Meaning of words "in" and "out" in Kotlin generics

August 13, 2024

I learn kotlin generics but naming makes me crazy.

Let consider 2 code snippets in java and in Kotlin which do the same job:

1. java

    public void copy(List<? extends Number> from, List<? super Number> to) {
        assert from.size() == to.size();
        for (int i = 0; i < from.size(); ++i) {
            to.add(from.get(i));
        }
    }

Everything is clear.

from is a list of items of type which inherits from Number. So it could be List<Double> List <Short> etc. I understand why extends word is used

to is a list of items of type which is a supertype of Number. So it could List<Number> or List<Serializable> or List<Object>

2.Kotlin

The similar code for Kotlin:

    fun copy(from: Array<out Number>, to: Array<in Number>) {
        assert(from.size == to.size)
        for (i in from.indices)
            to[i] = from[i]
    }

From my point of view it looks counterintuitive.
Why Array<out Number> is Array of subtypes of Number ?

Why Array<in Number> is Array of supertypes of Number ?

I am sure creaters had some logic and I want to undesit.