[英]Invert a Map with redundant values to produce a multimap
給定這樣的 map,我們有一年中每周的頻率計數:
Map.of(
DayOfWeek.MONDAY , 52 ,
DayOfWeek.TUESDAY , 52 ,
DayOfWeek.WEDNESDAY, 53 ,
DayOfWeek.THURSDAY , 53 ,
DayOfWeek.FRIDAY , 52 ,
DayOfWeek.SATURDAY , 52 ,
DayOfWeek.SUNDAY , 52
)
…或作為文本:
{周一=52,周二=52,周三=53,周四=53,周五=52,周六=52,周日=52}
...我如何反轉以生成不同數字的多重映射,每個數字都導致擁有該數字的DayOfWeek
的集合(列表?集合?)?
結果應該等同於這段代碼的結果:
Map.of(
53 , List.of( DayOfWeek.WEDNESDAY , DayOfWeek.THURSDAY ) ,
52 , List.of( DayOfWeek.MONDAY , DayOfWeek.TUESDAY , DayOfWeek.FRIDAY , DayOfWeek.SATURDAY , DayOfWeek.SUNDAY )
)
我想使用直接的Java生成生成的多圖,而不需要額外的庫,例如Eclipse Collections或Google Guava 。 這些庫可能會使這更容易,但我很好奇是否可以使用僅使用內置 Java 的解決方案。 否則,我的問題與Guava 完全相同:通過反轉 Map 來構造多圖。 鑒於現代 Java 中的新流和多地圖功能,我希望現在這是可能的,而當時還沒有。
我看到了與此類似的各種現有問題。 但沒有一個適合我的情況,這似乎是一種相當普遍的情況。 例如,這個問題忽略了原始值是冗余/多重的問題,因此需要一個多重映射。 諸如此類或此類的其他涉及Google Guava。
以下作品使用 Java 9 或更高版本:
@Test
void invertMap()
{
Map<DayOfWeek, Integer> map = Map.of(
DayOfWeek.MONDAY, 52,
DayOfWeek.TUESDAY, 52,
DayOfWeek.WEDNESDAY, 53,
DayOfWeek.THURSDAY, 53,
DayOfWeek.FRIDAY, 52,
DayOfWeek.SATURDAY, 52,
DayOfWeek.SUNDAY, 52
);
Map<Integer, Set<DayOfWeek>> flipped = new TreeMap<>();
map.forEach((dow, count) ->
flipped.computeIfAbsent(count, (key) ->
EnumSet.noneOf(DayOfWeek.class)).add(dow));
Map<Integer, Set<DayOfWeek>> flippedStream = map.entrySet().stream()
.collect(Collectors.groupingBy(
Map.Entry::getValue,
TreeMap::new,
Collectors.mapping(
Map.Entry::getKey,
Collectors.toCollection(
() -> EnumSet.noneOf(DayOfWeek.class)))));
Map<Integer, Set<DayOfWeek>> expected = Map.of(
53, EnumSet.of(
DayOfWeek.WEDNESDAY,
DayOfWeek.THURSDAY),
52, EnumSet.of(
DayOfWeek.MONDAY,
DayOfWeek.TUESDAY,
DayOfWeek.FRIDAY,
DayOfWeek.SATURDAY,
DayOfWeek.SUNDAY)
);
Assert.assertEquals(expected, flipped);
Assert.assertEquals(expected, flippedStream);
}
如果您願意使用第三方庫,以下代碼將適用於Eclipse Collections :
@Test
void invertEclipseCollectionsMap()
{
MutableMap<DayOfWeek, Integer> map =
Maps.mutable.<DayOfWeek, Integer>empty()
.withKeyValue(DayOfWeek.MONDAY, 52)
.withKeyValue(DayOfWeek.TUESDAY, 52)
.withKeyValue(DayOfWeek.WEDNESDAY, 53)
.withKeyValue(DayOfWeek.THURSDAY, 53)
.withKeyValue(DayOfWeek.FRIDAY, 52)
.withKeyValue(DayOfWeek.SATURDAY, 52)
.withKeyValue(DayOfWeek.SUNDAY, 52);
SetMultimap<Integer, DayOfWeek> flipped = map.flip();
Assert.assertEquals(flipped.get(52), Set.of(
DayOfWeek.MONDAY,
DayOfWeek.TUESDAY,
DayOfWeek.FRIDAY,
DayOfWeek.SATURDAY,
DayOfWeek.SUNDAY));
Assert.assertEquals(flipped.get(53), Set.of(
DayOfWeek.WEDNESDAY,
DayOfWeek.THURSDAY));
}
注意:我是 Eclipse Collections 的提交者。
使用流,您可以將 map 拆分為其條目,然后翻轉條目和組:
numberOfDaysInYear.entrySet().stream()
.collect(groupingBy(Map.Entry::getValue), mapping(Map.Entry::getKey, toList()));
根據您更新的評論,要求優化實際上不在您的原始問題中,
numberOfDaysInYear.entrySet().stream()
.collect(groupingBy(
Map.Entry::getValue,
TreeMap::new,
mapping(Map.Entry::getKey, toCollection(() -> EnumSet.of(DayOfWeek.class)))
));
Collectors.toMap
在這種情況下,您可以使用方法Collectors.toMap(keyMapper,valueMapper,mergeFunction)
並生成多圖,其中值可以是列表或集合:
List
: Map<Integer, List<DayOfWeek>> inverted = map.entrySet().stream().collect(Collectors.toMap( // key of the new map entry -> entry.getValue(), // value of the new map entry -> List.of(entry.getKey()), // merging two values, ie lists (list1, list2) -> { List<DayOfWeek> list = new ArrayList<>(); list.addAll(list1); list.addAll(list2); return list; }));
Set
: Map<Integer, Set<DayOfWeek>> inverted = map.entrySet().stream().collect(Collectors.toMap( // key of the new map entry -> entry.getValue(), // value of the new map entry -> Set.of(entry.getKey()), // merging two values, ie sets (set1, set2) -> { Set<DayOfWeek> set = new HashSet<>(); set.addAll(set1); set.addAll(set2); return set; }));
另請參閱:收集基於多個字段的 id 列表
請參考以下代碼:
@Test
void testMap() {
Map<DayOfWeek, Integer> map = new HashMap<>();
map.put(DayOfWeek.MONDAY, 52);
map.put(DayOfWeek.TUESDAY, 52);
map.put(DayOfWeek.WEDNESDAY, 53);
map.put(DayOfWeek.THURSDAY, 53);
map.put(DayOfWeek.FRIDAY, 52);
map.put(DayOfWeek.SATURDAY, 52);
map.put(DayOfWeek.SUNDAY, 52);
Map<Integer, List<DayOfWeek>> result = new HashMap<>();
for (Map.Entry<DayOfWeek, Integer> entry : map.entrySet()) {
if (result.containsKey(entry.getValue())) {
List list = result.get(entry.getValue());
list.add(entry.getKey());
result.put(entry.getValue(), list);
} else {
List list = new ArrayList();
list.add(entry.getKey());
result.put(entry.getValue(), list);
}
}
System.out.println(result);
}
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