Java – get all rejected additions to set

I'm not very good at hashsets. I want to know if I can use them The addall method retrieves duplicates deleted by hashes So in my code, it asks the user to enter a value of up to 20, and if it is - 1, it interrupts Then delete the duplicates and display the completed list But now I want to display a list of deleted duplicates For example: 2 = 3 repetitions, 5 = 5 repetitions This is my code:

import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.Scanner;
import java.util.Set;

public class Question {
    public static void main(String[] args) {
        ArrayList<Integer> entries = new ArrayList<Integer>();
        Scanner input = new Scanner(system.in);
        int counter = 0;
        int sentinel = -1;
        while (entries.size() <= 20) {

            System.out.println("Please enter some numbers into the entries list");
            entries.add(input.nextInt());
            if (entries.get(counter) == sentinel) {
                entries.remove(counter);
                break;
            }
            System.out.println("You've added: " + entries.get(counter) + ". So far your list is as shows: " + entries);
            counter++;

        }

        System.out.println("The list is: " + entries);
        System.out.println("Checking for Duplicates...");

        Set<Integer> num = new LinkedHashSet<>(entries);
        num.addAll(entries); // The addAll method deletes duplicates
        System.out.println(num);
        entries.clear();
        entries.addAll(num);

        System.out.println("The Finished list is: " + entries);

    }

}

Solution

Unfortunately, set does not track all items it rejects You must do this manually Quick and dirty methods are as follows:

Set<Integer> noDuplicates = new LinkedHashSet<Integer>();
List<Integer> rejects = new ArrayList<Integer>();
for(Integer entry : entry) {
   if(!noDuplicates.add(entry)) { rejects.add(entry) }
}

Note that when trying to add a duplicate element to the set, the add method returns false and rejects the add request