Stream分组并获取某字段最大数据，求和

DrawRewardServiceImpl.java

//语法上：
 Map<Long, ActivityOrderEntity> orderMaps = orders.stream()
     .collect(Collectors.groupingBy(ActivityOrderEntity::getUserId,
           Collectors.collectingAndThen( 
              Collectors.reducing((c1, c2) -> c1.getRoundNum() > c2.getRoundNum() ? c1 : c2), Optional::get)
      ));
//推荐以下：
Map<Long, ActivityOrderEntity> orderMaps = orders.stream()
    .collect(Collectors.toMap(ActivityOrderEntity::getUserId,
      Function.identity(), (c1, c2) -> c1.getRoundNum() > c2.getRoundNum() ? c1 : c2));
// 分组求和
fundTradeFlowList.stream()
    .filter(r -> FundBizCodeEnum.BUY.getCode().equals(r.getBizCode()))
    .collect(Collectors.groupingBy(FundTradeFlowBO::getFundCode,
            Collectors.reducing(BigDecimal.ZERO, FundTradeFlowBO::getAmount, this::sumBigDecimal)));
 private BigDecimal sumBigDecimal(BigDecimal assetA, BigDecimal assetB) {
        if (assetA == null && assetB == null) {
            return BigDecimal.ZERO;
        }
        if (assetA == null) {
            return assetB;
        }
        if (assetB == null) {
            return assetA;
        }
        return assetA.add(assetB);
    }