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("、"));