Group a list of objects by an attribute and save them on a list in the same object

Question

I'm trying to group some records. I have this entity:

@Getter
@Setter
@AllArgsConstructor
@NoArgsConstructor
@Builder
@EqualsAndHashCode
@ToString
public class Candidate {

  private Integer personId;

  private String codeHswCandId;

  private List<String> codeHswCandIdRelated;

  private LocalDateTime updateDate;

}

And a list of them:

LocalDate today = LocalDate.of(2023,2,16);
LocalTime now = LocalTime.now();
LocalDate yesterday = LocalDate.of(2023,2,15);
LocalDate beforeYesterday = LocalDate.of(2023,2,14);

List<Candidate> candidateList = List.of(
    Candidate.builder().personId(1).codeHswCandId("1").updateDate(LocalDateTime.of(today, now)).build(),
    Candidate.builder().personId(1).codeHswCandId("2").updateDate(LocalDateTime.of(yesterday, now)).build(),
    Candidate.builder().personId(1).codeHswCandId("3").updateDate(LocalDateTime.of(beforeYesterday, now)).build(),
    Candidate.builder().personId(2).codeHswCandId("4").updateDate(LocalDateTime.of(today, now)).build(),
    Candidate.builder().personId(2).codeHswCandId("5").updateDate(LocalDateTime.of(yesterday, now)).build(),
    Candidate.builder().personId(2).codeHswCandId("6").updateDate(LocalDateTime.of(beforeYesterday, now)).build()
);

I need to group them into this structure:

List<Candidate> output = List.of(
    Candidate.builder().personId(1).codeHswCandId("1").codeHswCandIdRelated(List.of("2","3")).build(),
    Candidate.builder().personId(2).codeHswCandId("4").codeHswCandIdRelated(List.of("5","6")).build()
);

Group by the most recent personId with closest updateDate. I let his codeHswCandId and the other on codeHswCandIdRelated list.

Answer 1

Assuming you are using Java 12 or higher,

• use the teeing collector and collect to two maps:
• first map using personId as key and mapping to the object having the max updateDate,
• second map grouping by personId and mapping to a list of codeHswCandId
• and then iterate over the entries of the first map and build Candidate objects using personId from the key of the entry, codeHswCandId from the value of the entry and a filterd list of codeHswCandIdRelated strings from the value of the second map having the same key as the entry in the first map.

code snippet:

List<Candidate> result =
candidateList.stream()
             .collect(Collectors.teeing(
                     Collectors.toMap(Candidate::getPersonId,
                                      Function.identity(),
                                      BinaryOperator.maxBy(Comparator.comparing(Candidate::getUpdateDate))),
                     Collectors.groupingBy(Candidate::getPersonId,
                                           Collectors.mapping(Candidate::getCodeHswCandId,Collectors.toList())),
                     (map1, map2) -> map1.entrySet()
                                         .stream()
                                         .map(entry -> Candidate.builder()
                                                                .personId(entry.getKey())
                                                                .codeHswCandId(entry.getValue().getCodeHswCandId())
                                                                .codeHswCandIdRelated(map2.get(entry.getKey())
                                                                                          .stream()
                                                                                          .filter(cd -> !cd.equals(entry.getValue().getCodeHswCandId()))
                                                                                          .collect(Collectors.toList()))
                                                                .build())
                                         .collect(Collectors.toList())));

Update

For Java 11 you can still use a simillar approach, except you need to stream over the input twice to build the maps:

Map<Integer, Candidate> personIdToCandidates =
        candidateList.stream()
                     .collect(Collectors.toMap(Candidate::getPersonId,
                                               Function.identity(),
                                               BinaryOperator.maxBy(
                                                       Comparator.comparing(Candidate::getUpdateDate))));

Map<Integer, List<String>> personIdToCodeHsw =
        candidateList.stream()
                     .collect(Collectors.groupingBy(Candidate::getPersonId,
                                                    Collectors.mapping(Candidate::getCodeHswCandId,
                                                                       Collectors.toList())));

List<Candidate> result =
        personIdToCandidates.entrySet()
                            .stream()
                            .map(entry -> Candidate.builder()
                                                   .personId(entry.getKey())
                                                   .codeHswCandId(entry.getValue().getCodeHswCandId())
                                                   .codeHswCandIdRelated(personIdToCodeHsw.get(entry.getKey())
                                                                                          .stream()
                                                                                          .filter(cd -> !cd.equals(entry.getValue().getCodeHswCandId()))
                                                                                          .collect(Collectors.toList()))
                                                   .build())
                            .collect(Collectors.toList());