Primary constructor call expected

Question

this is example of inheritance in Kotlin.

open class Person() {
    var _name : String = ""
    var _age : Int = 0

    constructor(name: String, age : Int)  : this(){
        _name = name
        _age = age
    }
}

class Student() : Person() {
    var _university : String = ""

    constructor (name : String, age: Int, university: String) : super(name, age){
        _university = university
    } 
}

fun main() {
   var person = Person("a", 10)
   var student = Student("b", 18, "MIT")
   println("${person._name} : ${person._age}")
   println("${student._university}")
}

How can I resolve error "Primary constructor call expected" in this case?

Answer 1

If a class has a primary constructor, you must delegate the secondary constructors to the primary. Which is the case for Student class which is not doing so.

First you should remove the boilerplate of Person class following Kotlin's idioms (using optional parameters rather than constructor overloading).

// This one's completely optional to do, skip if you don't want to follow idioms
open class Person(
    var name : String = ""
    var age : Int = 0
)

And then Student class should have two separate constructors, because if you take a primary constructor then other constructors must delegate to it:

class Student: Person {
    var university: String = ""
    
    constructor() : super()

    constructor (name: String, age: Int, university: String) : super(name, age) {
        this.university = university
    }
}