Question

Map<Integer, HashMap<String, Integer>> map = new HashMap<Integer, 

HashMap<String, Integer>>();
map.put(1, new HashMap<>());
map.get(1).put("123",5);
map.get(1).put("124",3);
// i store (id, isbn, rate) in Hashmap in Hashmap

map.put(2, new HashMap<>());
map.get(2).put("123",5);
map.get(2).put("122",2);

我如何从isbn获取ID？

示例我想获取阅读isbn 123的用户的ID？谢谢。

Answer 1

您需要按步骤进行思考：

遍历第一对地图
对于每个对象，遍历其对（第二级地图）
如果您发现一对isbn好的商品，请保存1-lvl地图的ID

您可以按照以下方法构建方法，并像这样调用

List<Integer> listId = getIdFromIsbn("123", map);

static List<Integer> getIdFromIsbn(String isbn, Map<Integer, Map<String, Integer>> map) {
    List<Integer> list = new ArrayList<>();

    for (Map.Entry<Integer, Map<String, Integer>> entry : map.entrySet()) {
        Map<String, Integer> value = entry.getValue();
        for (Map.Entry<String, Integer> subEntry : value.entrySet()) {
            if (subEntry.getKey().equals(isbn)) {
                list.add(entry.getKey());
            }
        }
    }

    return list;
}

使用Stream和lambda，它看起来像：

static List<Integer> getIdFromIsbn(String isbn, Map<Integer, Map<String, Integer>> map) {
    return map.entrySet()                           // Set<Entry<Integer,Map<String,Integer>>>
            .stream()                               // Stream<Entry<Integer,Map<String,Integer>>>
            .flatMap(entry -> entry.getValue().entrySet() // Set<Entry<String,Integer>>
                    .stream()                             // Stream<Entry<String,Integer>>
                    .map(Map.Entry::getKey)               // Stream<String> 
                    .filter(isbn::equals)                 // Stream<String> 
                    .map(subEntry -> entry.getKey()))     // Stream<Integer>
            .collect(Collectors.toList());                // List<Integer>
}

Answer 2

您可以直接迭代地图：

 SELECT dt2.id,
       dt2.NAME,
       dt2.state,
       dt2.type,
       dt2.count
FROM   (SELECT dt1.id,
               dt1.NAME,
               dt1.state,
               dt1.type,
               dt1.count,
               Row_number()
                 OVER (
                   partition BY dt1.state
                   ORDER BY dt1.count DESC) AS row_num
        FROM   (SELECT c.id,
                       c.NAME,
                       c.state,
                       t.NAME   AS type,
                       Count(*) AS count
                FROM   bookings_facilities AS f
                       JOIN bookings AS b
                         ON b.id = f.booking_id
                       JOIN clients AS c
                         ON c.id = b.client_id
                       JOIN client_types AS t
                         ON c.type = t.id
                WHERE  t.NAME = 'School'
                GROUP  BY c.id,
                          c.NAME,
                          c.state,
                          type) AS dt1) AS dt2
WHERE  dt2.row_num = 1

Answer 3

遍历第一级地图，然后遍历嵌套地图以执行检查。

以下代码部分将打印给定id的所有targetIsbn

    for (Entry<Integer, HashMap<String, Integer>> entry : map.entrySet()) {
        int id = entry.getKey();
        Map<String, Integer> value = entry.getValue();

        for (Entry<String, Integer> subEntry : value.entrySet()) {
            String isbn = subEntry.getKey();
            int rate = subEntry.getValue();
            if (isbn.equals(targetIsbn))
                System.out.println("given isbn found for id " + id);
        }
    }

Hashmap搜索中的Hashmap

3 个答案: