[英]Java streams: groupingBy and flatMapping keys
假設我有一個國家對象列表,其中包含該國家/地區使用的語言列表,如下所示:
class Country {
List<String> languages;
}
我想以以下格式創建地圖: Map<String, List<Country>>
,這樣每種語言都映射到 Country 對象列表。 例如:
"French" -> [Country:France, Country:Canada],
"English" -> [Country:UK, Country:US]
性能在這里是一個問題,所以我想避免多次迭代和查找。 我曾嘗試使用groupingBy
,但如何flatMap
鍵集?
例如,這導致Map<List<String>, List<Country>>
:
countries.stream()
.collect(Collectors.groupingBy(country -> country.getLanguages(), toList()));
由於您似乎關心性能,因此不要將流用於這個簡單的任務:
Map<String, List<Country>> countriesByLanguage = new HashMap<>();
for (Country country : countries) {
for (String language : country.getLanguages()) {
countriesByLanguage.computeIfAbsent(language, k -> new ArrayList<>())
.add(country);
}
}
您可以使用流中的流來完成,如下所示:首先迭代國家列表,然后迭代嵌套的語言列表並准備«language, country»
對,然后收集它們以進行映射:
public static void main(String[] args) {
List<Country> countries = List.of(
new Country("France", List.of("French")),
new Country("Canada", List.of("French")),
new Country("UK", List.of("English")),
new Country("US", List.of("English")));
Map<String, List<Country>> map = countries.stream()
// Stream<Map.Entry<String,Country>>
.flatMap(country -> country.getLanguages().stream()
.map(lang -> Map.entry(lang, country)))
.collect(Collectors.toMap(
// key - language
Map.Entry::getKey,
// value - List<Country>
entry -> new ArrayList<>(List.of(entry.getValue())),
// merge duplicates, if any
(list1, list2) -> {
list1.addAll(list2);
return list1;
}
));
// output
map.forEach((k, v) -> System.out.println(k + "=" + v));
//English=[Country:UK, Country:US]
//French=[Country:France, Country:Canada]
}
static class Country {
String name;
List<String> languages;
public Country(String name, List<String> languages) {
this.name = name;
this.languages = languages;
}
public List<String> getLanguages() {
return languages;
}
@Override
public String toString() {
return "Country:" + name;
}
}
這樣做:
countries.stream()
.flatMap(country -> country.getLanguages()
.stream()
.map(lang -> new SimpleEntry<>(lang,
new ArrayList<>(Arrays.asList(country)))))
.collect(Collectors.toMap(
Entry::getKey,
Entry::getValue,
(l1, l2) -> {
l1.addAll(l2);
return l2;
}));
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