kotlin半生對象

伴侶對象 (Companion object)

• If you need a function or a property to be tied to a class rather than to instances of it (similar to static in java), you can declare it inside a companion object:

  如果需要將函數或屬性綁定到類而不是實例(類似于java中的static)，則可以在同伴對象中聲明它：

• You can omit the name, in which case the name defaults to Companion,

  您可以省略名稱，在這種情況下，名稱默認為Companion，

  companion object <Optional Name>{
  //Companion Object Body
  }
  
  

• Companion objects members can only be accessed via the containing class name, not via instances of the class.

  伴侶對象成員只能通過包含的類名稱訪問，而不能通過類的實例訪問。

• A class has only one companion object.

  一個類只有一個伴隨對象。

• Companion object initializes when class is loaded, (typically when first time reference from other code).

  伴侶對象在加載類時初始化(通常是在第一次從其他代碼引用時)。

• Companion object has its own init block.

  伴隨對象具有其自己的init塊。

• The companion object is a singleton.

  伴隨對象是單例。

Kotlin中的伴隨對象特征程序 (Program for companion object features in Kotlin)

package com.includehelp
//Declare class
class Car{
//class init block
init {
println("Init Block of Class")
}
//Make companion object
companion object {
//companion object init block
init {
println("Init Block of Companion object")
}
//property of companion object
val name="Tata Altroz !! "
//function in companion object
fun printName(){
println("Your Car name : $name")
}
}
}
//Main Function, Entry Point of Program
fun main(){
//Call method with Class name, 
//without create Instance of class, 
//like static method in java
Car.printName()
//access Property using class name
val nameLen = Car.name.length
println("Car Name Length : $nameLen")
}

Output:

輸出：

Init Block of Companion object
Your Car name : Tata Altroz !! 
Car Name Length : 15

翻譯自: https://www.includehelp.com/kotlin/companion-object-features.aspx

kotlin半生對象