Groovy: Constructor invocation

There are four ways to invoke the constructor.

a.   Normal Java way using new operator

b.   Using coercion with as keyword

c.    Using coercion in assignment

d.   Using named parameters

For example, take below Employee class, I defined one constructor that takes employee name and id as arguments.

class Employee{

         String name

         int id

         Employee(name, id){

                  this.name = name

                  this.id = id

         }

        

}

Using new operator

Employee emp1 = new Employee("Krishna", 123)

Using coercion with as keyword

Employee emp2 = ["Krishna", 123] as Employee

Using coercion in assignment

Employee emp3 = ["Krishna", 123]

Find the below working application.

HelloWorld.groovy

class Employee{
 String name
 int id

 Employee(name, id){
  this.name = name
  this.id = id
 }
 
}

Employee emp1 = new Employee("Krishna", 123)
Employee emp2 = ["Krishna", 123] as Employee
Employee emp3 = ["Krishna", 123]

void printEmployee(Employee emp){
 println "name: ${emp.name}, id: ${emp.id}"
}

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

Output

name: Krishna, id: 123

name: Krishna, id: 123

name: Krishna, id: 123

Using named parameters

If a class has default constructor, then you can create objects by passing parameters in the form of a map (property/value pairs)

HelloWorld.groovy

class Employee{
 String name
 int id
}

Employee emp1 = new Employee(name : "Krishna", id : 123)
Employee emp2 = new Employee(name : "Krishna")
Employee emp3 = new Employee()


void printEmployee(Employee emp){
 println "name: ${emp.name}, id: ${emp.id}"
}

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

Output

name: Krishna, id: 123

name: Krishna, id: 0

name: null, id: 0

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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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