Refactor a method to using stream APIIs there a concise way to iterate over a stream with indices in Java 8?Why does Stream<T> not implement Iterable<T>?Should I always use a parallel stream when possible?Custom thread pool in Java 8 parallel streamHow to Convert a Java 8 Stream to an Array?Convert Iterable to Stream using Java 8 JDKWhat's the difference between map and flatMap methods in Java 8?Why are Java Streams once-off?How to sum a list of integers with java streams?Populate a map conditionally using streams - Java 8

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Refactor a method to using stream API


Is there a concise way to iterate over a stream with indices in Java 8?Why does Stream<T> not implement Iterable<T>?Should I always use a parallel stream when possible?Custom thread pool in Java 8 parallel streamHow to Convert a Java 8 Stream to an Array?Convert Iterable to Stream using Java 8 JDKWhat's the difference between map and flatMap methods in Java 8?Why are Java Streams once-off?How to sum a list of integers with java streams?Populate a map conditionally using streams - Java 8













8















I am currently told to count all .sql files which are hangin' out on some servers. Manually solving this very basic task is not an option, instead, I wrote some code that makes use of a SimpleFileVisitor<Path> and stores all the sql files found along with its parent path in a Map<Path, List<Path>>.



Now I want to receive the total amount of sql files found independent from their locations. I got it working with an enhanced for loop (almost the classic way):



public int getTotalAmountOfSqlFiles(Map<Path, List<Path>> sqlFilesInDirectories) 
int totalAmount = 0;

for (Path directory : sqlFilesInDirectories.keySet())
List<Path> sqlFiles = sqlFilesInDirectories.get(directory);
totalAmount += sqlFiles.size();


return totalAmount;



The question is now, how can I do the same using the stream API?



I wasn't able to get compilable code to work which isn't obviously doing the wrong thing.

The following line looks like a good idea to me, but not to the compiler, unfortunately.



totalAmount = sqlFilesInDirectories.entrySet().stream().map(List::size).sum();


The compiler says




Cannot infer type argument(s) for <R> map(Function<? super T,? extends R>




Does anyone know what I am doing wrong (and maybe provide some educated stream-API using solution)?










share|improve this question




























    8















    I am currently told to count all .sql files which are hangin' out on some servers. Manually solving this very basic task is not an option, instead, I wrote some code that makes use of a SimpleFileVisitor<Path> and stores all the sql files found along with its parent path in a Map<Path, List<Path>>.



    Now I want to receive the total amount of sql files found independent from their locations. I got it working with an enhanced for loop (almost the classic way):



    public int getTotalAmountOfSqlFiles(Map<Path, List<Path>> sqlFilesInDirectories) 
    int totalAmount = 0;

    for (Path directory : sqlFilesInDirectories.keySet())
    List<Path> sqlFiles = sqlFilesInDirectories.get(directory);
    totalAmount += sqlFiles.size();


    return totalAmount;



    The question is now, how can I do the same using the stream API?



    I wasn't able to get compilable code to work which isn't obviously doing the wrong thing.

    The following line looks like a good idea to me, but not to the compiler, unfortunately.



    totalAmount = sqlFilesInDirectories.entrySet().stream().map(List::size).sum();


    The compiler says




    Cannot infer type argument(s) for <R> map(Function<? super T,? extends R>




    Does anyone know what I am doing wrong (and maybe provide some educated stream-API using solution)?










    share|improve this question


























      8












      8








      8


      1






      I am currently told to count all .sql files which are hangin' out on some servers. Manually solving this very basic task is not an option, instead, I wrote some code that makes use of a SimpleFileVisitor<Path> and stores all the sql files found along with its parent path in a Map<Path, List<Path>>.



      Now I want to receive the total amount of sql files found independent from their locations. I got it working with an enhanced for loop (almost the classic way):



      public int getTotalAmountOfSqlFiles(Map<Path, List<Path>> sqlFilesInDirectories) 
      int totalAmount = 0;

      for (Path directory : sqlFilesInDirectories.keySet())
      List<Path> sqlFiles = sqlFilesInDirectories.get(directory);
      totalAmount += sqlFiles.size();


      return totalAmount;



      The question is now, how can I do the same using the stream API?



      I wasn't able to get compilable code to work which isn't obviously doing the wrong thing.

      The following line looks like a good idea to me, but not to the compiler, unfortunately.



      totalAmount = sqlFilesInDirectories.entrySet().stream().map(List::size).sum();


      The compiler says




      Cannot infer type argument(s) for <R> map(Function<? super T,? extends R>




      Does anyone know what I am doing wrong (and maybe provide some educated stream-API using solution)?










      share|improve this question
















      I am currently told to count all .sql files which are hangin' out on some servers. Manually solving this very basic task is not an option, instead, I wrote some code that makes use of a SimpleFileVisitor<Path> and stores all the sql files found along with its parent path in a Map<Path, List<Path>>.



      Now I want to receive the total amount of sql files found independent from their locations. I got it working with an enhanced for loop (almost the classic way):



      public int getTotalAmountOfSqlFiles(Map<Path, List<Path>> sqlFilesInDirectories) 
      int totalAmount = 0;

      for (Path directory : sqlFilesInDirectories.keySet())
      List<Path> sqlFiles = sqlFilesInDirectories.get(directory);
      totalAmount += sqlFiles.size();


      return totalAmount;



      The question is now, how can I do the same using the stream API?



      I wasn't able to get compilable code to work which isn't obviously doing the wrong thing.

      The following line looks like a good idea to me, but not to the compiler, unfortunately.



      totalAmount = sqlFilesInDirectories.entrySet().stream().map(List::size).sum();


      The compiler says




      Cannot infer type argument(s) for <R> map(Function<? super T,? extends R>




      Does anyone know what I am doing wrong (and maybe provide some educated stream-API using solution)?







      java java-8 sum java-stream






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited yesterday









      Stefan Zobel

      2,48231931




      2,48231931










      asked yesterday









      deHaardeHaar

      2,66961729




      2,66961729






















          4 Answers
          4






          active

          oldest

          votes


















          11














          Not sure why people are involving keySet here when the required sum is only of List size contained in values. Just sum the size of all values.



          return sqlFilesInDirectories.values().stream().mapToInt(List::size).sum();


          And even the for loop version should be simply this,



          for (List<Path> list : sqlFilesInDirectories.values()) 
          totalAmount += list.size();



          As iterating over keyset and then getting value from map isn't really required and will not be better performance wise.






          share|improve this answer
































            4














            It's because in stream you're taking whole entries instead of values. This should do it:



            totalAmount = sqlFilesInDirectories.values().stream().map(List::size).sum();





            share|improve this answer






























              3














              Other answers provide the shortest way to sum all entries, but if you need the amount of scripts per Path you can use the following:



              Map<Path, Integer> amountOfFilesForPath =
              files.entrySet().stream().collect(Collectors.groupingBy(Map.Entry::getKey,
              Collectors.summingInt(value -> value.getValue().size())));


              And you can also get the total value:



              int sum = amountOfFilesForPath.values().stream().mapToInt(Integer::intValue).sum();





              share|improve this answer






























                1














                Try



                int totalAmount = sqlFilesInDirectories.keySet().stream().map(sqlFilesInDirectories::get).mapToInt(List::size).sum();





                share|improve this answer






















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                  4 Answers
                  4






                  active

                  oldest

                  votes








                  4 Answers
                  4






                  active

                  oldest

                  votes









                  active

                  oldest

                  votes






                  active

                  oldest

                  votes









                  11














                  Not sure why people are involving keySet here when the required sum is only of List size contained in values. Just sum the size of all values.



                  return sqlFilesInDirectories.values().stream().mapToInt(List::size).sum();


                  And even the for loop version should be simply this,



                  for (List<Path> list : sqlFilesInDirectories.values()) 
                  totalAmount += list.size();



                  As iterating over keyset and then getting value from map isn't really required and will not be better performance wise.






                  share|improve this answer





























                    11














                    Not sure why people are involving keySet here when the required sum is only of List size contained in values. Just sum the size of all values.



                    return sqlFilesInDirectories.values().stream().mapToInt(List::size).sum();


                    And even the for loop version should be simply this,



                    for (List<Path> list : sqlFilesInDirectories.values()) 
                    totalAmount += list.size();



                    As iterating over keyset and then getting value from map isn't really required and will not be better performance wise.






                    share|improve this answer



























                      11












                      11








                      11







                      Not sure why people are involving keySet here when the required sum is only of List size contained in values. Just sum the size of all values.



                      return sqlFilesInDirectories.values().stream().mapToInt(List::size).sum();


                      And even the for loop version should be simply this,



                      for (List<Path> list : sqlFilesInDirectories.values()) 
                      totalAmount += list.size();



                      As iterating over keyset and then getting value from map isn't really required and will not be better performance wise.






                      share|improve this answer















                      Not sure why people are involving keySet here when the required sum is only of List size contained in values. Just sum the size of all values.



                      return sqlFilesInDirectories.values().stream().mapToInt(List::size).sum();


                      And even the for loop version should be simply this,



                      for (List<Path> list : sqlFilesInDirectories.values()) 
                      totalAmount += list.size();



                      As iterating over keyset and then getting value from map isn't really required and will not be better performance wise.







                      share|improve this answer














                      share|improve this answer



                      share|improve this answer








                      edited yesterday

























                      answered yesterday









                      Pushpesh Kumar RajwanshiPushpesh Kumar Rajwanshi

                      10.8k21230




                      10.8k21230























                          4














                          It's because in stream you're taking whole entries instead of values. This should do it:



                          totalAmount = sqlFilesInDirectories.values().stream().map(List::size).sum();





                          share|improve this answer



























                            4














                            It's because in stream you're taking whole entries instead of values. This should do it:



                            totalAmount = sqlFilesInDirectories.values().stream().map(List::size).sum();





                            share|improve this answer

























                              4












                              4








                              4







                              It's because in stream you're taking whole entries instead of values. This should do it:



                              totalAmount = sqlFilesInDirectories.values().stream().map(List::size).sum();





                              share|improve this answer













                              It's because in stream you're taking whole entries instead of values. This should do it:



                              totalAmount = sqlFilesInDirectories.values().stream().map(List::size).sum();






                              share|improve this answer












                              share|improve this answer



                              share|improve this answer










                              answered yesterday









                              AndronicusAndronicus

                              5,91021733




                              5,91021733





















                                  3














                                  Other answers provide the shortest way to sum all entries, but if you need the amount of scripts per Path you can use the following:



                                  Map<Path, Integer> amountOfFilesForPath =
                                  files.entrySet().stream().collect(Collectors.groupingBy(Map.Entry::getKey,
                                  Collectors.summingInt(value -> value.getValue().size())));


                                  And you can also get the total value:



                                  int sum = amountOfFilesForPath.values().stream().mapToInt(Integer::intValue).sum();





                                  share|improve this answer



























                                    3














                                    Other answers provide the shortest way to sum all entries, but if you need the amount of scripts per Path you can use the following:



                                    Map<Path, Integer> amountOfFilesForPath =
                                    files.entrySet().stream().collect(Collectors.groupingBy(Map.Entry::getKey,
                                    Collectors.summingInt(value -> value.getValue().size())));


                                    And you can also get the total value:



                                    int sum = amountOfFilesForPath.values().stream().mapToInt(Integer::intValue).sum();





                                    share|improve this answer

























                                      3












                                      3








                                      3







                                      Other answers provide the shortest way to sum all entries, but if you need the amount of scripts per Path you can use the following:



                                      Map<Path, Integer> amountOfFilesForPath =
                                      files.entrySet().stream().collect(Collectors.groupingBy(Map.Entry::getKey,
                                      Collectors.summingInt(value -> value.getValue().size())));


                                      And you can also get the total value:



                                      int sum = amountOfFilesForPath.values().stream().mapToInt(Integer::intValue).sum();





                                      share|improve this answer













                                      Other answers provide the shortest way to sum all entries, but if you need the amount of scripts per Path you can use the following:



                                      Map<Path, Integer> amountOfFilesForPath =
                                      files.entrySet().stream().collect(Collectors.groupingBy(Map.Entry::getKey,
                                      Collectors.summingInt(value -> value.getValue().size())));


                                      And you can also get the total value:



                                      int sum = amountOfFilesForPath.values().stream().mapToInt(Integer::intValue).sum();






                                      share|improve this answer












                                      share|improve this answer



                                      share|improve this answer










                                      answered yesterday









                                      GlainsGlains

                                      1,166719




                                      1,166719





















                                          1














                                          Try



                                          int totalAmount = sqlFilesInDirectories.keySet().stream().map(sqlFilesInDirectories::get).mapToInt(List::size).sum();





                                          share|improve this answer



























                                            1














                                            Try



                                            int totalAmount = sqlFilesInDirectories.keySet().stream().map(sqlFilesInDirectories::get).mapToInt(List::size).sum();





                                            share|improve this answer

























                                              1












                                              1








                                              1







                                              Try



                                              int totalAmount = sqlFilesInDirectories.keySet().stream().map(sqlFilesInDirectories::get).mapToInt(List::size).sum();





                                              share|improve this answer













                                              Try



                                              int totalAmount = sqlFilesInDirectories.keySet().stream().map(sqlFilesInDirectories::get).mapToInt(List::size).sum();






                                              share|improve this answer












                                              share|improve this answer



                                              share|improve this answer










                                              answered yesterday









                                              Praveen EPraveen E

                                              55047




                                              55047



























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