find Duplicate Element from a List of Integer without using Stream's distinct method

Recently this question is asked in Interview :
How to find the Duplicate Elements from a List of Integer without using java Stream’s distinct method ?

This should be done by Java’s Stream API but should not use distinct() method.

for e.g List listOfInt = new ArrayList();

>Solution :

You can do this using frequency collectors. Have not looked at optimization , but this will not use distinct

import java.util.Set;
import java.util.List;
import java.util.ArrayList;
import static java.util.stream.Collectors.toSet;
import java.util.Collections;

public class DetectDuplicates{
    public static void main(String args[]) {
      int x=10;
      int y=25;
      int z=x+y;
      List<Integer> companyIds = new ArrayList<Integer>();
      companyIds.add(1);
      companyIds.add(1);
      companyIds.add(2);
      companyIds.add(3);
      
      Set<Integer> duplicateCompanies = companyIds
                .stream()
                .filter(company -> Collections.frequency(companyIds, company) > 1)
                .collect(toSet());
      System.out.println("Duplicate companies " + duplicateCompanies);
    }
}

This will print

Duplicate companies [1, 3]