Extract duplicate objects from a List in Java 8

Question

This code removes duplicates from the original list, but I want to extract the duplicates from the original list -> not removing them (this package name is just part of another project):

Given:

a Person pojo:

package at.mavila.learn.kafka.kafkaexercises;

import org.apache.commons.lang3.builder.ToStringBuilder;

public class Person {

private final Long id;
private final String firstName;
private final String secondName;


private Person(final Builder builder) {
    this.id = builder.id;
    this.firstName = builder.firstName;
    this.secondName = builder.secondName;
}


public Long getId() {
    return id;
}

public String getFirstName() {
    return firstName;
}

public String getSecondName() {
    return secondName;
}

public static class Builder {

    private Long id;
    private String firstName;
    private String secondName;

    public Builder id(final Long builder) {
        this.id = builder;
        return this;
    }

    public Builder firstName(final String first) {
        this.firstName = first;
        return this;
    }

    public Builder secondName(final String second) {
        this.secondName = second;
        return this;
    }

    public Person build() {
        return new Person(this);
    }


}

@Override
public String toString() {
    return new ToStringBuilder(this)
            .append("id", id)
            .append("firstName", firstName)
            .append("secondName", secondName)
            .toString();
}
}

Duplication extraction code.

Notice here we filter the id and the first name to retrieve a new list, I saw this code someplace else, not mine:

package at.mavila.learn.kafka.kafkaexercises;

import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

import static java.util.Objects.isNull;

public final class DuplicatePersonFilter {


private DuplicatePersonFilter() {
    //No instances of this class
}

public static List<Person> getDuplicates(final List<Person> personList) {

   return personList
           .stream()
           .filter(duplicateByKey(Person::getId))
           .filter(duplicateByKey(Person::getFirstName))
           .collect(Collectors.toList());

}

private static <T> Predicate<T> duplicateByKey(final Function<? super T, Object> keyExtractor) {
    Map<Object,Boolean> seen = new ConcurrentHashMap<>();
    return t -> isNull(seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE));

}

}

The test code. If you run this test case you will get [alex, lolita, elpidio, romualdo].

I would expect to get instead [romualdo, otroRomualdo] as the extracted duplicates given the id and the firstName:

package at.mavila.learn.kafka.kafkaexercises;


import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.ArrayList;
import java.util.List;

import static org.junit.Assert.*;

public class DuplicatePersonFilterTest {

private static final Logger LOGGER = LoggerFactory.getLogger(DuplicatePersonFilterTest.class);



@Test
public void testList(){

    Person alex = new Person.Builder().id(1L).firstName("alex").secondName("salgado").build();
    Person lolita = new Person.Builder().id(2L).firstName("lolita").secondName("llanero").build();
    Person elpidio = new Person.Builder().id(3L).firstName("elpidio").secondName("ramirez").build();
    Person romualdo = new Person.Builder().id(4L).firstName("romualdo").secondName("gomez").build();
    Person otroRomualdo = new Person.Builder().id(4L).firstName("romualdo").secondName("perez").build();


    List<Person> personList = new ArrayList<>();

    personList.add(alex);
    personList.add(lolita);
    personList.add(elpidio);
    personList.add(romualdo);
    personList.add(otroRomualdo);

    final List<Person> duplicates = DuplicatePersonFilter.getDuplicates(personList);

    LOGGER.info("Duplicates: {}",duplicates);

}

}

In my job I was able to get the desired result it by using Comparator using TreeMap and ArrayList, but this was creating a list then filtering it, passing the filter again to a newly created list, this looks bloated code, (and probably inefficient)

Does someone has a better idea how to extract duplicates?, not remove them.

Thanks in advance.

Update

Thanks everyone for your answers

To remove the duplicate using same approach with the uniqueAttributes:

  public static List<Person> removeDuplicates(List<Person> personList) {
    return getDuplicatesMap(personList).values().stream()
            .filter(duplicates -> duplicates.size() > 1)
            .flatMap(Collection::stream)
            .collect(Collectors.toList());
}

private static Map<String, List<Person>> getDuplicatesMap(List<Person> personList) {
    return personList.stream().collect(groupingBy(DuplicatePersonFilter::uniqueAttributes));
}

private static String uniqueAttributes(Person person){

    if(Objects.isNull(person)){
        return StringUtils.EMPTY;
    }

    return (person.getId()) + (person.getFirstName()) ;
}

Update 2

But also the answer provided by @brett-ryan is correct:

public static List<Person> extractDuplicatesWithIdentityCountingV2(final List<Person> personList){

        List<Person> duplicates = personList.stream()
                .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
                .entrySet().stream()
                .filter(n -> n.getValue() > 1)
                .flatMap(n -> nCopies(n.getValue().intValue(), n.getKey()).stream())
                .collect(toList());

        return duplicates;

    }

EDIT

Above code can be found under:

https://gitlab.com/totopoloco/marco_utilities/-/tree/master/duplicates_exercises

Please see:

Usage: https://gitlab.com/totopoloco/marco_utilities/-/blob/master/duplicates_exercises/src/test/java/at/mavila/exercises/duplicates/lists/DuplicatePersonFilterTest.java

Implementation: https://gitlab.com/totopoloco/marco_utilities/-/blob/master/duplicates_exercises/src/main/java/at/mavila/exercises/duplicates/lists/DuplicatePersonFilter.java

Answer 1

To indentify duplicates, no method I know of is better suited than Collectors.groupingBy(). This allows you to group the list into a map based on a condition of your choice.

Your condition is a combination of id and firstName. Let's extract this part into an own method in Person:

String uniqueAttributes() {
  return id + firstName;
}

The getDuplicates() method is now quite straightforward:

public static List<Person> getDuplicates(final List<Person> personList) {
  return getDuplicatesMap(personList).values().stream()
      .filter(duplicates -> duplicates.size() > 1)
      .flatMap(Collection::stream)
      .collect(Collectors.toList());
}

private static Map<String, List<Person>> getDuplicatesMap(List<Person> personList) {
  return personList.stream().collect(groupingBy(Person::uniqueAttributes));
}

• The first line calls another method getDuplicatesMap() to create the map as explained above.
• It then streams over the values of the map, which are lists of persons.
• It filters out everything except lists with a size greater than 1, i.e. it finds the duplicates.
• Finally, flatMap() is used to flatten the stream of lists into one single stream of persons, and collects the stream to a list.

---

An alternative, if you truly identify persons as equal if the have the same id and firstName is to go with the solution by Jonathan Johx and implement an equals() method.