Hello, I had difficulties finishing this task because I could not wrap my head around on how to add object to various arraylists. But I checked other questions and answers to them and saw that I only need 1 array list, of children. My questions are: 1. How does the following expression work: object.ArrayList.add(object) - father.children.add(child1) - I don't remember having encountered this expression before. 2. I tried to print the arraylist, to see if it contains all of the objects added, but it would not print. Why is that? 
public class Solution {

    public static void main(String[] args) {
        //write your code here
        Human grandfather1 = new Human("Grandpapi1", true, 87, new ArrayList<Human>());
        Human grandfather2 = new Human("Grandpapi2", true, 84, new ArrayList<Human>());
        Human grandmother1 = new Human("Granmami1", false, 82, new ArrayList<Human>());
        Human grandmother2 = new Human("Grandmami2", false, 65,new ArrayList<Human>());
        Human father = new Human("Papi", true, 56,new ArrayList<Human>() );
        Human mother = new Human("Mami", false, 56,new ArrayList<Human>() );
        Human child1 = new Human("Kiddo1", true, 28, new ArrayList<Human>() );
        Human child2 = new Human("Kiddo2", false, 33, new ArrayList<Human>());
        Human child3 = new Human("Kiddo3", true, 30, new ArrayList<Human>() );


        ArrayList<Human> children = new ArrayList<>();

        father.children.add(child1);
        father.children.add(child2);
        father.children.add(child3);
        mother.children.add(child1);
        mother.children.add(child2);
        mother.children.add(child3);
        grandfather1.children.add(father);
        grandmother1.children.add(father);
        grandfather2.children.add(mother);
        grandmother2.children.add(mother);

        System.out.println(father);
        System.out.println(grandfather1);
        System.out.println(grandfather2);
        System.out.println(grandmother1);
        System.out.println(grandmother2);
        System.out.println(mother);
        System.out.println(child1);
        System.out.println(child2);
        System.out.println(child3);

        for (int i = 0; i < children.size(); i++) System.out.println(children.get(i));


    }

    public static class Human {
        //write your code here
        String name ;
        boolean sex ;
        int age;
        ArrayList<Human> children;

        Human (){}

        Human (String name, boolean sex, int age){
            this.name = name;
            this.sex = sex;
            this.age = age;
        }

        Human (String name, boolean sex, int age, ArrayList<Human> children){
            this.name = name;
            this.sex = sex;
            this.age = age;
            this.children = children;
        }

        public String toString() {
            String text = "";
            text += "Name: " + this.name;
            text += ", sex: " + (this.sex ? "male" : "female");
            text += ", age: " + this.age;

            int childCount = this.children.size();
            if (childCount > 0) {
                text += ", children: " + this.children.get(0).name;

                for (int i = 1; i < childCount; i++) {
                    Human child = this.children.get(i);
                    text += ", " + child.name;
                }
            }
            return text;
        }
    }

}