Unable to initialize kotlin class member from secondary function - kotlin

Question

class Person constructor(val first_name: String){
init{
println("Welcome, ${first_name}")
}
val last_name: String
constructor(fname: String, lname: String): this(fname){
this.last_name = lname
}
fun fullNameGreeting(){
println("Welcome, ${first_name} ${last_name}")
}
}
fun main() {
val kotlin = "🙂"
println(kotlin)
val adam: Person = Person("Adam", "Kim")
}
I get following error in Kotlin Playground:-
Property must be initialized or be abstract
Val cannot be reassigned
I am just trying it after reading the documentation on Kotlin website. I am still learning but I am not able to solve this problem. Please Help.

Answer 1

Property must be initialized or be abstract:
You got this error because you didn't initialize a value to last_name, you can't just declare the name and the type of the variable, you need to assign a default value to it like that val last_name: String = ""
Val cannot be reassigned:
And for this error because you set last_name as a value by using val and that means that you can't change it anymore, but you are trying to change it here this.last_name = lname, so to be able to change last_name you need to set is a variable by using var, like that var last_name: String = ""
So your final code should look like this:
class Person constructor(val first_name: String){
init{
println("Welcome, ${first_name}")
}
var last_name: String = ""
constructor(fname: String, lname: String): this(fname){
this.last_name = lname
}
fun fullNameGreeting(){
println("Welcome, ${first_name} ${last_name}")
}
}
fun main() {
val kotlin = "🙂"
println(kotlin)
val adam: Person = Person("Adam", "Kim")
}
If you want to create an instance of Person with only first_name or both first_name and last_name, you can use this approach:
class Person constructor(
val first_name: String,
val last_name: String = ""
){
init{
println("Welcome, ${first_name}")
}
fun fullNameGreeting(){
println("Welcome, ${first_name} ${last_name}")
}
}
This way you can either call Person("Your first name"), or Person("Your first name", "Your last name")

Answer 2

I'd suggest switching the constructors around. Then you can do
class Person constructor(val first_name: String, val last_name: String){
init{
println("Welcome, ${first_name}")
}
constructor(fname: String): this(fname, "")
fun fullNameGreeting(){
println("Welcome, ${first_name} ${last_name}")
}
}
or better yet, make the second parameter optional, like
class Person constructor(val first_name: String, val last_name: String = ""){
init{
println("Welcome, ${first_name}")
}
fun fullNameGreeting(){
println("Welcome, ${first_name} ${last_name}")
}
}

Answer 3

It seems that val is used for constant and var for variable (overwritable) properties. If you replace vals in the class with vars, it works.