将 Set<V> 转换为 Map<String, Set<String>
Convert Set<V> to Map<String, Set<String>
我关注collection:
Set<Map.Entry<Event, Long>> entries
事件 POJO:
public class Event{
private long epoch;
private List<Pair<String, String> eventParams;
}
我想将条目 collection 转换为 Map<String, Set<String>>
示例:
List<Pair<String, String> eventParams = Arrays.asList(Pair.of("abc","123"), Pair.of("abc","456"));
已转换 collection:
Map<String, Set<String>> converted = ["abc", ["123", "456"]]
我试过以下方法:
entries.stream().flatMap(x -> x.getKey().getEventParams().stream())
.collect(Collectors.groupingBy(Pair::getKey, Collectors.toSet(Pair::getValue)));
但是,我收到错误消息:无法应用收集器中的 toSet。
正确的做法是什么?
替换
Collectors.toSet(Pair::getValue)
和
Collectors.mapping(Pair::getValue, Collectors.toSet())
问题是 Collectors.toSet()
doesn't have any parameters, it operates on the type defined by the input stream. Collectors.mapping(mapper, downstream)
通过 "applying a mapping function to each input element before accumulation" 改变了这种行为。
我关注collection:
Set<Map.Entry<Event, Long>> entries
事件 POJO:
public class Event{
private long epoch;
private List<Pair<String, String> eventParams;
}
我想将条目 collection 转换为 Map<String, Set<String>>
示例:
List<Pair<String, String> eventParams = Arrays.asList(Pair.of("abc","123"), Pair.of("abc","456"));
已转换 collection:
Map<String, Set<String>> converted = ["abc", ["123", "456"]]
我试过以下方法:
entries.stream().flatMap(x -> x.getKey().getEventParams().stream())
.collect(Collectors.groupingBy(Pair::getKey, Collectors.toSet(Pair::getValue)));
但是,我收到错误消息:无法应用收集器中的 toSet。
正确的做法是什么?
替换
Collectors.toSet(Pair::getValue)
和
Collectors.mapping(Pair::getValue, Collectors.toSet())
问题是 Collectors.toSet()
doesn't have any parameters, it operates on the type defined by the input stream. Collectors.mapping(mapper, downstream)
通过 "applying a mapping function to each input element before accumulation" 改变了这种行为。