Finding duplicated objects by two properties

user_8429747 0
Viewed 10360

Considering that I have a list of Person objects like this :

Class Person {
  String fullName;
  String occupation;
  String hobby;
  int salary;
}

Using java8 streams, how can I get list of duplicated objects only by fullName and occupation property?

java-8 java
4 Answers

By using java-8 Stream() and Collectors.groupingBy() on firstname and occupation

List<Person> duplicates = list.stream()
    .collect(Collectors.groupingBy(p -> p.getFullName() + "-" + p.getOccupation(), Collectors.toList()))
    .values()
    .stream()
    .filter(i -> i.size() > 1)
    .flatMap(j -> j.stream())
    .collect(Collectors.toList());
user_99591520

I need to find if they were any duplicates in fullName - occupation pair, which has to be unique

Based on this comment it seems that you don't really care about which Person objects were duplicated, just that there were any.

In that case you can use a stateful anyMatch:

Collection<Person> input = new ArrayList<>();

Set<List<String>> seen = new HashSet<>();
boolean hasDupes = input.stream()
                        .anyMatch(p -> !seen.add(List.of(p.fullName, p.occupation)));

You can use a List as a 'key' for a set which contains the fullName + occupation combinations that you've already seen. If this combination is seen again you immediately return true, otherwise you finish iterating the elements and return false.

user_36991390

I offer solution with O(n) complexity. I offer to use Map to group given list by key (fullName + occupation) and then retrieve duplicates.

public static List<Person> getDuplicates(List<Person> persons, Function<Person, String> classifier) {
    Map<String, List<Person>> map = persons.stream()
                                           .collect(Collectors.groupingBy(classifier, Collectors.mapping(Function.identity(), Collectors.toList())));

    return map.values().stream()
              .filter(personList -> personList.size() > 1)
              .flatMap(List::stream)
              .collect(Collectors.toList());
}

Client code:

List<Person> persons = Collections.emptyList();
List<Person> duplicates = getDuplicates(persons, person -> person.fullName + ':' + person.occupation);
user_34613970

First implement equals and hashCode in your person class and then use.

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

Set<Person> duplicates=personList.stream().filter(p -> Collections.frequency(personList, p) ==2)
                .collect(Collectors.toSet());

If objects are more than 2 then you use Collections.frequency(personList, p) >1 in filter predicate.

user_46231290
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