How to get object field for max value in another field in grouped by one more field?

user_9273157 0
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I have list of entities that looks like this:

class History {
    public String code;
    public Integer version;
    public LocalDateTime creationDate;
}

For each code with max version I wanna to get creationDate. I did it like this, but I think that it can be done somehow easier...

Map<String, History> historyMaxVersion = histories.stream()
              .collect(Collectors.toMap(History::getCode, Function.identity(),
               BinaryOperator.maxBy(Comparator.comparing(History::getVersion))));

Map<String, LocalDateTime> historiesLastUpdatedDates = historyMaxVersion.entrySet().stream()
.collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue()
                        .getCreationDate()));
java-stream java-11 java
1 Answers

You can do like this:

After grouping by each code then use collectingAndThen collectors and find max item based on version amount and at the end use map to extract creationCode.

import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.maxBy;

Map<String, LocalDateTime> result = histories.stream()
      .collect(Collectors.groupingBy(History::getCode,
            collectingAndThen(maxBy(Comparator.comparing(History::getVersion)),
                  history -> history.map(History::getCreationDate).orElse(null))));
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