Java generics and Kotlin in/out

AddIntToObjectsNotWorking.java

// Why are Java generics invariant?

// invariant means List<Object> is NOT a parent of List<String>.
// if it was the case, then:

List<String> strs = new ArrayList<String>();
List<Object> objs = strs; // !!! A compile-time error here saves us from a runtime exception later.
objs.add(1); // Put an Integer into a list of Strings
String s = strs.get(0); // !!! ClassCastException: Cannot cast Integer to String

AddStringsToObjects.java

package ru.maksim.sample;

interface MyCollection<E> {

    // void addAll(MyCollection<E> items);
  
    void addAll(MyCollection<? extends E> items);
}


package ru.maksim.sample;

public class Client {

    void copyAll(MyCollection<Object> to, MyCollection<String> from) {
        to.addAll(from); 
        // since "to" expects an instance of MyCollection<Object> and 
        // MyCollection<String> is not a child of MyCollection<Object>
        // "? extends E" needs to be used.
    }
}

public class Client {

    void copyAll(MyCollection<Number> to, MyCollection<Double> from, MyCollection<Integer> from2) {
        // since "to" expects an instance of MyCollection<Number> and 
        // MyCollection<Double> is not a child of MyCollection<Number>
        // "? extends E" needs to be used.
        to.addAll(from2);
        to.addAll(from);
    }
}