Здравствуйте. Подскажете почему в конце метода пишется return null? Написал, потому что IDEA предложила. Сам не понимаю для чего оно там. 
public String toString() {
            if (mother == null && father == null)
                return "Cat name is " + name + ", no mother, no father ";
            else if (mother != null && father == null)
                return "Cat name is " + name + ", mother is " + mother.name + ", no father";
            else if (mother == null && father != null)
                return "Cat name is " + name + ", no mother, father is " + father.name;
            else if (mother != null && father != null)
                return "Cat name is " + name + ", mother is " + mother.name + ", father is " + father.name;
            return null;
        }