Java: groupingBy subvalue as value

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up vote
6
down vote

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Let's say, I have an object Person with fields of type FirstName and LastName. Now I also have a List<Person> and I like to use streams.



Now I want to generate a Map<FirstName, List<LastName>> in order to group people with the same first name. How do I go about this without writing much code? My approach so far is



personList
.stream()
.collect(Collectors.groupingBy(
 Person::getFirstName,
 person -> person.getLastName() // this seems to be wrong
));


but it seems this is the wrong way to assign the value of the map. What should I change? Or should I perhaps use .reduce with new HashMap<FirstName, List<LastName>>() as initial value and then aggregate to it by putting elements inside?






java java-stream







share|improve this question



























    up vote
    6
    down vote

    favorite
    1












    Let's say, I have an object Person with fields of type FirstName and LastName. Now I also have a List<Person> and I like to use streams.



    Now I want to generate a Map<FirstName, List<LastName>> in order to group people with the same first name. How do I go about this without writing much code? My approach so far is



    personList
    .stream()
    .collect(Collectors.groupingBy(
     Person::getFirstName,
     person -> person.getLastName() // this seems to be wrong
    ));


    but it seems this is the wrong way to assign the value of the map. What should I change? Or should I perhaps use .reduce with new HashMap<FirstName, List<LastName>>() as initial value and then aggregate to it by putting elements inside?






    java java-stream







    share|improve this question

























      up vote
      6
      down vote

      favorite
      1









      up vote
      6
      down vote

      favorite
      1






      1





      Let's say, I have an object Person with fields of type FirstName and LastName. Now I also have a List<Person> and I like to use streams.



      Now I want to generate a Map<FirstName, List<LastName>> in order to group people with the same first name. How do I go about this without writing much code? My approach so far is



      personList
      .stream()
      .collect(Collectors.groupingBy(
       Person::getFirstName,
       person -> person.getLastName() // this seems to be wrong
      ));


      but it seems this is the wrong way to assign the value of the map. What should I change? Or should I perhaps use .reduce with new HashMap<FirstName, List<LastName>>() as initial value and then aggregate to it by putting elements inside?






      java java-stream







      share|improve this question















      Let's say, I have an object Person with fields of type FirstName and LastName. Now I also have a List<Person> and I like to use streams.



      Now I want to generate a Map<FirstName, List<LastName>> in order to group people with the same first name. How do I go about this without writing much code? My approach so far is



      personList
      .stream()
      .collect(Collectors.groupingBy(
       Person::getFirstName,
       person -> person.getLastName() // this seems to be wrong
      ));


      but it seems this is the wrong way to assign the value of the map. What should I change? Or should I perhaps use .reduce with new HashMap<FirstName, List<LastName>>() as initial value and then aggregate to it by putting elements inside?






      java java-stream




      java java-stream






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited 2 hours ago

























      asked 2 hours ago









      Phil

      1,75811628




      1,75811628






















          3 Answers
          3






          active

          oldest

          votes

















          up vote
          5
          down vote



          accepted










          personList.stream()
           .collect(Collectors.groupingBy(
           Person::getFirstName,
           Collectors.mapping(Person::getLastName, Collectors.toList())));


          You are looking for a downstream collector with groupingBy






          share|improve this answer





























            up vote
            5
            down vote













            This should work for you :



            Map<String, List<String>> map = personList.stream()
             .collect(Collectors.groupingBy(Person::getFirstName, 
             Collectors.mapping(Person::getLastName, Collectors.toList())));





            share|improve this answer



























              up vote
              0
              down vote













              I think you are looking for something like this:



              Map<String, Map<String, List>> map = personList.stream()
               .collect(groupingBy(Person::getFirstName, groupingBy(Person::getLastName)));


              The double grouping gives you a map of a map. That's the trick.






              share|improve this answer
















              • 1




                Thank you, but the value was supposed to be a simple list, not another map.
                – Phil
                2 hours ago






              • 1




                I see it. I voted up nullpointer's response. That's the one.
                – Perimosh
                2 hours ago











              Your Answer





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              3 Answers
              3






              active

              oldest

              votes








              3 Answers
              3






              active

              oldest

              votes









              active

              oldest

              votes






              active

              oldest

              votes








              up vote
              5
              down vote



              accepted










              personList.stream()
               .collect(Collectors.groupingBy(
               Person::getFirstName,
               Collectors.mapping(Person::getLastName, Collectors.toList())));


              You are looking for a downstream collector with groupingBy






              share|improve this answer


























                up vote
                5
                down vote



                accepted










                personList.stream()
                 .collect(Collectors.groupingBy(
                 Person::getFirstName,
                 Collectors.mapping(Person::getLastName, Collectors.toList())));


                You are looking for a downstream collector with groupingBy






                share|improve this answer
























                  up vote
                  5
                  down vote



                  accepted







                  up vote
                  5
                  down vote



                  accepted






                  personList.stream()
                   .collect(Collectors.groupingBy(
                   Person::getFirstName,
                   Collectors.mapping(Person::getLastName, Collectors.toList())));


                  You are looking for a downstream collector with groupingBy






                  share|improve this answer














                  personList.stream()
                   .collect(Collectors.groupingBy(
                   Person::getFirstName,
                   Collectors.mapping(Person::getLastName, Collectors.toList())));


                  You are looking for a downstream collector with groupingBy







                  share|improve this answer














                  share|improve this answer



                  share|improve this answer








                  edited 56 mins ago

























                  answered 2 hours ago









                  Eugene

                  64.4k989152




                  64.4k989152






















                      up vote
                      5
                      down vote













                      This should work for you :



                      Map<String, List<String>> map = personList.stream()
                       .collect(Collectors.groupingBy(Person::getFirstName, 
                       Collectors.mapping(Person::getLastName, Collectors.toList())));





                      share|improve this answer
























                        up vote
                        5
                        down vote













                        This should work for you :



                        Map<String, List<String>> map = personList.stream()
                         .collect(Collectors.groupingBy(Person::getFirstName, 
                         Collectors.mapping(Person::getLastName, Collectors.toList())));





                        share|improve this answer






















                          up vote
                          5
                          down vote










                          up vote
                          5
                          down vote









                          This should work for you :



                          Map<String, List<String>> map = personList.stream()
                           .collect(Collectors.groupingBy(Person::getFirstName, 
                           Collectors.mapping(Person::getLastName, Collectors.toList())));





                          share|improve this answer












                          This should work for you :



                          Map<String, List<String>> map = personList.stream()
                           .collect(Collectors.groupingBy(Person::getFirstName, 
                           Collectors.mapping(Person::getLastName, Collectors.toList())));






                          share|improve this answer












                          share|improve this answer



                          share|improve this answer










                          answered 2 hours ago









                          nullpointer

                          32.1k1064133




                          32.1k1064133




















                              up vote
                              0
                              down vote













                              I think you are looking for something like this:



                              Map<String, Map<String, List>> map = personList.stream()
                               .collect(groupingBy(Person::getFirstName, groupingBy(Person::getLastName)));


                              The double grouping gives you a map of a map. That's the trick.






                              share|improve this answer
















                              • 1




                                Thank you, but the value was supposed to be a simple list, not another map.
                                – Phil
                                2 hours ago






                              • 1




                                I see it. I voted up nullpointer's response. That's the one.
                                – Perimosh
                                2 hours ago















                              up vote
                              0
                              down vote













                              I think you are looking for something like this:



                              Map<String, Map<String, List>> map = personList.stream()
                               .collect(groupingBy(Person::getFirstName, groupingBy(Person::getLastName)));


                              The double grouping gives you a map of a map. That's the trick.






                              share|improve this answer
















                              • 1




                                Thank you, but the value was supposed to be a simple list, not another map.
                                – Phil
                                2 hours ago






                              • 1




                                I see it. I voted up nullpointer's response. That's the one.
                                – Perimosh
                                2 hours ago













                              up vote
                              0
                              down vote










                              up vote
                              0
                              down vote









                              I think you are looking for something like this:



                              Map<String, Map<String, List>> map = personList.stream()
                               .collect(groupingBy(Person::getFirstName, groupingBy(Person::getLastName)));


                              The double grouping gives you a map of a map. That's the trick.






                              share|improve this answer












                              I think you are looking for something like this:



                              Map<String, Map<String, List>> map = personList.stream()
                               .collect(groupingBy(Person::getFirstName, groupingBy(Person::getLastName)));


                              The double grouping gives you a map of a map. That's the trick.







                              share|improve this answer












                              share|improve this answer



                              share|improve this answer










                              answered 2 hours ago









                              Perimosh

                              158211




                              158211







                              • 1




                                Thank you, but the value was supposed to be a simple list, not another map.
                                – Phil
                                2 hours ago






                              • 1




                                I see it. I voted up nullpointer's response. That's the one.
                                – Perimosh
                                2 hours ago













                              • 1




                                Thank you, but the value was supposed to be a simple list, not another map.
                                – Phil
                                2 hours ago






                              • 1




                                I see it. I voted up nullpointer's response. That's the one.
                                – Perimosh
                                2 hours ago








                              1




                              1




                              Thank you, but the value was supposed to be a simple list, not another map.
                              – Phil
                              2 hours ago




                              Thank you, but the value was supposed to be a simple list, not another map.
                              – Phil
                              2 hours ago




                              1




                              1




                              I see it. I voted up nullpointer's response. That's the one.
                              – Perimosh
                              2 hours ago





                              I see it. I voted up nullpointer's response. That's the one.
                              – Perimosh
                              2 hours ago


















                               

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