Grouping by a Map value in Java 8 using streams

Question

I have a List of Map that i want to group it by the key nom using java streams.

[
      {
        "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"
      }
    ]

so i want as a result something like this

{  
   "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"
      }
   ]
}

As a beginner in using streams I have made some research I found some codes like that

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

but that doesn't do the job for me

Answer 1

Seems like you're simply looking for:

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

or if you don't want each Map<String, String> in the result set to contain the "nom" entry then you can do as follows:

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())));