问如何合并两个Hashmap并对相同键的值求和？

EN

For all keys in table 2
  v2 = that key's value in table 2.
  v1 = that key's value in table 1.
  If v1 is null
    Set key and v2 pair into table 1.
  Otherwise
    Sum the two values
    Set key and sum pair in table 1.
Done.

public static List<EnergyUsageData> process(final Collection<EnergyUsageData> data) {
    final Map<EnergyType, Map<YearMonth, BigDecimal>> tempMap = new HashMap<>();
    final List<EnergyUsageData> energyUsageData = data.stream()
            .flatMap(d -> d.getBillsMap().entrySet().stream().map(
                    dd -> new FlatData(d.getType(), dd.getKey(), dd.getValue())))
            .filter(d -> {
                final Map<YearMonth, BigDecimal> map = tempMap.get(d.type);
                if (map != null) {
                    final BigDecimal amount = map.get(d.yearMonth);
                    map.put(d.yearMonth, amount.add(d.amount));
                    return false;
                }
                return true;
            })
            .map(d -> {
                final HashMap<YearMonth, BigDecimal> billsMap = new HashMap<>();
                billsMap.put(d.yearMonth, d.amount);
                tempMap.put(d.type, billsMap);
                return new EnergyUsageData(d.type, billsMap);
                //Assuming Constructor of EnergyUsageData will hold
                // the reference of billsMap and will not copy billsMap
            })
            .collect(Collectors.toList());
    return energyUsageData;
}

final class FlatData {
    public final EnergyType type;
    public final YearMonth yearMonth;
    public final BigDecimal amount;

    FlatData(EnergyType type, YearMonth yearMonth, BigDecimal amount) {
        this.type = type;
        this.yearMonth = yearMonth;
        this.amount = amount;
    }
}