Strange mistake! Why not like the implementation of the toString method for the class Dog 
package com.codegym.task.task05.task0525;

public class Solution {

    public static void main(String[] args) {
        Duck duck1 = new Duck();
        Duck duck2 = new Duck();
        System.out.println(duck1);
        System.out.println(duck2);

        Cat cat1 = new Cat();
        Cat cat2 = new Cat();
        Dog dog1 = new Dog();
        Dog dog2 = new Dog();
        System.out.println(cat1);
        System.out.println(cat2);
        System.out.println(dog1);
        System.out.println(dog2);

    }

    public static class Duck {
        public String toString() {
            return "Duck";
        }
    }

        public static class Cat {
            public String toString() {
            return "Cat";
        }
    }

        public static class Dog {
            public String toString() {
            return "Doc";
        }
    }
}
Everything is displayed on the screen correctly!