Не компилируется программа, что-то связанное с null.

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 grandfather = new Cat(grandfatherName, null, null); String grandmotherName = reader.readLine(); Cat grandmother = new Cat(grandmotherName, null, null); String fatherName = reader.readLine(); Cat father = new Cat(fatherName, null, grandfather); String motherName = reader.readLine(); Cat mother = new Cat(motherName, grandmother, null); String sonName = reader.readLine(); Cat son = new Cat(sonName, mother, father); String daughterName = reader.readLine(); Cat daughter = new Cat(daughterName, mother, father); System.out.println(grandfather.toString()); System.out.println(grandmother.toString()); System.out.println(father.toString()); System.out.println(mother.toString()); System.out.println(son.toString()); System.out.println(daughter.toString()); } public static class Cat { private String name; private Cat parent1; private Cat parent2; Cat(String name, Cat parent1, Cat parent2) { this.name = name; this.parent1 = parent1; this.parent2 = parent2; } @Override public String toString() { String s = ""; if (parent1 == null && parent2 == null) s = "The cat's name is " + name + ", no mother, no father"; if (parent1 == null) s = "The cat's name is " + name + ", no mother, father is " + parent2.name; if (parent2 == null) s = "The cat's name is " + name + ", mother is " + parent1.name + ", no father"; if (parent1 != null && parent2 != null) s = "The cat's name is " + name + ", mother is " + parent1.name + ", father is " + parent2.name; return s; } } }