CODEWITHSUNDEEP
constructorkotlin
Jan Slominski
Jan Slominski

Reputation: 2978

Kotlin: 'val' on secondary constructor parameter is not allowed

I have following class:

class Person(val name: String) {
    private var surname: String = "Unknown"

    constructor(name: String, surname: String) : this(name) {
        this.surname = surname
    }
}

But when I want to have the name parameter immutable in second constructor:

constructor(val name: String, surname: String) : this(name) {
    this.surname = surname
}

I have the following compile-time error:

Kotlin: 'val' on secondary constructor parameter is not allowed

Can someone explain why is Kotlin compiler not allowing to do this?

Upvotes: 14

Views: 8269

Answers (4)

flamewave000
flamewave000

Reputation: 616

The currently accepted answer is correct in explaining why your initial attempt did not work. As such, given your particular scenario, I would inverse the solution and make your secondary constructor the primary, and make that second parameter have a default value.

data class Person(val name: String, val surname: String = "Unknown")

Also, if the class's purpose is to simply hold data, I would make it a data class to improve its handling.

Upvotes: 1

Mahdi A
Mahdi A

Reputation: 11

You can define the variable as val or var in the class you inherit from

   open class Human(val name: String) constructor(name: String) {
            open fun showInfo()
        {
            println("Show Info")
        }
        }

class Person:Human {

constructor(name: String) : super(name)
    private var surname: String = "Unknown"
  override fun showInfo() {
        println("$name And surname is $surname")
    }
    
}

Upvotes: 0

s1m0nw1
s1m0nw1

Reputation: 81879

In addition to the great answer of yole, the documentation is pretty clear as well:

Note that parameters of the primary constructor can be used in the initializer blocks. They can also be used in property initializers declared in the class body. [...] In fact, for declaring properties and initializing them from the primary constructor, Kotlin has a concise syntax:

class Person(val firstName: String, val lastName: String, var age: Int) {
    // ...
}

Much the same way as regular properties, the properties declared in the primary constructor can be mutable (var) or read-only (val).

This all does not apply to secondary constructors.

Upvotes: 4

yole
yole

Reputation: 97148

Parameters in Kotlin are always immutable. Marking a constructor parameter as a val turns it into a property of a class, and this can only be done in the primary constructor, because the set of properties of a class cannot vary depending on the constructor used to create an instance of the class.

Upvotes: 32

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