Java流按内部数组分组

Question

我有以下类的对象列表:

class Response {
     public String shortName; 
     public String[] types;
}

我想使用流对类型进行分组操作。例如如果我给出如下的响应列表[{"阿拉斯加", ["state", "admin level1"]}, {"New Jersey", ["state", "admin level2"]}]结果应该是这样的 map :

{"state":["Alaska", "New Jersey"], "admin level1": ["Alaska"], "admin level2": "New Jersey"}

Answer 1

map 将每个 Response 中的字符串数组转换为 SimpleEntry，将其展平并使用 groupingBy mapping 作为下游收集器。

Map<String, List<String>> resultSet = 
      responses.stream()
               .flatMap(e -> Arrays.stream(e.getTypes()).map(a -> new AbstractMap.SimpleEntry<>(a, e.getShortName())))
               .collect(Collectors.groupingBy(AbstractMap.SimpleEntry::getKey,
                        Collectors.mapping(AbstractMap.SimpleEntry::getValue, 
                                            Collectors.toList())));

如果您希望结果按照帖子中显示的顺序显示，那么您需要将结果转储到 LinkedHashMap 中:

...
...
.collect(Collectors.groupingBy(AbstractMap.SimpleEntry::getKey,
       LinkedHashMap::new, // a supplier providing a new empty map into which the results will be inserted
       Collectors.mapping(AbstractMap.SimpleEntry::getValue, Collectors.toList())));