Java streams SQL laadne GROUP BY

Kui vaja teha Java mingi class listidega SQL laadne GROUP BY arvutus

InventoryPositionsViewRate on algne klass mis on vaja summeerida tInventorySummaryViewRate listiks

private void calculateSummary(final List<InventoryPositionsViewRate> ipwr) {
        // Calculates sum GROUP BY branch, currency
        this.listInventorySummaryViewRate = ipwr.parallelStream()
                .collect(Collectors.groupingBy(Function.identity(),
                        () -> new TreeMap<>(Comparator.<InventoryPositionsViewRate, String> comparing(InventoryPositionsViewRate::getBranchDisplayName)
                                .thenComparing(InventoryPositionsViewRate::getCurrencyCode)
                                .thenComparing(InventoryPositionsViewRate::getRate)), //getRate is nessesary for localAmount calculating
                        Collectors.summingDouble(foo -> foo.getAmount().doubleValue())))
                .entrySet().stream()
                .map(iswr -> new InventoryPositionsSummaryRate(iswr.getKey().getBranchDisplayName(),
                        iswr.getKey().getCurrencyCode(), BigDecimal.valueOf(iswr.getValue()), iswr.getKey().getRate()))
                .collect(Collectors.toList());
    }