How to get a List from a HashMap<String,List>












6














I want to extract a List<E> from a Map<String,List<E>> (E is a random Class) using stream().



I want a simple one-line method using java 8's stream.



What I have tried until now : 



HashMap<String,List<E>> map = new HashMap<>();

List<E> list = map.values(); // does not compile

list = map.values().stream().collect(Collectors.toList()); // does not compile





java collections java-8 java-stream collectors







share|improve this question





























    6














    I want to extract a List<E> from a Map<String,List<E>> (E is a random Class) using stream().



    I want a simple one-line method using java 8's stream.



    What I have tried until now : 



    HashMap<String,List<E>> map = new HashMap<>();
    
List<E> list = map.values(); // does not compile
    
list = map.values().stream().collect(Collectors.toList()); // does not compile





    java collections java-8 java-stream collectors







    share|improve this question



























      6












      6








      6


      3





      I want to extract a List<E> from a Map<String,List<E>> (E is a random Class) using stream().



      I want a simple one-line method using java 8's stream.



      What I have tried until now : 



      HashMap<String,List<E>> map = new HashMap<>();
      
List<E> list = map.values(); // does not compile
      
list = map.values().stream().collect(Collectors.toList()); // does not compile





      java collections java-8 java-stream collectors







      share|improve this question















      I want to extract a List<E> from a Map<String,List<E>> (E is a random Class) using stream().



      I want a simple one-line method using java 8's stream.



      What I have tried until now : 



      HashMap<String,List<E>> map = new HashMap<>();
      
List<E> list = map.values(); // does not compile
      
list = map.values().stream().collect(Collectors.toList()); // does not compile





      java collections java-8 java-stream collectors




      java collections java-8 java-stream collectors






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited 18 mins ago









      Nicholas K

      5,81951031




      5,81951031










      asked 11 hours ago









      Yassine Ben Hamida

      13111




      13111
























          6 Answers
          6






          active

          oldest

          votes


















          7














          map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:



          List<E> result = map.values()
          
                    .stream()
          
                    .flatMap(List::stream)
          
                    .collect(Collectors.toList());





          share|improve this answer





























            5














            Here's the Java9 solution,



            List<E> result = map.values().stream()
            
    .collect(Collectors.flatMapping(List::stream, Collectors.toList()));





            share|improve this answer

















            • 6




              Just the apiNote :- The flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.
              – nullpointer
              11 hours ago








            • 3




              It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
              – davidxxx
              11 hours ago






            • 2




              In fact it is not the Java 9 solution. It is a way possible with Java 9. But the Java 9 way to favor is of course the same as with Java 8 : stackoverflow.com/a/54054456/270371
              – davidxxx
              10 hours ago





















            4














            In addition to other answers:



            List<E> result = map.values()
            
                    .stream()
            
                    .collect(ArrayList::new, List::addAll, List::addAll);


            This could also do the trick.






            share|improve this answer





























              3














              Or use forEach 



               map.forEach((k,v)->list.addAll(v));


              or as Aomine commented use this 



              map.values().forEach(list::addAll);





              share|improve this answer



















              • 3




                good idea in showing a non-stream version. btw it would be better to iterate over the values since you're not doing anything with the k i.e. you can do List<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
                – Aomine
                11 hours ago



















              3














              You can use Collection.stream with flatMap as:



              Map<String, List<E>> map = new HashMap<>(); // program to interface
              
List<E> list = map.values()
              
                  .stream()
              
                  .flatMap(Collection::stream)
              
                  .collect(Collectors.toList());


              or use a non-stream version as:



              List<E> list = new ArrayList<>();
              
map.values().forEach(list::addAll)





              share|improve this answer































                2














                Simply use :-



                map.values().stream().flatMap(List::stream).collect(Collectors.toList());





                share|improve this answer



















                • 1




                  This wont' work, rather it will give you List<List<E>>
                  – Ravindra Ranwala
                  11 hours ago













                Your Answer






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                6 Answers
                6






                active

                oldest

                votes








                6 Answers
                6






                active

                oldest

                votes









                active

                oldest

                votes






                active

                oldest

                votes









                7














                map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:



                List<E> result = map.values()
                
                    .stream()
                
                    .flatMap(List::stream)
                
                    .collect(Collectors.toList());





                share|improve this answer


























                  7














                  map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:



                  List<E> result = map.values()
                  
                    .stream()
                  
                    .flatMap(List::stream)
                  
                    .collect(Collectors.toList());





                  share|improve this answer
























                    7












                    7








                    7






                    map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:



                    List<E> result = map.values()
                    
                    .stream()
                    
                    .flatMap(List::stream)
                    
                    .collect(Collectors.toList());





                    share|improve this answer












                    map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:



                    List<E> result = map.values()
                    
                    .stream()
                    
                    .flatMap(List::stream)
                    
                    .collect(Collectors.toList());






                    share|improve this answer












                    share|improve this answer



                    share|improve this answer










                    answered 11 hours ago









                    Aomine

                    40.7k73870




                    40.7k73870

























                        5














                        Here's the Java9 solution,



                        List<E> result = map.values().stream()
                        
    .collect(Collectors.flatMapping(List::stream, Collectors.toList()));





                        share|improve this answer

















                        • 6




                          Just the apiNote :- The flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.
                          – nullpointer
                          11 hours ago








                        • 3




                          It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                          – davidxxx
                          11 hours ago






                        • 2




                          In fact it is not the Java 9 solution. It is a way possible with Java 9. But the Java 9 way to favor is of course the same as with Java 8 : stackoverflow.com/a/54054456/270371
                          – davidxxx
                          10 hours ago


















                        5














                        Here's the Java9 solution,



                        List<E> result = map.values().stream()
                        
    .collect(Collectors.flatMapping(List::stream, Collectors.toList()));





                        share|improve this answer

















                        • 6




                          Just the apiNote :- The flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.
                          – nullpointer
                          11 hours ago








                        • 3




                          It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                          – davidxxx
                          11 hours ago






                        • 2




                          In fact it is not the Java 9 solution. It is a way possible with Java 9. But the Java 9 way to favor is of course the same as with Java 8 : stackoverflow.com/a/54054456/270371
                          – davidxxx
                          10 hours ago
















                        5












                        5








                        5






                        Here's the Java9 solution,



                        List<E> result = map.values().stream()
                        
    .collect(Collectors.flatMapping(List::stream, Collectors.toList()));





                        share|improve this answer












                        Here's the Java9 solution,



                        List<E> result = map.values().stream()
                        
    .collect(Collectors.flatMapping(List::stream, Collectors.toList()));






                        share|improve this answer












                        share|improve this answer



                        share|improve this answer










                        answered 11 hours ago









                        Ravindra Ranwala

                        8,52231634




                        8,52231634








                        • 6




                          Just the apiNote :- The flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.
                          – nullpointer
                          11 hours ago








                        • 3




                          It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                          – davidxxx
                          11 hours ago






                        • 2




                          In fact it is not the Java 9 solution. It is a way possible with Java 9. But the Java 9 way to favor is of course the same as with Java 8 : stackoverflow.com/a/54054456/270371
                          – davidxxx
                          10 hours ago
















                        • 6




                          Just the apiNote :- The flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.
                          – nullpointer
                          11 hours ago








                        • 3




                          It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                          – davidxxx
                          11 hours ago






                        • 2




                          In fact it is not the Java 9 solution. It is a way possible with Java 9. But the Java 9 way to favor is of course the same as with Java 8 : stackoverflow.com/a/54054456/270371
                          – davidxxx
                          10 hours ago










                        6




                        6




                        Just the apiNote :- The flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.
                        – nullpointer
                        11 hours ago






                        Just the apiNote :- The flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.
                        – nullpointer
                        11 hours ago






                        3




                        3




                        It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                        – davidxxx
                        11 hours ago




                        It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                        – davidxxx
                        11 hours ago




                        2




                        2




                        In fact it is not the Java 9 solution. It is a way possible with Java 9. But the Java 9 way to favor is of course the same as with Java 8 : stackoverflow.com/a/54054456/270371
                        – davidxxx
                        10 hours ago






                        In fact it is not the Java 9 solution. It is a way possible with Java 9. But the Java 9 way to favor is of course the same as with Java 8 : stackoverflow.com/a/54054456/270371
                        – davidxxx
                        10 hours ago













                        4














                        In addition to other answers:



                        List<E> result = map.values()
                        
                    .stream()
                        
                    .collect(ArrayList::new, List::addAll, List::addAll);


                        This could also do the trick.






                        share|improve this answer


























                          4














                          In addition to other answers:



                          List<E> result = map.values()
                          
                    .stream()
                          
                    .collect(ArrayList::new, List::addAll, List::addAll);


                          This could also do the trick.






                          share|improve this answer
























                            4












                            4








                            4






                            In addition to other answers:



                            List<E> result = map.values()
                            
                    .stream()
                            
                    .collect(ArrayList::new, List::addAll, List::addAll);


                            This could also do the trick.






                            share|improve this answer












                            In addition to other answers:



                            List<E> result = map.values()
                            
                    .stream()
                            
                    .collect(ArrayList::new, List::addAll, List::addAll);


                            This could also do the trick.







                            share|improve this answer












                            share|improve this answer



                            share|improve this answer










                            answered 11 hours ago









                            ETO

                            1,886422




                            1,886422























                                3














                                Or use forEach 



                                 map.forEach((k,v)->list.addAll(v));


                                or as Aomine commented use this 



                                map.values().forEach(list::addAll);





                                share|improve this answer



















                                • 3




                                  good idea in showing a non-stream version. btw it would be better to iterate over the values since you're not doing anything with the k i.e. you can do List<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
                                  – Aomine
                                  11 hours ago
















                                3














                                Or use forEach 



                                 map.forEach((k,v)->list.addAll(v));


                                or as Aomine commented use this 



                                map.values().forEach(list::addAll);





                                share|improve this answer



















                                • 3




                                  good idea in showing a non-stream version. btw it would be better to iterate over the values since you're not doing anything with the k i.e. you can do List<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
                                  – Aomine
                                  11 hours ago














                                3












                                3








                                3






                                Or use forEach 



                                 map.forEach((k,v)->list.addAll(v));


                                or as Aomine commented use this 



                                map.values().forEach(list::addAll);





                                share|improve this answer














                                Or use forEach 



                                 map.forEach((k,v)->list.addAll(v));


                                or as Aomine commented use this 



                                map.values().forEach(list::addAll);






                                share|improve this answer














                                share|improve this answer



                                share|improve this answer








                                edited 11 hours ago

























                                answered 11 hours ago









                                Hadi J

                                9,86231742




                                9,86231742








                                • 3




                                  good idea in showing a non-stream version. btw it would be better to iterate over the values since you're not doing anything with the k i.e. you can do List<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
                                  – Aomine
                                  11 hours ago














                                • 3




                                  good idea in showing a non-stream version. btw it would be better to iterate over the values since you're not doing anything with the k i.e. you can do List<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
                                  – Aomine
                                  11 hours ago








                                3




                                3




                                good idea in showing a non-stream version. btw it would be better to iterate over the values since you're not doing anything with the k i.e. you can do List<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
                                – Aomine
                                11 hours ago




                                good idea in showing a non-stream version. btw it would be better to iterate over the values since you're not doing anything with the k i.e. you can do List<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
                                – Aomine
                                11 hours ago











                                3














                                You can use Collection.stream with flatMap as:



                                Map<String, List<E>> map = new HashMap<>(); // program to interface
                                
List<E> list = map.values()
                                
                  .stream()
                                
                  .flatMap(Collection::stream)
                                
                  .collect(Collectors.toList());


                                or use a non-stream version as:



                                List<E> list = new ArrayList<>();
                                
map.values().forEach(list::addAll)





                                share|improve this answer




























                                  3














                                  You can use Collection.stream with flatMap as:



                                  Map<String, List<E>> map = new HashMap<>(); // program to interface
                                  
List<E> list = map.values()
                                  
                  .stream()
                                  
                  .flatMap(Collection::stream)
                                  
                  .collect(Collectors.toList());


                                  or use a non-stream version as:



                                  List<E> list = new ArrayList<>();
                                  
map.values().forEach(list::addAll)





                                  share|improve this answer


























                                    3












                                    3








                                    3






                                    You can use Collection.stream with flatMap as:



                                    Map<String, List<E>> map = new HashMap<>(); // program to interface
                                    
List<E> list = map.values()
                                    
                  .stream()
                                    
                  .flatMap(Collection::stream)
                                    
                  .collect(Collectors.toList());


                                    or use a non-stream version as:



                                    List<E> list = new ArrayList<>();
                                    
map.values().forEach(list::addAll)





                                    share|improve this answer














                                    You can use Collection.stream with flatMap as:



                                    Map<String, List<E>> map = new HashMap<>(); // program to interface
                                    
List<E> list = map.values()
                                    
                  .stream()
                                    
                  .flatMap(Collection::stream)
                                    
                  .collect(Collectors.toList());


                                    or use a non-stream version as:



                                    List<E> list = new ArrayList<>();
                                    
map.values().forEach(list::addAll)






                                    share|improve this answer














                                    share|improve this answer



                                    share|improve this answer








                                    edited 15 mins ago

























                                    answered 11 hours ago









                                    nullpointer

                                    43.4k1093178




                                    43.4k1093178























                                        2














                                        Simply use :-



                                        map.values().stream().flatMap(List::stream).collect(Collectors.toList());





                                        share|improve this answer



















                                        • 1




                                          This wont' work, rather it will give you List<List<E>>
                                          – Ravindra Ranwala
                                          11 hours ago


















                                        2














                                        Simply use :-



                                        map.values().stream().flatMap(List::stream).collect(Collectors.toList());





                                        share|improve this answer



















                                        • 1




                                          This wont' work, rather it will give you List<List<E>>
                                          – Ravindra Ranwala
                                          11 hours ago
















                                        2












                                        2








                                        2






                                        Simply use :-



                                        map.values().stream().flatMap(List::stream).collect(Collectors.toList());





                                        share|improve this answer














                                        Simply use :-



                                        map.values().stream().flatMap(List::stream).collect(Collectors.toList());






                                        share|improve this answer














                                        share|improve this answer



                                        share|improve this answer








                                        edited 17 mins ago

























                                        answered 11 hours ago









                                        Nicholas K

                                        5,81951031




                                        5,81951031








                                        • 1




                                          This wont' work, rather it will give you List<List<E>>
                                          – Ravindra Ranwala
                                          11 hours ago
















                                        • 1




                                          This wont' work, rather it will give you List<List<E>>
                                          – Ravindra Ranwala
                                          11 hours ago










                                        1




                                        1




                                        This wont' work, rather it will give you List<List<E>>
                                        – Ravindra Ranwala
                                        11 hours ago






                                        This wont' work, rather it will give you List<List<E>>
                                        – Ravindra Ranwala
                                        11 hours ago




















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