gistfile1.java

private static Map<Long, Long> multiDataPointsToPerTxnSummedValue(List<MultiDataPoint> countsList)
    {
        Map<DateTime, Map<Long, Long>> timeStampToTxnIdToCountsMap = new HashMap<>();
        for (MultiDataPoint multiDataPoint : countsList)
        {
            timeStampToTxnIdToCountsMap.put(multiDataPoint.getTimestamp(), convertToPerTxnData(multiDataPoint.getData()));
        }

        return timeStampToTxnIdToCountsMap.entrySet().stream()
                .flatMap(e -> e.getValue().entrySet().stream().map(ex ->
                                new Object()
                                {
                                    Long txId = ex.getKey();
                                    DataPoint dataPoint = new DataPoint(e.getKey(), ex.getValue());
                                }
                ))
                .collect(
                        groupingBy(t -> t.txId, mapping(t -> t.dataPoint, Collectors.summingLong(dataPoint -> dataPoint.getValue().longValue())))
                );
    }