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Question about the task

Equals and HashCode

Java Multithreading, Level 1, Lesson 5

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It seems to me that the expressions

if (obj == null || getClass() != obj.getClass()) return false;

and

if (!(obj instanceof Solution))   return false;

are equivalent. Is not it?

In the Solution class, fix the equals/hashCode methods according to the implementation rules for these methods (use your favorite search engine to check the details).
Both the first and last strings should be involved in the equals method's comparison and the calculation of the hashcode.
The main method is not tested.

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 return true if the first and last fields are equal to the passed object and the current one (don't forget that they might be null).
You must ensure that the HashSet of Solution elements behaves correctly.

package com.codegym.task.task21.task2104; import java.util.HashSet; import java.util.Objects; 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; } public boolean equals(Object obj) { if (this == obj) return true; if (obj == null || getClass() != obj.getClass()) return false; Solution sol = (Solution) obj; if (!Objects.equals(first, sol.first)) return false; return Objects.equals(last, sol.last); } public int hashCode() { int result = first != null ? first.hashCode() : 0; result = 31 * result + (last != null ? last.hashCode() : 0); return result; } public static void main(String[] args) { Set<Solution> s = new HashSet<>(); s.add(new Solution("Donald", "Duck")); // "Donald", "Duck" System.out.println(s.contains(new Solution("Donald", "Duck"))); } } /* Equals and HashCode In the Solution class, fix the equals/hashCode methods according to the implementation rules for these methods (use your favorite search engine to check the details). Both the first and last strings should be involved in the equals method's comparison and the calculation of the hashcode. The main method is not tested. Requirements: 1. The hashcodes of identical objects must be equal. 2. The equals method should check whether the passed object is equal to the current object (comparison using ==). 3. The equals method should check whether the passed object is a Solution object. 4. The equals method must return true if the first and last fields are equal to the passed object and the current one (don't forget that they might be null). 5. You must ensure that the HashSet of Solution elements behaves correctly. */

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Guadalupe Gagnon

Level 37 , Tampa, United States

1 August, 14:13

solution

Thomas is correct. Here is an article that helps explain: instanceof versus getClass() in the equals() method Imagine a scenario where you create a class to use as a key for a HashMap but the actual objects you are going to be passing to the HashMap are descendants of that class. The purpose is to use the parent class to search for any of the values. I'll post a semi simple example in code:

Guadalupe Gagnon

Level 37 , Tampa, United States

1 August, 14:14

useful

public class Solution {
    static int keyInt;

    static class KeyInt{
        int key;

        KeyInt() {
            key = keyInt++;
        }

        KeyInt(int x) {
            key = x;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            //if (!(o.getClass() == KeyInt.class)) return false;
            if (!(o instanceof KeyInt)) return false;
            KeyInt keyInt = (KeyInt) o;
            return key == keyInt.key;
        }

        @Override
        public int hashCode() {
            return Objects.hash(key);
        }
    }

    static class Key extends KeyInt {
        String keyString;

        Key(String key) {
            keyString = key;
        }

        @Override
        public String toString() {
            return "[Key: '" + keyString + "' ID: '" + key + "']";
        }
    }

    public static void main(String[] args) {
        HashMap<KeyInt, String> test = new HashMap<>();

        test.put(new Key("First"), "something");
        test.put(new Key("Second"), "something else");

        System.out.println(test);
        System.out.println(test.get(new KeyInt(1)));
    }
}

Guadalupe Gagnon

Level 37 , Tampa, United States

1 August, 14:31

useful

On line 50 I find a value using a Key of the parent class. In this way I could create numerous sub classes of the parent class and use them as keys. On line 18 you can see i commented out that if statement using the getClass() method. If you comment line 17 and uncomment line 18 and run the code again you will see that if now fails to return a value. I don't really know the practicality of this, but it could be useful when getting the keys instead of the values in certain situations where that extra bit of data is needed.

Thomas

Level 41 , Bayreuth, Germany

1 August, 07:18

useful

There are differences when it comes to inheritance. getClass() returns true only when both sides are of the same type, while instanceOf returns true also for descendants. So getClass is more restrictive while instanceOf may be more comfortable but give wrong results in times (eg. think of bus and sports car. Both being descendants of vehicle. If both have the same properties, then bus and sports car are equal). That behavior however also may be what you are looking for. Choose wisely ;) Why CodeGym insists on instanceOf? No clue. Maybe they just overlooked getClass. When I did the task I even checked if it may be a dedicated null check but it wasn't.