import java.util.ArrayList;
import java.util.List;

// List<Integer> no es subtipo de List<Number>.
public class Invariance {
    static double total(List<Number> numbers) {
        double t = 0;
        for (Number n : numbers) {
            t += n.doubleValue();
        }
        return t;
    }

    public static void main(String[] args) {
        List<Integer> ints = new ArrayList<>(List.of(1, 2, 3));
        List<Number> nums = ints;
        System.out.println(total(ints));
    }
}