java - 将 List 分组到 Map

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 java 流等效于:

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

谢谢!

编辑:我过度简化了问题 - CustomerOrder 包含一个订单，其中包含 orderAmount

我已经设法到达 Map<Gender, List<Long>>通过类似的东西:

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()));

我没有编译这个，但应该可以工作(有小的替换)。