Return Map of Map without side-effects using Java 8 Stream

Julez :

How do I put/add eServiceReportsMapByBatchFile with key oldReportId to eServiceReportMap without side-effects?

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;

That is, I want it to become like this:

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);

Thank you.

Naman :

You're looking forward mostly to Collectors.toMap which can be used as :

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