Java流:groupingBy和flatMapping键 [英] Java streams: groupingBy and flatMapping keys

问题描述

说我有一个国家对象列表，其中包含该国家使用的语言列表，例如:

Say I have a list of country objects that contain lists of languages spoken in that country, like so:

class Country {
    List<String> languages; 
}

我想以以下格式创建地图: Map< String，List< Country>> ，以便每种语言都映射到Country对象的列表.例如:

I would like to create a map in the following format: Map<String, List<Country>>, such that a each language maps to a list of Country objects. E.g.:

"French" -> [Country:France, Country:Canada],
"English" -> [Country:UK, Country:US]

这里的性能是一个问题，所以我想避免多次迭代和查找.我已经尝试过使用 groupingBy ，但是我该如何 flatMap 键集?

Performance is an issue here, so I would like to avoid multiple iterations and lookups. I have tried using groupingBy, but how do I flatMap the keyset?

例如，这将导致 Map< List< String>，List< Country>> :

countries.stream()
    .collect(Collectors.groupingBy(country -> country.getLanguages(), toList()));

推荐答案

由于您似乎关心性能，因此请勿将流用于此简单任务:

Since you seem to care about performance, don't use streams for this simple task:

Map<String, List<Country>> countriesByLanguage = new HashMap<>();
for (Country country : countries) {
    for (String language : country.getLanguages()) {
        countriesByLanguage.computeIfAbsent(language, k -> new ArrayList<>())
                           .add(country);
    }
}

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