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Kotlin program | Companion object features

belongs to collection: Kotlin Class and Object Programs

YasminRadwan26
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2022-07-14
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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:
  • You can omit the name, in which case the name defaults to 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.
  • A class has only one companion object.
  • Companion object initializes when class is loaded, (typically when first time reference from other code).
  • Companion object has its own init block.
  • The companion object is a singleton.
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YasminRadwan26
2022-07-14
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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:

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

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