在要初始化列表的映射中查找枚举值

Question

Given I have the following Map and call to writeMetadata(...) :

Map<Report, TransferStatus> transferStatusMap = sftpService.transfer(reportDeliverables);

dbMetadataWriter.writeMetadata(successReports);

ENUM as:

public enum TransferStatus {
    SUCCESS,
    FAILURE
}

Firstly, how can I create the successReport list with all Report with status TransferStatus.SUCCESS with Java 8?

Secondly, before call to dbMetadataWriter.writeMetadata(successReports); there should be a check to see if any values with success exist.

and finally, in general how should failed transfers be handled? Practically, what is best practice?

Answer 1

 List<Report> successReports = transferStatusMap.entrySet()
          .stream()
          .filter(e -> e.getValue() == TransferStatus.SUCCESS)
          .map(Entry::getKey)
          .collect(Collectors.toList());

 if(!reports.isEmpty()){
     dbMetadataWriter.writeMetadata(successReports);
 }

Or you could collect them both to a Map :

Map<TransferStatus, List<Report>> map = 
     transferStatusMap.entrySet()
                      .stream()
                      .collect(Collectors.groupingBy(
                            Entry::getValue,
                            Collectors.mapping(Entry::getKey, Collectors.toList()));

Answer 2

You can create the list of successReports as:

Map<Report, TransferStatus> transferStatusMap = sftpService.transfer(reportDeliverables);

List<Report> successReports = transferStatusMap.entrySet().stream().filter(e -> e.getValue().equals(TransferStatus.SUCCESS))
            .map(Map.Entry::getKey)
            .collect(Collectors.toList());

---

Similarily writeMetaData if any such entry in the map exists

Map<Report, TransferStatus> transferStatusMap = sftpService.transfer(reportDeliverables);

if(transferStatusMap.entrySet().stream().anyMatch(x->x.getValue().equals(TransferStatus.SUCCESS))) {
        dbMetadataWriter.writeMetadata(successReports);
}

Note : If these are consecutive or sequential statements, @Eugene's solution is better in doing isEmpty() check over the existing datatype.