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












8














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





























    8














    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



























      8












      8








      8


      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 1 hour ago









      Nicholas K

      5,81951031




      5,81951031










      asked 12 hours ago









      Yassine Ben Hamida

      14111




      14111
























          6 Answers
          6






          active

          oldest

          votes


















          8














          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





























            6














            Here's an alternate way to do it with Java-9 and above:



            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
              12 hours ago








            • 3




              It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
              – davidxxx
              12 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
              12 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














              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
                12 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























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                  6 Answers
                  6






                  active

                  oldest

                  votes








                  6 Answers
                  6






                  active

                  oldest

                  votes









                  active

                  oldest

                  votes






                  active

                  oldest

                  votes









                  8














                  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


























                    8














                    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
























                      8












                      8








                      8






                      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 12 hours ago









                      Aomine

                      40.7k73870




                      40.7k73870

























                          6














                          Here's an alternate way to do it with Java-9 and above:



                          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
                            12 hours ago








                          • 3




                            It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                            – davidxxx
                            12 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
                            12 hours ago


















                          6














                          Here's an alternate way to do it with Java-9 and above:



                          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
                            12 hours ago








                          • 3




                            It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                            – davidxxx
                            12 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
                            12 hours ago
















                          6












                          6








                          6






                          Here's an alternate way to do it with Java-9 and above:



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





                          share|improve this answer














                          Here's an alternate way to do it with Java-9 and above:



                          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








                          edited 29 mins ago









                          nullpointer

                          43.4k1093178




                          43.4k1093178










                          answered 12 hours ago









                          Ravindra Ranwala

                          8,53231634




                          8,53231634








                          • 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
                            12 hours ago








                          • 3




                            It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                            – davidxxx
                            12 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
                            12 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
                            12 hours ago








                          • 3




                            It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                            – davidxxx
                            12 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
                            12 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
                          12 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
                          12 hours ago






                          3




                          3




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




                          It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
                          – davidxxx
                          12 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
                          12 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
                          12 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 12 hours ago









                              ETO

                              1,886422




                              1,886422























                                  4














                                  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
                                    12 hours ago
















                                  4














                                  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
                                    12 hours ago














                                  4












                                  4








                                  4






                                  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 12 hours ago

























                                  answered 12 hours ago









                                  Hadi J

                                  9,87231742




                                  9,87231742








                                  • 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
                                    12 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
                                    12 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
                                  12 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
                                  12 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 1 hour ago

























                                      answered 12 hours ago









                                      nullpointer

                                      43.4k1093178




                                      43.4k1093178























                                          2














                                          Simply use :-



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





                                          share|improve this answer




























                                            2














                                            Simply use :-



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





                                            share|improve this answer


























                                              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 1 hour ago

























                                              answered 12 hours ago









                                              Nicholas K

                                              5,81951031




                                              5,81951031






























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