Kotlin primary and secondary constructors during inheritance

Question

I am learning Kotlin and I'm kind of stuck with the constructors. I've written a program to help me understand the concept. The program is as follows:

open class Animal(){
    var name :String = "noname";
    constructor(name:String):this(){
        this.name = name
    }
    open fun eat(){
        println("$name the animal is eating")
    }
}
class Dog: Animal{
    constructor():super()
    constructor(name : String):super(name)
    override fun eat(){
        println("$name the dog is eating")
    }
}
fun main (args:Array<String>){
    var a1 = Animal("foo")
    var a2 = Animal()
    var d1 = Dog("bar")
    var d2 = Dog()
    a1.eat()
    a2.eat()
    d1.eat()
    d2.eat()
}

Is it possible to modify the program so that the child class calls the parent's primary constructor without using another secondary constructor.
What I'm trying to achieve here is to not force the user to pass a parameter while creating objects using primary and secondary constructors.

Please forgive me if my question is dumb. I am novice here.

score 0 · Answer 1 · answered May 11 '18 at 11:20

0

In Kotlin, you can set the default value in the constructor itself:

open class Animal(var name: String = "noname"){
    open fun eat(){
        println("$name the animal is eating")
    }
}
class Dog(name: String = "noname"): Animal(name){
    override fun eat(){
        println("$name the dog is eating")
    }
}

This way, "noname" is assigned if no string is provided when invoking the constructor. Also, when you create a Dog object, it will always call the parent constructor.

answered May 11 '18 at 11:20

DAA

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Oh cool! Thanks. Although, would it have been possible without the default values? Just curious. – rdias002 May 11 '18 at 11:27
1

You could make 'name' nullable and set it to null in the constructor if no name is provided. If you don't want it nullable but still want two different constructors (with and without name), then you have to implement both constructors separately. – DAA May 11 '18 at 11:33
Implementing both constructors separately would mean the same thing that I did in the question? – rdias002 May 11 '18 at 11:49
That is correct, although you are setting a default value for name anyway in your question. Right now, the code you wrote and mine do the same thing – DAA May 11 '18 at 11:55
Okay... So what I take of this is that there is no other way to get the same output without using default values or the null thingy that you mentioned or two different secondary constructors. That right? Because what I was actually expecting was something of the sort: `class Dog(): Animal()` – rdias002 May 11 '18 at 12:17
class Dog(): Animal() works and means: Dog class has a constructor WITHOUT parameters that calls the constructor without parameters of the Animal parent class. You could do that (have empty constructors for both classes) and set the value of the name later with myDog.name = "Name". If you want to set the name in the constructor, you should use the code I wrote earlier. – DAA May 11 '18 at 12:27

Kotlin primary and secondary constructors during inheritance

1 Answers1