Не могу понять.

package com.javarush.task.task06.task0621; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; /* Родственные связи кошек */ public class Solution { public static void main(String[] args) throws IOException { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); String grfatherName = reader.readLine(); Cat catGrFather = new Cat(grfatherName, null, null); String grmotherName = reader.readLine(); Cat catGrMother = new Cat(grmotherName, null, null); String fatherName = reader.readLine(); Cat catFather = new Cat(fatherName, null, catGrFather); String motherName = reader.readLine(); Cat catMother = new Cat(motherName, catGrMother, null); String sonName = reader.readLine(); Cat catSon = new Cat(sonName, catMother, catFather); String daughterName = reader.readLine(); Cat catDaughter = new Cat(daughterName, catMother, catFather); System.out.println(catGrFather); System.out.println(catGrMother); System.out.println(catFather); System.out.println(catMother); System.out.println(catSon); System.out.println(catDaughter); } public static class Cat { private String name; private Cat mother; private Cat father; Cat(String name, Cat mother, Cat father) { this.name = name; this.mother = mother; this.father = father; } @Override public String toString() { if ((mother == null) && (father == null)) return "The cat's name is " + name + ", no mother, no father"; if (mother == null) return "The cat's name is " + name + ", no mother , father is " + father.name; if (father == null) return "The cat's name is " + name + " mother is " + mother.name +" no father "; else return "The cat's name is " + name + ", mother is " + mother.name + " , father is " + father.name ; } } }