What does actually happen in the clone() method?

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I don't think I understand this topic even after doing some reading on it. I will try to describe as good as I can what I think happens in my code. If you can see the mistake, I would be grateful for any hints. As far as I understand after clone() in the main method is called, the solution.clone() method is executed. In there I create a new solution object (called cl), this is using the default constructor (creating a soultion object with a Map<String, User> users instance). In code gmy's correct solution another Map<String, User> users is created, does anyone know why? Then I loop through all the map entries of the original solution object , for every entry I take the name and the user object and add the name and a clone of the user object to my cl clone's map. In the user clone method the default clone() is called, since only primitive types are being cloned. My result is: com.codegym.task.task21.task2107.Solution@d6c3b2cb com.codegym.task.task21.task2107.Solution@324b1279 {Hubert=com.codegym.task.task21.task2107.Solution$User@279f2327, Zapp=com.codegym.task.task21.task2107.Solution$User@2ff4acd0} {Hubert=com.codegym.task.task21.task2107.Solution$User@54bedef2, Zapp=com.codegym.task.task21.task2107.Solution$User@5caf905d} To me this looks like it actually worked, the solution as well as the users got cloned and don't share the same reference, but the validation fails. Any ideas?

package com.codegym.task.task21.task2107; import java.util.LinkedHashMap; import java.util.Map; import java.util.Objects; /* Deep cloning of a map */ public class Solution implements Cloneable{ public static void main(String[] args) { Solution solution = new Solution(); solution.users.put("Hubert", new User(172, "Hubert")); solution.users.put("Zapp", new User(41, "Zapp")); Solution clone = null; try { clone = (Solution) solution.clone(); System.out.println(solution); System.out.println(clone); System.out.println(solution.users); System.out.println(clone.users); } catch (CloneNotSupportedException e) { e.printStackTrace(System.err); } } @Override protected Object clone() throws CloneNotSupportedException { Solution cl = new Solution(); for(Map.Entry<String, User> m : this.users.entrySet()){ String name =m.getKey(); User u = m.getValue(); cl.users.put(name, (User)u.clone()); } return cl; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; Solution solution = (Solution) o; return Objects.equals(users, solution.users); } @Override public int hashCode() { return Objects.hash(users); } protected Map<String, User> users = new LinkedHashMap(); public static class User implements Cloneable { int age; String name; public User(int age, String name) { this.age = age; this.name = name; } @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); } } }

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Thomas

Level 41 , Bayreuth, Germany

9 December 2022, 21:49

if you want to compare two objects (equals) you need to override the equals and hashCode methods. Otherwise the default implementation of the Object class will be used (and that one is using, hmm, lets name it the memory address of the object for equality). And because you create a new object and compare that to the blueprint object the will be different as they have different memory addresses. -> you need equals and hashCode methods in the User class (check the requirements, it's there, too) In code gmy's correct solution another Map<String, User> users is created, does anyone know why? you answer that yourself few lines below: In the user clone method the default clone() is called, since only primitive types are being cloned. You create a new Solution object and therefore a new map. The CodeGym solution probably uses clone to get a new Solution object, hence now new map object is created, just the reference is copied.

Paul

Level 24 , Vienna, Austria

12 December 2022, 17:26

Thanks for the answer. yes, the codegym answer calles the super.clone() method, so that makes perfect sense now. It seems I forgot that map is also a regular object and not a primitive just because it is an instance variable. I am still confused about the first part, becuase I don't see where in the code the objects are compared. In the posted solution, I had my equals and hashCode methosds in the solution class, I created new ones for the user class, which let me pass the requirements. Still, if I add two statements: solution.equals(clone) still returns false but solutions.users.equals(clone.users) returns true.

Thomas

Level 41 , Bayreuth, Germany

12 December 2022, 18:43

That's cause you coded more than you should. Look at the main method. It prints the solution class (and it's clone). CG expects these to be different (you cloned the object and orig and clone have now different memory addresses -> Object.equals should be the output ( -> no overridden hashCode method required in Solution). equals() in the Solution class is also redundant as it is never invoked. Then main outputs the hashCode of the maps of the original Solution object and its clone. Here the hashCode method of the map!!! is invoked (the map has the hashCode method overridden). And that overridden map hashCode method calls the containing objects hashCode method, User hashCode method (you see, you need to override it here ;) ). So the map's hash is dependent of its elements hashes. And CG expects both to be equal.