How to find duplicate elements in Java Stream?

We can identify duplicate elements in a Java Stream by making use of the Collectors.groupingBy collector with Collectors.counting.

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Example {

    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 2, 3, 3, 5, 4);

        List<Integer> duplicates = numbers.stream()
                .collect(Collectors.groupingBy(e -> e, Collectors.counting()))
                .entrySet().stream()
                .filter(entry -> entry.getValue() > 1)
                .map(Map.Entry::getKey)
                .collect(Collectors.toList());

        System.out.println("Duplicate in " + numbers + " are : " + duplicates);

    }
}