Java 8 streams to find the duplicate elements?

There are multiple way in Java 8 streams to find the duplicate elements.

From filter() and HashSet() method :-

Create HashSet object of Integer type.
First convert Arrays to list using Arrays.asList method
Now get the Stream data from List using arrayList.stream() method.
After Streaming,inside filter are add duplicate elements.
From Collectors.toSet() method we are collect all duplicate elements.
Print the numbers from forEach() method.

From frequency and filter method :-

First convert Arrays to list using Arrays.asList method
Now get the Stream data from List using arrayList.stream() method.
Inside filter method collections.frequency() method find the frequency of element present in the list.
If frequency is greater than one means there are multiple duplicate elements.
From Collectors.toSet() method we are collect all duplicate elements.
Print the numbers from forEach() method.

Syntax :- public static int frequency(Collection<?> c, Object o)

				
					import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class FindDuplicateElements {
	public static void main(String[] args) {
		// from filter and hashset method
		List<Integer> listOne = Arrays.asList(18, 90, 22, 80, 98, 23, 87, 20, 80, 18, 22, 65, 21, 80);
		Set<Integer> hs = new HashSet<Integer>();
		System.out.println("from filter and hashset method :- ");
		listOne.stream().filter(r -> !hs.add(r)).collect(Collectors.toSet()).forEach(System.out::println);

		// from frequency and filter method
		System.out.println("from frequency and filter method :- ");
		List<Integer> listTwo = Arrays.asList(18, 90, 22, 80, 98, 23, 87, 20, 80, 18, 22, 65, 21);
		listTwo.stream().filter(e -> Collections.frequency(listTwo, e) > 1).collect(Collectors.toSet())
				.forEach(System.out::println);
	}
}

Output :-
from filter and hashset method :-
80
18
22
from frequency and filter method :-
80
18
22