如何将Map <String,List <String >>转换为java 8功能API中的Map <String,String>

Question

我有一张像波纹管的地图,

 [key = "car", value = ["bmw", "toyota"]]
 [key = "bike", value = ["honda", "kawasaki"]]

我想使用java 8功能apis将其转换为另一个地图,如bellow,

 [key = "bmw", value = "car"]
 [key = "toyota", value = "car"]
 [key = "honda", value = "bike"]
 [key = "kawasaki", value = "bike"]

Answer 1

将地图值展平为条目然后收集它们:

Map<String, String> m2 = map
    .entrySet()
    .stream()
    .flatMap(e -> e.getValue().stream().map(v -> new AbstractMap.SimpleEntry<>(v, e.getKey())))
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

这可以通过导入AbstractMap.SimpleEntry和缩短Map.Entry.