learn the syntax for constructors in Kotlin

Question

I am learning Kotlin programming language perfectly. I try to write code in different patterns and try to understand. However, I did not understand the thing. Can you help me, please? Here it is:

open class Parent {
    open val foo = 1
    init {
        println(foo)
    }
}
class Child: Parent() {
    override val foo =2
}
fun main() {
    Child()
}

In this code, 0 is the output. How will this be?

Answer 1

Now, let's with java understand why. In Java, it's impossible to override fields and under the hood in Kotlin is the same. When you override a property, in fact, you override a getter, not a field. For instance, you can override a property that doesn't have a field with a property that has a field. That's totally legal. However, when both a property from a superclass and an overridden property in a subclass have fields, that might lead to unexpected results. Let's see what bytecode is generated for the Kotlin class in my example. As usual, I'll look at the corresponding Java code instead for simplicity.

public class Parent {
    private final int foo = 1;
    public int getFoo() {return foo;}
    public Parent(){
        System.out.println(getFoo());
    }
}
public final class Child extends Parent {
    private final int foo = 2;
    public int getFoo() {return foo;}
}
public class Main
{
    public static void main (String[] args) {
        new Child();
    }
}

Note two things here. First, the foo get to is trivial, so a field and a getter correspond to the full property. Then because the property is open and can be overridden in a subclass, its usage inside the class is compiled to a getter code, not a field code. Now, the generated code for the child class. Note that the overridden property in the parent class is also compiled to a field and a getter, and now it's another field. What happens when you create an instance of the child class? At first at the parent constructor is called, the parent constructor initializes the first fulfilled with one. But inside the init section, an overridden getter is called which calls get foo from the child class. Because the field in the child class is not yet initialized, 0 is returned. That's why 0 is printed here.