stream如何实现分组后，多个字段分别求和？

参考如下假例代码,请修改再实现:

        List<UserDTO> users = userMapper.getUserAllListFromDb();
        Map<Integer, List<UserDTO>> mapList = users.stream().collect(Collectors.groupingBy(UserDTO::getGender));
        List<UserDTO> manUserList = mapList.get("1"); //假设代表男
        BigDecimal ageTotal = manUserList.stream().map(UserDTO::getAge).reduce(BigDecimal::add).orElse(BigDecimal.ZERO);
        BigDecimal lengthTotal = manUserList.stream().map(UserDTO::getLength).reduce(BigDecimal::add).orElse(BigDecimal.ZERO);
        System.out.println("总年龄:" + ageTotal);
        System.out.println("总身高:" + lengthTotal);

可以做一个 computeIfAbsent