[开发测试]Stream练习——groupingBy分组

 *需求说明：
 * 涉及一个对外提供服务的接口，支持调用方传入多个账户编号查询订单

public class Case_4 {

    @Data
    @AllArgsConstructor
    class Order {
        private Integer orderId;
        private String accountId;
    }

    /**
     * 模拟数据库查询
     * @param accountIds
     * @return
     */
    public List<Order> selectFromDB(List<String> accountIds){
        List<Order> orderList = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            orderList.add(
                    new Order(i,accountIds.get(i%accountIds.size( )))
            );
        }
        return orderList;
    }

    /**
     * 这个接口符合原需求，但是对调用方不够友好，因为查询出的数据还需要调用方去分组
     * @param accountIds
     * @return
     */
    public List<Order> queryOrderByAccountIds(List<String> accountIds){
        List<Order> orderList = selectFromDB(accountIds);
        return orderList;
    }

    /**
     * 这个接口不仅符合需求，且对结果进行了分类包装，让调用者一目了然
     * @param accountIds
     * @return
     */
    public Map<String,List<Order>> queryOrderByAccountIdsMap(List<String> accountIds){
        Map<String, List<Order>> result = Optional.ofNullable(selectFromDB(accountIds))
                .map(List::stream)
                .orElseGet(Stream::empty)
                .collect(Collectors.groupingBy(
                        order -> order.getAccountId()
                ));

        return result;
    }

    @Test
    public void test(){
        List<Order> orders_1 = queryOrderByAccountIds(Lists.newArrayList("张三", "李四", "王五"));
        Map<String, List<Order>> orders_2 = queryOrderByAccountIdsMap(
                Lists.newArrayList("张三", "李四", "王五")
        );

        System.out.println(JSON.toJSONString(orders_1,true));
        System.out.println("==========两种结果对比参考=============");
        System.out.println(JSON.toJSONString(orders_2,true));
    }
}

j结果展示

[
{"accountId":"张三","orderId":0},
{"accountId":"李四","orderId":1},
{"accountId":"王五","orderId":2},
{"accountId":"张三","orderId":3},
{"accountId":"李四","orderId":4},
{"accountId":"王五","orderId":5},
{"accountId":"张三","orderId":6},
{"accountId":"李四","orderId":7},
{"accountId":"王五","orderId":8},
{"accountId":"张三","orderId":9}
]
==========两种结果对比参考=============
{
"李四":
[{"accountId":"李四","orderId":1},
{"accountId":"李四","orderId":4},
{"accountId":"李四","orderId":7}],
"张三":
[{"accountId":"张三","orderId":0},
{"accountId":"张三","orderId":3},
{"accountId":"张三","orderId":6},
{"accountId":"张三","orderId":9}],
"王五":
[{"accountId":"王五","orderId":2},
{"accountId":"王五","orderId":5},
{"accountId":"王五","orderId":8}]
}