Kotlin: Secondary Constructors

Kotlin class can have more than one secondary constructors.

How to define secondary constructor?

You should use the keyword ‘constructor’ to define secondary constructor.

Syntax
constructor(arguments){

}

ConstructorDemo.kt

class Employee {
 var firstName: String = ""
 var lastName: String = ""

 constructor(firstName: String) {
  this.firstName = firstName
  this.lastName = firstName
 }

 constructor(firstName: String, lastName: String) {
  this.firstName = firstName
  this.lastName = lastName
 }
}

fun printEmployee(emp: Employee) {
 println("firstName : ${emp.firstName}, lastName : ${emp.lastName}")
}

fun main(args: Array<String>) {
 var emp1 = Employee("Hareesh", "Seeram")
 var emp2 = Employee("Praneeth")

 printEmployee(emp1)
 printEmployee(emp2)

}

Output

firstName : Hareesh, lastName : Seeram
firstName : Praneeth, lastName : Praneeth

Primary constructor must be called from secondary constructor explicitly. What I mean is, if a class has primary constructor, then the secondary constructor must delegate to the primary constructor either directly (or) indirectly using other secondary constructors.

class Employee(var firstName: String) {

         ....

         ....

        

         constructor(fName: String, lName: String) : this(fName) {

                 this.lastName = lName

                 this.id = ""

         }

}

In the above example, Employee class has a primary constructor, We are calling primary constructor using 'this' keyword, while defining secondary constructor.

ConstructorDemo.kt

class Employee(var firstName: String) {
 var lastName: String = ""
 var id: String = ""

 constructor(fName: String, lName: String) : this(fName) {
  this.lastName = lName
  this.id = ""
 }

 constructor(fName: String, lName: String, id: String) : this(fName) {
  this.lastName = lName
  this.id = id
 }
}

fun printEmployee(emp: Employee) {
 println("id : ${emp.id}, firstName : ${emp.firstName}, lastName : ${emp.lastName}")
}

fun main(args: Array<String>) {
 var emp1 = Employee("Krishna")
 var emp2 = Employee("Hareesh", "Seeram")
 var emp3 = Employee("Praneeth", "Sai", "123")

 printEmployee(emp1)
 printEmployee(emp2)
 printEmployee(emp3)

}

Output

id : , firstName : Krishna, lastName : 
id : , firstName : Hareesh, lastName : Seeram
id : 123, firstName : Praneeth, lastName : Sai

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