Use Collectors.groupingby to create a map to a set












8















I know how to create a Map<T, List<U>> , using Collectors.groupingBy:



Map<Key, List<Item>> listMap = items.stream().collect(Collectors.groupingBy(s->s.key));



How would I modify that code to create Map<Key, Set<Item>>? Or can I not do it using stream and so have to create it manually using a for loop etc.?






java set java-stream grouping







share|improve this question



























    8















    I know how to create a Map<T, List<U>> , using Collectors.groupingBy:



    Map<Key, List<Item>> listMap = items.stream().collect(Collectors.groupingBy(s->s.key));



    How would I modify that code to create Map<Key, Set<Item>>? Or can I not do it using stream and so have to create it manually using a for loop etc.?






    java set java-stream grouping







    share|improve this question

























      8












      8








      8








      I know how to create a Map<T, List<U>> , using Collectors.groupingBy:



      Map<Key, List<Item>> listMap = items.stream().collect(Collectors.groupingBy(s->s.key));



      How would I modify that code to create Map<Key, Set<Item>>? Or can I not do it using stream and so have to create it manually using a for loop etc.?






      java set java-stream grouping







      share|improve this question














      I know how to create a Map<T, List<U>> , using Collectors.groupingBy:



      Map<Key, List<Item>> listMap = items.stream().collect(Collectors.groupingBy(s->s.key));



      How would I modify that code to create Map<Key, Set<Item>>? Or can I not do it using stream and so have to create it manually using a for loop etc.?






      java set java-stream grouping




      java set java-stream grouping






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked yesterday









      simonalexander2005simonalexander2005

      1,93842957




      1,93842957
























          3 Answers
          3






          active

          oldest

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          12














          Use Collectors.toSet() as a downstream in groupingBy:



          Map<Key, Set<Item>> map = items.stream()
          
            .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));





          share|improve this answer

































            4














            You have to use a downstream collector like this:



            Map<Key, Set<Item>> listMap = items.stream()
            
    .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));





            share|improve this answer

































              3














              I also like the non-stream solution sometimes:



               Map<Key, Set<Item>> yourMap = new HashMap<>();
              
 items.forEach(x -> yourMap.computeIfAbsent(x.getKey(), ignoreMe -> new HashSet<>()).add(x));


              If you really wanted you could exercise to do the same via compute/merge methods too






              share|improve this answer

























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                3 Answers
                3






                active

                oldest

                votes








                3 Answers
                3






                active

                oldest

                votes









                active

                oldest

                votes






                active

                oldest

                votes









                12














                Use Collectors.toSet() as a downstream in groupingBy:



                Map<Key, Set<Item>> map = items.stream()
                
            .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));





                share|improve this answer






























                  12














                  Use Collectors.toSet() as a downstream in groupingBy:



                  Map<Key, Set<Item>> map = items.stream()
                  
            .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));





                  share|improve this answer




























                    12












                    12








                    12







                    Use Collectors.toSet() as a downstream in groupingBy:



                    Map<Key, Set<Item>> map = items.stream()
                    
            .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));





                    share|improve this answer















                    Use Collectors.toSet() as a downstream in groupingBy:



                    Map<Key, Set<Item>> map = items.stream()
                    
            .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));






                    share|improve this answer














                    share|improve this answer



                    share|improve this answer








                    edited yesterday

























                    answered yesterday









                    RuslanRuslan

                    2,451822




                    2,451822

























                        4














                        You have to use a downstream collector like this:



                        Map<Key, Set<Item>> listMap = items.stream()
                        
    .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));





                        share|improve this answer






























                          4














                          You have to use a downstream collector like this:



                          Map<Key, Set<Item>> listMap = items.stream()
                          
    .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));





                          share|improve this answer




























                            4












                            4








                            4







                            You have to use a downstream collector like this:



                            Map<Key, Set<Item>> listMap = items.stream()
                            
    .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));





                            share|improve this answer















                            You have to use a downstream collector like this:



                            Map<Key, Set<Item>> listMap = items.stream()
                            
    .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));






                            share|improve this answer














                            share|improve this answer



                            share|improve this answer








                            edited yesterday









                            nullpointer

                            43.2k10101200




                            43.2k10101200










                            answered yesterday









                            Ravindra RanwalaRavindra Ranwala

                            10.4k42040




                            10.4k42040























                                3














                                I also like the non-stream solution sometimes:



                                 Map<Key, Set<Item>> yourMap = new HashMap<>();
                                
 items.forEach(x -> yourMap.computeIfAbsent(x.getKey(), ignoreMe -> new HashSet<>()).add(x));


                                If you really wanted you could exercise to do the same via compute/merge methods too






                                share|improve this answer






























                                  3














                                  I also like the non-stream solution sometimes:



                                   Map<Key, Set<Item>> yourMap = new HashMap<>();
                                  
 items.forEach(x -> yourMap.computeIfAbsent(x.getKey(), ignoreMe -> new HashSet<>()).add(x));


                                  If you really wanted you could exercise to do the same via compute/merge methods too






                                  share|improve this answer




























                                    3












                                    3








                                    3







                                    I also like the non-stream solution sometimes:



                                     Map<Key, Set<Item>> yourMap = new HashMap<>();
                                    
 items.forEach(x -> yourMap.computeIfAbsent(x.getKey(), ignoreMe -> new HashSet<>()).add(x));


                                    If you really wanted you could exercise to do the same via compute/merge methods too






                                    share|improve this answer















                                    I also like the non-stream solution sometimes:



                                     Map<Key, Set<Item>> yourMap = new HashMap<>();
                                    
 items.forEach(x -> yourMap.computeIfAbsent(x.getKey(), ignoreMe -> new HashSet<>()).add(x));


                                    If you really wanted you could exercise to do the same via compute/merge methods too







                                    share|improve this answer














                                    share|improve this answer



                                    share|improve this answer








                                    edited yesterday

























                                    answered yesterday









                                    EugeneEugene

                                    70.7k9102169




                                    70.7k9102169






























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