stream 示例
//分组，返回mapping
Collectors.groupingBy(AccountPayRecord::getMode, HashMap::new,
Collectors.mapping(AccountPayRecord::getTotalFee, Collectors.reducing(new Money(), (d1, d2) -> d1.add(d2)))));
//排序，拼接
installmentIdList.stream().map(id -> installmentMap.get(id))
.sorted(Comparator.comparing(OrderInstallment::getLogicNumber))
.map(in -> in.getLogicNumber() + "段").collect(Collectors.joining("、"));
