I was able to get my solution verified, but strangely enough I got the following error in the output area: 
java.lang.NullPointerException
	at com.codegym.task.task07.task0726.Solution.main(Solution.java:22)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
My solution is as follows: 
package com.codegym.task.task07.task0726;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;

/*
Cat code won't compile

*/

public class Solution {
    public final static ArrayList<Cat> CATS = new ArrayList<>();

    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        while (true) {
            String name = reader.readLine();

            if (name.isEmpty()) break;

            int age = Integer.parseInt(reader.readLine());
            int weight = Integer.parseInt(reader.readLine());
            int tail = Integer.parseInt(reader.readLine());

            Cat cat = new Cat(name, age, weight, tail);
            CATS.add(cat);
        }

        printList();
    }

    public static void printList() {
        for (Cat cat : CATS) {
            System.out.println(cat);
        }
    }

    public static class Cat {
        private String name;
        private int age;
        private int weight;
        private int tailLength;

        Cat(String name, int age, int weight, int tailLength) {
            this.name = name;
            this.age = age;
            this.weight = weight;
            this.tailLength = tailLength;
        }

        @Override
        public String toString() {
            return "Cat's name: " + name + ", age: " + age + ", weight: " + weight + ", tail: " + tailLength;
        }
    }
}
Does anyone have any idea what's wrong? Is my solution really correct, or was there a mistake that was not caught by the verification system?