Пример: 
дедушка Вася
бабушка Мурка
папа Котофей
мама Василиса
сын Мурчик
дочь Пушинка
The cat's name is дедушка Вася, no mother, no father
The cat's name is бабушка Мурка, no mother, no father
The cat's name is папа Котофей, no mother, father is дедушка Вася
The cat's name is мама Василиса, mother is бабушка Мурка, no father
The cat's name is сын Мурчик, mother is мама Василиса, father is папа Котофей
The cat's name is дочь Пушинка, mother is мама Василиса, father is папа Котофей
Валидатор: 
Строка про папу (третья) должна соответствовать условию.
Код: 
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 grandFatherName = reader.readLine();
        Cat catGrandFather = new Cat(grandFatherName, null, null);

        String grandMotherName = reader.readLine();
        Cat catGrandMother = new Cat(grandMotherName, null,null);

        String fatherName = reader.readLine();
        Cat catFather = new Cat(fatherName, catGrandFather, null);

        String motherName = reader.readLine();
        Cat catMother = new Cat(motherName, null, catGrandMother);

        String sonName = reader.readLine();
        Cat catSon = new Cat(sonName, catFather, catMother);

        String daughterName = reader.readLine();
        Cat catDaughter = new Cat(daughterName, catFather, catMother);

        System.out.println(catGrandFather);
        System.out.println(catGrandMother);

        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 father, Cat mother) {
            this.name = name;
            this.mother = mother;
            this.father = father;
        }

        @Override
        public String toString() {

            if (mother == null) {
                if (father == null)
                    return "The cat's name is " + name + ", no mother, no father";
                else
                    return "The cat's name is " + name + ", no mother, father is " + father.name;
            }

            else {
                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;
            }

        }
    }

}