使用流在Java 8中按Map值进行分组

Question

我有一个地图列表,我想通过使用java流的键名称对它进行分组.

[
      {
        "dateDebut": "2018-07-01T00:00:00.000+0000",
        "nom": "Julien Mannone",
        "etat": "Impayé"
      },
      {
        "dateDebut": "2018-08-01T00:00:00.000+0000",
        "nom": "Julien Mannone",
        "etat": "Impayé"
      },          
      {
        "dateDebut": "2018-10-01T00:00:00.000+0000",
        "nom": "Mathiew Matic",
        "etat": "payé"
      },           
      {
        "dateDebut": "2018-10-01T00:00:00.000+0000",
        "nom": "Ash Moon",
        "etat": "payé"
      }
    ]

所以我想要这样的结果

{  
   "Julien Mannone":[  
      {  
         "dateDebut":"2018-07-01T00:00:00.000+0000",
         "etat":"Impayé"
      },
      {  
         "dateDebut":"2018-08-01T00:00:00.000+0000",
         "etat":"Impayé"
      }
   ],
   "Mathiew Matic":[  
      {  
         "dateDebut":"2018-10-01T00:00:00.000+0000",
         "etat":"payé"
      }
   ],
   "Ash Moon":[  
      {  
         "dateDebut":"2018-10-01T00:00:00.000+0000",
         "etat":"payé"
      }
   ]
}

作为使用流的初学者,我做了一些研究,我发现了一些类似的代码

Map<String, List<Map>> afterFormatting =
        beforeFormatting.stream()
                .flatMap(m -> m.entrySet().stream())
                .collect(groupingBy(Map.Entry::getKey, mapping(Map.Entry::getValue, toList())));

但这对我来说不起作用

Answer 1

好像你只是在寻找:

Map<String, List<Map<String, String>>> afterFormatting = 
            beforeFormatting.stream()
                            .collect(Collectors.groupingBy(map -> map.get("nom")));

或者如果您不希望Map<String, String>结果集中的每个都包含"nom"条目,那么您可以执行以下操作:

Map<String, List<Map<String, String>>> afterFormatting = 
     beforeFormatting.stream()
                     .collect(Collectors.groupingBy(map -> map.get("nom"), 
                           Collectors.mapping(map -> { 
                                Map<String, String> temp = new HashMap<>(map);
                                temp.remove("nom");
                               return temp;
                     }, Collectors.toList())));