Java 8 - find duplicate elements in List

In this article, we would like to show you how to find duplicate elements in List working with Streams in Java 8.

Practical example

In this example, we filter letters stream and use Collections.frequency() to find duplicate elements.

import java.util.*;
import java.util.stream.Collectors;

public class Example {

    public static void main(String[] args) {
        // create list of strings
        List<String> letters = Arrays.asList("A", "B", "C", "B", "A");

        // find duplicate elements
        letters.stream().filter(i -> Collections.frequency(letters, i) > 1)
                .collect(Collectors.toSet()).forEach(System.out::println);
    }
}

Output:

A
B