将List <CustomerOrder>分组到Map <Gender,Money>中

Question

public class CustomerOrder {

    private final Gender gender;
    private final Order order;

    public Gender getGender() {
        return gender;
    }

    public Order getOrder() {
        return order;
    }

}

public class Order {

    private final long orderAmount;

    public long getOrderAmount() {
        return orderAmount;
    }

}

我想实现相当于java的java流:

SELECT Gender, SUM(orderAmount) FROM orders
GROUP BY Gender;

谢谢!

编辑:我简化了问题 - CustomerOrder包含一个Order,其中包含orderAmount

我设法通过以下方式获得Map<Gender, List<Long>>:

Map<Gender, List<Long>> map = orders.stream()
    .collect(Collectors.groupingBy(CustomerOrder::getGender))

Answer 1

list.stream().collect(Collectors.groupingBy(CustomerOrder::getGender,
            Collectors.summingLong(CustomerOrder::getOrderAmmount));

编辑

list.stream().collect(Collectors.groupingBy(CustomerOrder::getGender,
            Collectors.summingLong(co -> co.getOrder().getAmmount()));

我没有编译它,但应该工作(有少量替换).