How to convert for each loops into Java streams and lambda function?

Question

I have a use-case in Java where I need to populate one of the lists (say x) based on the id of the other list(say y) and to fetch the result from that list.

List<LightRecruiterScholarResponse> responses = eventScholarRepository.findScholarDetailsByEventId(eventId);
List<InterviewDto> interviewResults = interviewRepository.getInterviewResultByRoundIdAndScholarId();

for (LightRecruiterScholarResponse response : responses) {
    String val = null;

    for (InterviewDto dto : interviewResults) {
        if (dto.getId().equals(response.getScholarId())) {
            val = dto.getInterviewResult();
            break;
        }
    }

    response.setInterviewStatus(val);
}

Does this answer your question? Stream Filter of 1 list based on another list — silentsudo
– silentsudo, Commented Sep 8, 2021 at 6:53

Youcef LAIDANI · Accepted Answer · 2021-09-08 06:54:39Z

1

You can use:

Map<String, String> map = interviewResults.stream()
      .collect(Collectors.toMap(InterviewDto::getId, InterviewDto::getInterviewResult));

responses.forEach(response ->
        response.setInterviewStatus(
                map.getOrDefault(response.getScholarId(), null)));

The idea is to create a Map of Id for key and InterviewResult for value, and then for each element in responses you set InterviewStatus which you can find it in the map by ScholarId which can replace if (dto.getId().equals(response.getScholarId()))

answered Sep 8, 2021 at 6:54

Youcef LAIDANI

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Comments

Oleg Cherednik · Accepted Answer · 2021-09-08 07:15:49Z

1

This could be done straightforward:

List<LightRecruiterScholarResponse> responses =
         eventScholarRepository.findScholarDetailsByEventId(eventId);
List<InterviewDto> interviewResults =
         interviewRepository.getInterviewResultByRoundIdAndScholarId();

responses.forEach(response -> response.setInterviewStatus(
                 interviewResults.stream()
                        .filter(dto -> dto.getId().equals(response.getScholarId()))
                        .map(InterviewDto::getInterviewResult)
                        .findFirst().orElse(null)));

This is not very efficient, because you iterate over interviewResults for every response. To fix it, you can build Map and use it:

List<LightRecruiterScholarResponse> responses =
         eventScholarRepository.findScholarDetailsByEventId(eventId);
Map<String, String> interviewResults =
         interviewRepository.getInterviewResultByRoundIdAndScholarId().stream()
                    .collect(Collectors.toMap(InterviewDto::getId,
                                              InterviewDto::getInterviewResult)); 
responses.forEach(response ->
   response.setInterviewStatus(interviewResults.get(response.getScholarId())));

edited Sep 8, 2021 at 7:15

answered Sep 8, 2021 at 6:54

Oleg Cherednik

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2 Comments

Sugata Kar Over a year ago

@oleg.cherednik I tried the first approach but it seems to be giving NulPointerException. Basically, I need to set the interviewResult field for the list named responses and return that list from the method.

Sugata Kar Over a year ago

@oleg.cherednik in my use case there might be a situation where there can be multiple null values being returned by the method getInterviewResult.

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