java - Map.Entry::getKey 在 groupby 下抛出错误

Question

Map.Entry::getKey 为以下代码抛出错误

Map<String,List <String> > bikes= new HashMap<>();

bikes.put("ROBBIN",Arrays.asList("FZ","APACHE","HONDA"));
bikes.put("VIN",Arrays.asList("FZ","HONDA"));
bikes.put("GRACE",Arrays.asList("APACHE","HONDA"));
bikes.put("RUBBY",Arrays.asList("FZ","BUS","HONDA"));


Map<String, List<String>> group1;
bikes.entrySet().stream()
        .flatMap(x->x.getValue().stream())
        .collect(Collectors.groupingBy(
                     Function.identity(), 
                     Collectors.mapping(Map.Entry::getKey,
                                        Collectors.toList())
                ));

Answer 1

flatMap(x->x.getValue().stream())转换您的Stream<Map.Entry<String,List <String>>>到 Stream<String> ，所以你没有 Map.Entry不再是了。

看起来您想反转输入 Map (即将输入 List 的值的 Map 元素设为输出 Map 的键)。

这可以按如下方式完成:

Map<String, List<String>> group1 =
    bikes.entrySet()
         .stream().flatMap(e->e.getValue()
                               .stream()
                               .map(v->new SimpleEntry<String,String>(v,e.getKey())))
         .collect(Collectors.groupingBy(Map.Entry::getKey, 
                                        Collectors.mapping(Map.Entry::getValue,
                                                           Collectors.toList())));

现在flatMap变换每个 Map.Entry<String,List<String>>多个 Map.Entry<String,String>实例，每个实例都包含一个键，该键的值是 List<String>和一个值，它是原始 Map.Entry 的键.

这将导致以下结果 Map :

{BUS=[RUBBY], FZ=[ROBBIN, VIN, RUBBY], APACHE=[GRACE, ROBBIN], HONDA=[GRACE, ROBBIN, VIN, RUBBY]}