How do I create two different compliementary lists using same input












8














In my previous question - How to filter the age while grouping in map with list I was able to find the name to age groups using List<User> users. Now I am trying to find the different User groups from ages according to the threshold. I tried this 



List<User> userAboveThreshold = users.stream().filter(u -> u.getAge() > 21).collect(toList());

List<User> userBelowThreshold = users.stream().filter(u -> u.getAge() <= 21).collect(toList());


This time it works I can see using



userAboveThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));

userBelowThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));


But I have to access the users list again to find the complimentary list. Can this not be done simpler?






java java-stream







share|improve this question



























    8














    In my previous question - How to filter the age while grouping in map with list I was able to find the name to age groups using List<User> users. Now I am trying to find the different User groups from ages according to the threshold. I tried this 



    List<User> userAboveThreshold = users.stream().filter(u -> u.getAge() > 21).collect(toList());
    
List<User> userBelowThreshold = users.stream().filter(u -> u.getAge() <= 21).collect(toList());


    This time it works I can see using



    userAboveThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));
    
userBelowThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));


    But I have to access the users list again to find the complimentary list. Can this not be done simpler?






    java java-stream







    share|improve this question

























      8












      8








      8


      2





      In my previous question - How to filter the age while grouping in map with list I was able to find the name to age groups using List<User> users. Now I am trying to find the different User groups from ages according to the threshold. I tried this 



      List<User> userAboveThreshold = users.stream().filter(u -> u.getAge() > 21).collect(toList());
      
List<User> userBelowThreshold = users.stream().filter(u -> u.getAge() <= 21).collect(toList());


      This time it works I can see using



      userAboveThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));
      
userBelowThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));


      But I have to access the users list again to find the complimentary list. Can this not be done simpler?






      java java-stream







      share|improve this question













      In my previous question - How to filter the age while grouping in map with list I was able to find the name to age groups using List<User> users. Now I am trying to find the different User groups from ages according to the threshold. I tried this 



      List<User> userAboveThreshold = users.stream().filter(u -> u.getAge() > 21).collect(toList());
      
List<User> userBelowThreshold = users.stream().filter(u -> u.getAge() <= 21).collect(toList());


      This time it works I can see using



      userAboveThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));
      
userBelowThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));


      But I have to access the users list again to find the complimentary list. Can this not be done simpler?






      java java-stream




      java java-stream






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked Dec 20 '18 at 13:26









      Mani

      2351320




      2351320
























          2 Answers
          2






          active

          oldest

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          7














          List.removeAll



          You can use removeAll to obtain the complimentary list.



          List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'
          
userBelowThreshold.removeAll(userAboveThreshold);


          Note: This would require overridden equals and hashCode implementation for User.




          Collectors.partitioningBy



          On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:



          Map<Boolean, List<User>> userAgeMap = users.stream()
          
        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));
          
List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);
          
List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);





          share|improve this answer



















          • 1




            For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
            – Alexander
            Dec 20 '18 at 18:47



















          6














          You're after the partitioningBy collector:



          Map<Boolean, List<User>> result = 
          
             users.stream().collect(partitioningBy(u -> u.getAge() > 21));


          Then use it as follows:



          List<User> userAboveThreshold = result.get(true);
          
List<User> userBelowThreshold = result.get(false);





          share|improve this answer

















          • 2




            @Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
            – Eugene
            Dec 20 '18 at 13:58











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          2 Answers
          2






          active

          oldest

          votes








          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          7














          List.removeAll



          You can use removeAll to obtain the complimentary list.



          List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'
          
userBelowThreshold.removeAll(userAboveThreshold);


          Note: This would require overridden equals and hashCode implementation for User.




          Collectors.partitioningBy



          On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:



          Map<Boolean, List<User>> userAgeMap = users.stream()
          
        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));
          
List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);
          
List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);





          share|improve this answer



















          • 1




            For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
            – Alexander
            Dec 20 '18 at 18:47
















          7














          List.removeAll



          You can use removeAll to obtain the complimentary list.



          List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'
          
userBelowThreshold.removeAll(userAboveThreshold);


          Note: This would require overridden equals and hashCode implementation for User.




          Collectors.partitioningBy



          On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:



          Map<Boolean, List<User>> userAgeMap = users.stream()
          
        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));
          
List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);
          
List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);





          share|improve this answer



















          • 1




            For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
            – Alexander
            Dec 20 '18 at 18:47














          7












          7








          7






          List.removeAll



          You can use removeAll to obtain the complimentary list.



          List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'
          
userBelowThreshold.removeAll(userAboveThreshold);


          Note: This would require overridden equals and hashCode implementation for User.




          Collectors.partitioningBy



          On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:



          Map<Boolean, List<User>> userAgeMap = users.stream()
          
        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));
          
List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);
          
List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);





          share|improve this answer














          List.removeAll



          You can use removeAll to obtain the complimentary list.



          List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'
          
userBelowThreshold.removeAll(userAboveThreshold);


          Note: This would require overridden equals and hashCode implementation for User.




          Collectors.partitioningBy



          On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:



          Map<Boolean, List<User>> userAgeMap = users.stream()
          
        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));
          
List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);
          
List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);






          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited Dec 20 '18 at 13:37

























          answered Dec 20 '18 at 13:27









          nullpointer

          43.1k1093178




          43.1k1093178








          • 1




            For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
            – Alexander
            Dec 20 '18 at 18:47














          • 1




            For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
            – Alexander
            Dec 20 '18 at 18:47








          1




          1




          For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
          – Alexander
          Dec 20 '18 at 18:47




          For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
          – Alexander
          Dec 20 '18 at 18:47













          6














          You're after the partitioningBy collector:



          Map<Boolean, List<User>> result = 
          
             users.stream().collect(partitioningBy(u -> u.getAge() > 21));


          Then use it as follows:



          List<User> userAboveThreshold = result.get(true);
          
List<User> userBelowThreshold = result.get(false);





          share|improve this answer

















          • 2




            @Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
            – Eugene
            Dec 20 '18 at 13:58
















          6














          You're after the partitioningBy collector:



          Map<Boolean, List<User>> result = 
          
             users.stream().collect(partitioningBy(u -> u.getAge() > 21));


          Then use it as follows:



          List<User> userAboveThreshold = result.get(true);
          
List<User> userBelowThreshold = result.get(false);





          share|improve this answer

















          • 2




            @Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
            – Eugene
            Dec 20 '18 at 13:58














          6












          6








          6






          You're after the partitioningBy collector:



          Map<Boolean, List<User>> result = 
          
             users.stream().collect(partitioningBy(u -> u.getAge() > 21));


          Then use it as follows:



          List<User> userAboveThreshold = result.get(true);
          
List<User> userBelowThreshold = result.get(false);





          share|improve this answer












          You're after the partitioningBy collector:



          Map<Boolean, List<User>> result = 
          
             users.stream().collect(partitioningBy(u -> u.getAge() > 21));


          Then use it as follows:



          List<User> userAboveThreshold = result.get(true);
          
List<User> userBelowThreshold = result.get(false);






          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered Dec 20 '18 at 13:26









          Aomine

          40.3k73870




          40.3k73870








          • 2




            @Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
            – Eugene
            Dec 20 '18 at 13:58














          • 2




            @Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
            – Eugene
            Dec 20 '18 at 13:58








          2




          2




          @Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
          – Eugene
          Dec 20 '18 at 13:58




          @Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
          – Eugene
          Dec 20 '18 at 13:58


















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