java - 使用 Java 8 Stream 返回无副作用的 Map of Map

Question

如何将带有键 oldReportId 的 eServiceReportsMapByBatchFile 放入/添加到 eServiceReportMap 而没有副作用？

Map<String, Map<String, Set<EServiceReport>>> eServiceReportMap = new HashMap<>();
reports.forEach(report -> {
    String oldReportId = report.getOldId();
        Map<String, Set<EServiceReport>> eServiceReportsMapByBatchFile = // processing of batch files
        ...
    eServiceReportMap.put(oldReportId, eServiceReportsMapByBatchFile);
});

return eServiceReportMap;

也就是我想让它变成这样:

return reports.stream()
    .map(report -> {
        String oldReportId = report.getOldId();
        Map<String, Set<EServiceReport>> eServiceReportsMapByBatchFile = // processing of batch files
        ...
        // I don't know how and what to return here
    }).collect(// I don't know what to do here);

谢谢。

Answer 1

您最期待的是 Collectors.toMap，它可以用作:

return reports.stream()
        .collect(Collectors.toMap(report -> report.getOldId(),
                        report -> {
                    // batch processing for eServiceReportsMapByBatchFile
                            return eServiceReportsMapByBatchFile;
                }));