abstract class Animal<A extends Animal<A>>
{
    ... someAbstractMethodDefinitions
}

class Duck<D extends Duck<D>> extends Animal<D>
{
    public int getBeekSize()
    {
         return beekSize;
    }

    public int getHeadSize()
    {
         return headSize;
    }
}

class Mallard extends Duck<Mallard>
{
    .. someCode
}

List<Duck> ducks = Arrays.asList(new Duck(), new Duck());
Collections.sort(
        ducks,
        Comparator.comparing(Duck::getBeekSize,Comparator.nullsFirst(Comparator.naturalOrder()))
            .thenComparing(Duck::getHeadSize, Comparator.nullsFirst(Comparator.naturalOrder())));

.thenComparing(Duck::getHeadSize, Comparator.nullsFirst(Comparator.naturalOrder())));

List<Duck> ducks = Arrays.asList(new Duck(), new Duck());
Collections.sort(
        ducks,
        Comparator.comparing(Duck::getBeekSize,Comparator.nullsFirst(Comparator.naturalOrder()));