Kotlin variable initialization for child class behaves weird for initializing variable with value 0

Question

I have created the following class hierarchy:

open class A {
    init {
        f()
    }

    open fun f() {
        println("In A f")
    }
}

class B : A() {
    var x: Int = 33

    init {
        println("x: " + x)
    }

    override fun f() {
        x = 1
        println("x in f: "+ x)
    }

    init {
        println("x2: " + x)
    }
}

fun main() {
    println("Hello World!!")
    val b = B()
    println("in main x : " + b.x)
}

The output of this code is

Hello World!!
x in f: 1
x: 33
x2: 33
in main x : 33

But if I change the initialization of x from

var x: Int = 33

to

var x: Int = 0

the output shows the invocation of the method in contrast to the output above:

Hello World!!
x in f: 1
x: 1
x2: 1
in main x : 1

Does anyone know why the initialization with 0 causes a different behaviour than the one with another value?

Answer 1

super class is initialized before sub class.

The constructor call of B calls the constructor of A, which calls the function f printing "x in f: 1", after A is initialized, the rest of B is initialized.

So essentially, the setting of the value is being overwritten.

(When you initialize primitives with their zero value in Kotlin, they technically just dont initialize at all)

You can observe this "overwrite" behavior by changing the signature from

var x: Int = 0 to var x: Int? = 0

Since x is no longer the primitive int, the field actually gets initialized to a value, producing the output:

Hello World!!
x in f: 1
x: 0
x2: 0
in main x : 0