Подскажите, пожалуйста, что не так

package com.javarush.task.task21.task2104; import java.util.HashSet; import java.util.Set; /* Equals and HashCode */ public class Solution { private final String first, last; public Solution(String first, String last) { this.first = first; this.last = last; } @Override public boolean equals(Object n) { if(n == this) return true;//Рефлексивность: объект должен равняться себе самому // if (n == null || !n.getClass().getSimpleName().equals("Solution")) return false; if(n == null || !(n instanceof Solution)) return false; Solution other = (Solution) n; return ((first == other.first || other.first != null && first.equals(other.first)) && last == other.last || other.last != null && last.equals(other.last)); } @Override public int hashCode() { int hash = 7; hash = hash + (this.first != null ? this.first.hashCode() : 0); hash = hash + (this.last != null ? this.last.hashCode() : 0); return hash; } public static void main(String[] args) { // Solution first = new Solution("first", "last"); // Solution second = new Solution("first", "last"); // System.out.println(first.equals(second)); Set<Solution> s = new HashSet<>(); s.add(new Solution("Donald", "Duck")); System.out.println(s.contains(new Solution("Donald", "Duck"))); } }