Why the numbers superior to 10 are displayed in my code?

Question about the task

Greater than 10? You're not a good fit for us

Java Syntax, Level 8, Lesson 8

Resolved

Hello, I don't understand why my method is not working well.

public static HashSet<Integer> removeAllNumbersGreaterThan10(HashSet<Integer> set) {
       HashSet<Integer> set2 = new HashSet<>();
       for(Integer element : set){
           if(element <= 10){
           set2.add(element);
           }
       }
       return set2;
   }

I don't understand why I have the numbers superior to 10 in the output. (I choose to have an output to understand better even if it's not asked). Thanks!

package com.codegym.task.task08.task0814; import java.util.HashSet; import java.util.Set; /* Greater than 10? You're not a good fit for us */ public class Solution { public static HashSet<Integer> createSet() { HashSet<Integer> set = new HashSet<>(); set.add(1); set.add(2); set.add(3); set.add(4); set.add(5); set.add(6); set.add(7); set.add(8); set.add(9); set.add(10); set.add(11); set.add(12); set.add(13); set.add(14); set.add(15); set.add(16); set.add(17); set.add(18); set.add(19); set.add(20); return set; } public static HashSet<Integer> removeAllNumbersGreaterThan10(HashSet<Integer> set) { HashSet<Integer> set2 = new HashSet<>(); for(Integer element : set){ if(element < 10){ set2.add(element); } } return set2; } public static void main(String[] args) { HashSet<Integer> set = createSet(); removeAllNumbersGreaterThan10(set); for(Integer element : set){ System.out.println(element); } } }

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Gellert Varga

Level 23 , Szekesfehervar, Hungary

26 December 2021, 20:46

solution

In my opinion, it is absolutely the right approach to put a lot of Sys.o.print commands in the code to see exactly what the program is doing. (Just don't forget to comment them out before you validate the code...) Here's what happens in your program: - The removeAllNumbersGreaterThan10 method does not remove the elements greater than 10 from the original set, but creates a new set (set2 local variable) and into it puts the elements not greater than 10 . - The method returns the reference of this set2 object, but this reference is lost in line 50, you not put it into any variable. So the set2 object goes to the garbage collector, destroyed. - After that, you print the elements of the original set object on line 51 (for each loop), which still contains 20 elements. (PS. Hint: the task asks you to remove the necessary elements from the original set in the method.)

Alexandre Desbois

Level 10 , Chamarandes-Choignes, France

27 December 2021, 06:36

Thanks, I didn't know how to remove but now thanks to another exercise I think I know. I will create a copy of the list to iterate through it and remove the elements of the copy from the original list. Thanks for you help.

Gellert Varga

Level 23 , Szekesfehervar, Hungary

27 December 2021, 19:57

useful

Yes, this is is the one way; and the other way is to create an Iterator object, and to use its remove() method during iteration:

import java.util.*;
public class Test {
    public static void main(String[] args) {

        Set<String> set = new HashSet<>();
        set.add("Rain");
        set.add("In");
        set.add("Spain");
        set.add("and");
        set.add("in");
        set.add("France");

        // Get an iterator for the set:
        Iterator<String> iterator = set.iterator();

        // Iteration of all the elements:
        while (iterator.hasNext()) {  //  Check if there is another element

           // Get the current element and move to the next one:
           String text = iterator.next();

           // remove an element, if...:
           if (text.toLowerCase().equals("in")) { // change all of chars in the string to lower case
              iterator.remove();
           }
        }
        // display the remainder elements:
        for (String st : set) {
            System.out.println(st);
            // From the output you can see that the "In" and "in" elements have been removed from the set.
        }
    }
}