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Kotlin program | Example of Secondary Constructor

belongs to collection: Kotlin Class and Object Programs

YasminRadwan26
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2022-07-14
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Secondary Constructor

  • Kotlin class can also declare, Secondary Constructor in the class body, Prefixed with constructor keyboard
  • If the class has a primary constructor then each secondary constructor needs to delegate primary constructor.
  • Delegation of another constructor of the same class is done using the 'this' keyword.
  • NOTE: Delegation to the primary constructor happens as the first statement of a secondary constructor and code in initializer blocks effectively becomes part of the primary constructor, so all initializer blocks and property initializer is executed before the secondary constructor body.
  • If Kotlin class has no primary constructor, the delegation still happens implicitly, and the initializer blocks are still executed
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YasminRadwan26
2022-07-14
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Program to demonstrate the example of Secondary Constructor in Kotlin

package com.includehelp

// Declared Class with Parameterized 
// primary constructor
class Animal(name:String){
    // Property Declaration
    private var name: String?=null
    private var type: String?=null

    // Init Block, effectively becomes part of 
    // the primary constructor
    init{
        this.name=name
        println("Init Block")
    }

    // Secondary Constructor, calls/delegates the 
    // primary constructor using 'this'
    constructor(name:String,type:String) : this(name){
        this.type=type
        println("Secondary Constructor")
    }

    //fun to print class property data
    fun printData(){
        println("Animal -> Name : $name  and Type : $type")
    }
}

// Declared class without primary Constructor, 
// delegation still happens implicitly,
// and the initializer blocks are still executed 
// before Secondary Constructor execution
class Sample{
    // First Init Block
    init{
        println("Init Block 1")
    }

    // Secondary Constructor
    constructor(a: Int,b:Int){
        println("Secondary Constructor : A=$a and B=$b")
    }

    // Second Init block
    init {
        println("Init Block 2")
    }
}

// Main function, Entry Point of Program
fun main(args: Array<String>){
    // Create Class object to call Primary constructor 
    // of Animal Class
    val animal = Animal("Cobra")
    animal.printData()

    // Create Class object to call Secondary constructor 
    // of Animal Class
    val animal2 = Animal("Python","Snake")
    animal2.printData()

    // Create Class object to call Secondary constructor 
    // of Sample Class with named parameter
    Sample(a=10, b=20)
}

Output:

Init Block
Animal -> Name : Cobra  and Type : null
Init Block
Secondary Constructor
Animal -> Name : Python  and Type : Snake
Init Block 1
Init Block 2
Secondary Constructor : A=10 and B=20

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