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 28 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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                                                  No ethernet ip address in my vocore2

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                                                  0 I just flashed my vocore2u with openwrt 18.06.1, and plug it with an ethernet cable to access network while setting up wireless access-point for my phone. The AP is working and connected to the network, but the ip in my phone is in the subnet of another router. And there's no ip assigned to my vocore2 so I cannot ssh into it. After connecting to vocore2 with serial terminal, ping 8.8.8.8 is also failed. How can I make my vocore2 gain ip from the router and setting up the network well? root@OpenWrt:/etc/config# ifconfig apcli0 Link encap:Ethernet HWaddr BA:D8:12:07:01:71 inet6 addr: fe80::b8d8:12ff:fe07:171/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 erro
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