Finding duplicates in a list

CollectionDuplicateTest.java

package barky;

import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

public class CollectionDuplicateTest {

  @Test
  void findDuplicatesOldSchool() {

    final List<Integer> integers = List.of(1, 2, 3, 3, 3, 2, 4, 5, 6, 2, 5, 7, 8, 9);

    final Set<Integer> newValue = new HashSet<>();
    final Set<Integer> collector = new HashSet<>();
    for(final Integer integer: integers) {
      if (!newValue.add(integer)) {
        collector.add(integer);
      }
    }

    Assertions.assertArrayEquals(collector.toArray(), List.of(2, 3, 5).toArray());
  }
  
  @Test
  void findDuplicatesStreamy() {

    final List<Integer> integers = List.of(1, 2, 3, 3, 3, 2, 4, 5, 6, 2, 5, 7, 8, 9);

    final Set<Integer> newValue = new HashSet<>();
    final Set<Integer> collector =
        integers.stream()
          .filter(v -> !newValue.add(v))
          .collect(Collectors.toSet());

    Assertions.assertArrayEquals(collector.toArray(), List.of(2, 3, 5).toArray());
  }

  @Test
  void findDuplicatesStreamStreamyToo() {
    
    final List<Integer> integers = List.of(1, 2, 3, 3, 3, 2, 4, 5, 6, 2, 5, 7, 8, 9);

    final Set<Integer> collector =
        integers.stream()
          .filter(v -> Collections.frequency(integers, v) > 1)
          .collect(Collectors.toSet());

    Assertions.assertArrayEquals(collector.toArray(), List.of(2, 3, 5).toArray());
  }

  /**
   * Not my idea, added because it is nice and totally immutable 
   * in the stream and is not working with any external objects.
   * https://stackoverflow.com/a/52296246/460544
   */
  @Test
  void findDuplicatesStreamOnly() {
    
    final List<Integer> integers = List.of(1, 2, 3, 3, 3, 2, 4, 5, 6, 2, 5, 7, 8, 9);

    final Set<Integer> collector =
        integers.stream()
          .collect(Collectors.groupingBy(Function.identity()))
          .entrySet()
          .stream()
          .filter(e -> e.getValue().size() > 1)
          .map(Map.Entry::getKey)
          .collect(Collectors.toSet());

    Assertions.assertArrayEquals(collector.toArray(), List.of(2, 3, 5).toArray());
  }

}