleton

2. 定義與建立發生在同一時間

3. object 可以有 properties, methods, initializers 但就是不能有 constructors

   class Course(val id: Int, val title: String)
   
   object Courses{
       var allCourses = arrayListOf<Course>()
   }
   
   fun enrole(courseName: String) {
       Courses.allCourses
       .filter { it.title == courseName }
       .firstOrNull()
   }

4. object 可以在 class 內出現，而此時他的 scope 會是該 class 內

5. companion object: Factory object and static members

   class Person {
       companion object {
           fun createNormalPerson(name: String): NormalPerson {
               return NormalPerson(name)
           }
       }
   }
   
   val normalPerson = Person.createNormalPerson("XXX")

6. @JvmStatic to mark as static method for Java compiler

參考資料

• Sealed Class