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Question about the task

Bug in equals/hashCode

Java Multithreading, Level 1, Lesson 5

Under discussion

I tested my code:

Set<Solution> s = new HashSet<>();
       Date d = new Date();
       Solution a = new Solution();
       Solution sol = new Solution(3, "ala", 2.2, d, a);
       System.out.println(s.add(sol));
       System.out.println(s.add(sol));

output is: true false

Correct the implementation errors in the Solution's classes equals and hashCode methods.

Requirements:

The hashcodes of identical objects must be equal.
The equals method should check whether the passed object is equal to the current object (comparison using ==).
The equals method should check whether the passed object is a Solution object.
The equals method must check all of the fields of the passed object and the current object (note that some of them may be null).
You must ensure that the HashSet of Solution elements behaves correctly.
The Solution class must implement the hashCode method.

package com.codegym.task.task21.task2106; import java.time.LocalDate; import java.time.format.DateTimeFormatter; import java.util.Date; import java.util.HashSet; import java.util.Objects; import java.util.Set; /* Bug in equals/hashCode */ public class Solution { private int anInt; private String string; private double aDouble; private Date date; private Solution solution; public Solution() { } public Solution(int anInt, String string, double aDouble, Date date, Solution solution) { this.anInt = anInt; this.string = string; this.aDouble = aDouble; this.date = date; this.solution = solution; } @Override public boolean equals(Object o) { if (this == o) { return true; } if (o instanceof Solution) { return false; } if (o == null) { return false; } Solution solution1 = (Solution) o; return Objects.equals(this.anInt, solution1.anInt) && Objects.equals(this.string, solution1.string) && Objects.equals(this.aDouble, solution1.aDouble) && Objects.equals(this.date, solution1.date) && Objects.equals(this.solution, solution1.solution); } @Override public int hashCode() { int result; long temp; result = anInt; temp = aDouble != +0.0d ? Double.doubleToLongBits(aDouble) : 0L; result = 31 * result + (int) (temp ^ (temp >>> 32)); result = 31 * result + (solution != null ? solution.hashCode() : 0); return result; } public static void main(String[] args) { } }

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Fadi Alsaidi

Level 34 , Carrollton, TX, USA

11 July 2020, 23:15

I know it's been a while, but I thought i would help. aInt and aDouble are primitive data type and therefore, you only need to check if they are not euqal. so add those line belwo and remove them from Objects.equal() method if (Double.compare(solution1.aDouble, aDouble) != 0) return false; if (anInt != solution1.anInt) return false;

KIN SOCHEAT

Level 34 , Phnom Penh, Cambogia

8 April 2020, 09:50

then delete this if (o == null) { return false; }

Azja

Level 32 , Krakow, Poland

9 May 2019, 13:09

When you pass Solution object this statement return false

if (o instanceof Solution) {
            return false;
        }

but should be like this. When non Solution object pass immediately return false

if (!(o instanceof Solution)) {
            return false;
        }