Java 8 filtering list of objects by unique name while only keeping highest ID?

Question

Assume we have a person class with fields:

Class Person {
  private String name;
  private Integer id (this one is unique);
}

And then we have a List<Person> people such that:

['Jerry', 993]
['Tom', 3]
['Neal', 443]
['Jerry', 112]
['Shannon', 259]
['Shannon', 533]

How can I make a new List<Person> uniqueNames such that it filters for unique names only AND keeps the highest ID of that name.

So the end list would look like:

['Jerry', 993]
['Tom', 3]
['Neal', 443]
['Shannon', 533]

Answer 1

Collectors.groupingBy + Collectors.maxBy should do the trick to build the map of persons grouped by name and then selecting the max value:

List<Person> persons = Arrays.asList(
    new Person("Jerry", 123),
    new Person("Tom", 234),
    new Person("Jerry", 456),
    new Person("Jake", 789)
);

List<Person> maxById = persons
    .stream()
    .collect(Collectors.groupingBy(
        Person::getName, 
        Collectors.maxBy(Comparator.comparingInt(Person::getID))
    ))
    .values() // Collection<Optional<Person>>
    .stream() // Stream<Optional<Person>>
    .map(opt -> opt.orElse(null))
    .collect(Collectors.toList());

System.out.println(maxById);

Output:

[789: Jake, 234: Tom, 456: Jerry]

---

Update

is there a way to get a separate list of the Person object who were deleted because they were duplicates within this stream()?

It may be better to collect the grouped items in a list which should be converted then in some wrapper class providing information about the maxById person and the list of deduped persons:

class PersonList {
    private final Person max;
    private final List<Person> deduped;
    
    public PersonList(List<Person> group) {
        this.max = Collections.max(group, Comparator.comparingInt(Person::getID));
        this.deduped = new ArrayList<>(group);
        this.deduped.removeIf(p -> p.getID() == max.getID());
    }
    
    @Override
    public String toString() {
        return "{max: " + max + "; deduped: " + deduped + "}";
    }
}

Then the persons should be collected like this:

List<PersonList> maxByIdDetails = new ArrayList<>(persons
    .stream()
    .collect(Collectors.groupingBy(
        Person::getName, 
        LinkedHashMap::new,
        Collectors.collectingAndThen(
            Collectors.toList(), PersonList::new
        )
    ))
    .values()); // Collection<PersonList>

maxByIdDetails.forEach(System.out::println);

Output:

{max: 456: Jerry; deduped: [123: Jerry]}
{max: 234: Tom; deduped: []}
{max: 789: Jake; deduped: []}

---

Update 2

Getting list of duplicated persons:

List<Person> duplicates = persons
    .stream()
    .collect(Collectors.groupingBy(Person::getName))
    .values() // Collection<List<Person>>
    .stream() // Stream<List<Person>>
    .map(MyClass::removeMax)
    .flatMap(List::stream) // Stream<Person>
    .collect(Collectors.toList()); // List<Person>

System.out.println(duplicates);

Output:

[123: Jerry]

where removeMax may be implemented like this:

private static List<Person> removeMax(List<Person> group) {
    List<Person> dupes = new ArrayList<>();
    Person max = null;

    for (Person p : group) {
        Person duped = null;
        if (null == max) {
            max = p;
        } else if (p.getID() > max.getID()) {
            duped = max;
            max = p;
        } else {
            duped = p;
        }
        if (null != duped) {
            dupes.add(duped);
        }
    }
    return dupes;
}

Or, providing that hashCode and equals are implemented properly in class Person, the difference between the two lists may be calculated using removeAll:

List<Person> duplicates2 = new ArrayList<>(persons);
duplicates2.removeAll(maxById);
System.out.println(duplicates2);

Answer 2

You could try:

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

persons.stream()
       .collect(
          groupingBy(
            Person::getName, 
            collectingAndThen(
              maxBy(comparingInt(Person::getId)), 
              Optional::get
            )
          )
       )
       .values()
       ;

• You group by name
• Then you request the max of grouped person (per name)
• Then you returns the values (since groupingBy returns a Map<String, Optional<Person>>, the collectAndThen call's Optional::get).

Note that this will list unique names, but not duplicates names.

Answer 3

You can use Collectors#toMap like this.

record Person(String name, Integer id) {}

public static void main(String[] args) {
    List<Person> input = List.of(
        new Person("Jerry", 993),
        new Person("Tom", 3),
        new Person("Neal", 443),
        new Person("Jerry", 112),
        new Person("Shannon", 259),
        new Person("Shannon", 533));

    List<Person> output = input.stream()
        .collect(Collectors.toMap(Person::name, Function.identity(),
            (a, b) -> a.id() > b.id() ? a : b, LinkedHashMap::new))
        .values().stream().toList();

    for (Person e : output)
        System.out.println(e);
}

output:

Person[name=Jerry, id=993]
Person[name=Tom, id=3]
Person[name=Neal, id=443]
Person[name=Shannon, id=533]

You can omit the , LinkedHashMap::new if you don't care about the order.

Answer 4

is there a way to get a separate list of the Person object who were deleted because they were duplicates within this stream()?

private static final Map<String, Person> highestIds = new HashMap<>();
private static final List<Person> duplicates = new ArrayList<>();

public static void main(String[] args) {
    for (Person person : people) {
        Person result = highestIds.get(person.name);
        if (isPresent(result) && person.id > result.id) {
            duplicates.add(result);
            highestIds.put(person.name, person);
        } else if (result == null) {
            highestIds.put(person.name, person);
        } else {
            duplicates.add(person);
        }
    }
    System.out.println("Highest ids:");
    highestIds.values().forEach(System.out::println);
    System.out.println("Duplicates:");
    duplicates.forEach(System.out::println);
}

private static boolean isPresent(Person result) {
    return result != null;
}