Constructing list from multi level map in Java 8












6















I have a multi-level map as follows:



 Map<String, Map<String, Student> outerMap = 

    {"cls1" : {"xyz" : Student(rollNumber=1, name="test1")}, 

     "cls2" : {"abc" : Student(rollNumber=2, name="test2")}}


Now I want to construct a list of string from the above map as follows:



["In class cls1 xyz with roll number 1",

 "In class cls2 abc with roll number 2"]


I have written as follows, but this is not working, in this context I have gone through the post as well: Java 8 Streams - Nested Maps to List, but did not get much idea. 



 List<String> classes = outerMap.keySet();

 List<String> studentList = classes.stream()

      .map(cls -> 

           outerMap.get(cls).keySet().stream()

           .map(student -> "In class "+ cls +

            student + " with roll number " +

            outerMap.get(cls).get(student).getRollNum() +"n"

            ).collect(Collectors.toList());





java list java-8 hashmap







share|improve this question



























    6















    I have a multi-level map as follows:



     Map<String, Map<String, Student> outerMap = 
    
    {"cls1" : {"xyz" : Student(rollNumber=1, name="test1")}, 
    
     "cls2" : {"abc" : Student(rollNumber=2, name="test2")}}


    Now I want to construct a list of string from the above map as follows:



    ["In class cls1 xyz with roll number 1",
    
 "In class cls2 abc with roll number 2"]


    I have written as follows, but this is not working, in this context I have gone through the post as well: Java 8 Streams - Nested Maps to List, but did not get much idea. 



     List<String> classes = outerMap.keySet();
    
 List<String> studentList = classes.stream()
    
      .map(cls -> 
    
           outerMap.get(cls).keySet().stream()
    
           .map(student -> "In class "+ cls +
    
            student + " with roll number " +
    
            outerMap.get(cls).get(student).getRollNum() +"n"
    
            ).collect(Collectors.toList());





    java list java-8 hashmap







    share|improve this question

























      6












      6








      6


      1






      I have a multi-level map as follows:



       Map<String, Map<String, Student> outerMap = 
      
    {"cls1" : {"xyz" : Student(rollNumber=1, name="test1")}, 
      
     "cls2" : {"abc" : Student(rollNumber=2, name="test2")}}


      Now I want to construct a list of string from the above map as follows:



      ["In class cls1 xyz with roll number 1",
      
 "In class cls2 abc with roll number 2"]


      I have written as follows, but this is not working, in this context I have gone through the post as well: Java 8 Streams - Nested Maps to List, but did not get much idea. 



       List<String> classes = outerMap.keySet();
      
 List<String> studentList = classes.stream()
      
      .map(cls -> 
      
           outerMap.get(cls).keySet().stream()
      
           .map(student -> "In class "+ cls +
      
            student + " with roll number " +
      
            outerMap.get(cls).get(student).getRollNum() +"n"
      
            ).collect(Collectors.toList());





      java list java-8 hashmap







      share|improve this question














      I have a multi-level map as follows:



       Map<String, Map<String, Student> outerMap = 
      
    {"cls1" : {"xyz" : Student(rollNumber=1, name="test1")}, 
      
     "cls2" : {"abc" : Student(rollNumber=2, name="test2")}}


      Now I want to construct a list of string from the above map as follows:



      ["In class cls1 xyz with roll number 1",
      
 "In class cls2 abc with roll number 2"]


      I have written as follows, but this is not working, in this context I have gone through the post as well: Java 8 Streams - Nested Maps to List, but did not get much idea. 



       List<String> classes = outerMap.keySet();
      
 List<String> studentList = classes.stream()
      
      .map(cls -> 
      
           outerMap.get(cls).keySet().stream()
      
           .map(student -> "In class "+ cls +
      
            student + " with roll number " +
      
            outerMap.get(cls).get(student).getRollNum() +"n"
      
            ).collect(Collectors.toList());





      java list java-8 hashmap




      java list java-8 hashmap






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked 1 hour ago









      JoyJoy

      1,09472352




      1,09472352
























          3 Answers
          3






          active

          oldest

          votes


















          4














          You can simply use Map.forEach for this operation as:



          List<String> messages = new ArrayList<>();
          
outerMap.forEach((cls, students) ->
          
        students.forEach((name, student) -> 
          
                messages.add(convertToMessage(cls, name, student.getRollNumber()))));


          where convertToMessage is a util as :



          // this could be made cleaner using 'format'
          
String convertToMessage(String cls, String studentName, String rollNumber) {
          
    return "In class" + cls + "--> " + studentName + " with roll number: " + rollNumber;
          
}





          share|improve this answer

































            1














            One method use Java 8 Stream and lambda function:



            String format = "In class %s %s with roll number %d";
            
List<String> result = new ArrayList<>();
            
outerMap.entrySet().stream()
            
    .forEach(v -> {
            
        String className = v.getKey();
            
        v.getValue().entrySet().stream()
            
            .forEach(stringStudentEntry -> result.add(String.format(format,className,stringStudentEntry.getKey(),stringStudentEntry.getValue().getRollNumber())));
            
    });





            share|improve this answer































              1














              You may do it like so,



              List<String> formattedOutput = outerMap
              
    .entrySet().stream()
              
    .flatMap(e -> e.getValue().entrySet().stream().map(se -> "In class " + e.getKey()
              
        + " " + se.getKey() + " with roll number " + se.getValue().getRollNumber()))
              
    .collect(Collectors.toList());


              You have to use the flatMap operator instead of map operator.






              share|improve this answer





















              • 1





                @nullpointer Thanks for pointing that out. I have updated my answer.

                – Ravindra Ranwala
                43 mins 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









              4














              You can simply use Map.forEach for this operation as:



              List<String> messages = new ArrayList<>();
              
outerMap.forEach((cls, students) ->
              
        students.forEach((name, student) -> 
              
                messages.add(convertToMessage(cls, name, student.getRollNumber()))));


              where convertToMessage is a util as :



              // this could be made cleaner using 'format'
              
String convertToMessage(String cls, String studentName, String rollNumber) {
              
    return "In class" + cls + "--> " + studentName + " with roll number: " + rollNumber;
              
}





              share|improve this answer






























                4














                You can simply use Map.forEach for this operation as:



                List<String> messages = new ArrayList<>();
                
outerMap.forEach((cls, students) ->
                
        students.forEach((name, student) -> 
                
                messages.add(convertToMessage(cls, name, student.getRollNumber()))));


                where convertToMessage is a util as :



                // this could be made cleaner using 'format'
                
String convertToMessage(String cls, String studentName, String rollNumber) {
                
    return "In class" + cls + "--> " + studentName + " with roll number: " + rollNumber;
                
}





                share|improve this answer




























                  4












                  4








                  4







                  You can simply use Map.forEach for this operation as:



                  List<String> messages = new ArrayList<>();
                  
outerMap.forEach((cls, students) ->
                  
        students.forEach((name, student) -> 
                  
                messages.add(convertToMessage(cls, name, student.getRollNumber()))));


                  where convertToMessage is a util as :



                  // this could be made cleaner using 'format'
                  
String convertToMessage(String cls, String studentName, String rollNumber) {
                  
    return "In class" + cls + "--> " + studentName + " with roll number: " + rollNumber;
                  
}





                  share|improve this answer















                  You can simply use Map.forEach for this operation as:



                  List<String> messages = new ArrayList<>();
                  
outerMap.forEach((cls, students) ->
                  
        students.forEach((name, student) -> 
                  
                messages.add(convertToMessage(cls, name, student.getRollNumber()))));


                  where convertToMessage is a util as :



                  // this could be made cleaner using 'format'
                  
String convertToMessage(String cls, String studentName, String rollNumber) {
                  
    return "In class" + cls + "--> " + studentName + " with roll number: " + rollNumber;
                  
}






                  share|improve this answer














                  share|improve this answer



                  share|improve this answer








                  edited 30 mins ago

























                  answered 1 hour ago









                  nullpointernullpointer

                  45.5k1096185




                  45.5k1096185

























                      1














                      One method use Java 8 Stream and lambda function:



                      String format = "In class %s %s with roll number %d";
                      
List<String> result = new ArrayList<>();
                      
outerMap.entrySet().stream()
                      
    .forEach(v -> {
                      
        String className = v.getKey();
                      
        v.getValue().entrySet().stream()
                      
            .forEach(stringStudentEntry -> result.add(String.format(format,className,stringStudentEntry.getKey(),stringStudentEntry.getValue().getRollNumber())));
                      
    });





                      share|improve this answer




























                        1














                        One method use Java 8 Stream and lambda function:



                        String format = "In class %s %s with roll number %d";
                        
List<String> result = new ArrayList<>();
                        
outerMap.entrySet().stream()
                        
    .forEach(v -> {
                        
        String className = v.getKey();
                        
        v.getValue().entrySet().stream()
                        
            .forEach(stringStudentEntry -> result.add(String.format(format,className,stringStudentEntry.getKey(),stringStudentEntry.getValue().getRollNumber())));
                        
    });





                        share|improve this answer


























                          1












                          1








                          1







                          One method use Java 8 Stream and lambda function:



                          String format = "In class %s %s with roll number %d";
                          
List<String> result = new ArrayList<>();
                          
outerMap.entrySet().stream()
                          
    .forEach(v -> {
                          
        String className = v.getKey();
                          
        v.getValue().entrySet().stream()
                          
            .forEach(stringStudentEntry -> result.add(String.format(format,className,stringStudentEntry.getKey(),stringStudentEntry.getValue().getRollNumber())));
                          
    });





                          share|improve this answer













                          One method use Java 8 Stream and lambda function:



                          String format = "In class %s %s with roll number %d";
                          
List<String> result = new ArrayList<>();
                          
outerMap.entrySet().stream()
                          
    .forEach(v -> {
                          
        String className = v.getKey();
                          
        v.getValue().entrySet().stream()
                          
            .forEach(stringStudentEntry -> result.add(String.format(format,className,stringStudentEntry.getKey(),stringStudentEntry.getValue().getRollNumber())));
                          
    });






                          share|improve this answer












                          share|improve this answer



                          share|improve this answer










                          answered 43 mins ago









                          TongChenTongChen

                          21529




                          21529























                              1














                              You may do it like so,



                              List<String> formattedOutput = outerMap
                              
    .entrySet().stream()
                              
    .flatMap(e -> e.getValue().entrySet().stream().map(se -> "In class " + e.getKey()
                              
        + " " + se.getKey() + " with roll number " + se.getValue().getRollNumber()))
                              
    .collect(Collectors.toList());


                              You have to use the flatMap operator instead of map operator.






                              share|improve this answer





















                              • 1





                                @nullpointer Thanks for pointing that out. I have updated my answer.

                                – Ravindra Ranwala
                                43 mins ago
















                              1














                              You may do it like so,



                              List<String> formattedOutput = outerMap
                              
    .entrySet().stream()
                              
    .flatMap(e -> e.getValue().entrySet().stream().map(se -> "In class " + e.getKey()
                              
        + " " + se.getKey() + " with roll number " + se.getValue().getRollNumber()))
                              
    .collect(Collectors.toList());


                              You have to use the flatMap operator instead of map operator.






                              share|improve this answer





















                              • 1





                                @nullpointer Thanks for pointing that out. I have updated my answer.

                                – Ravindra Ranwala
                                43 mins ago














                              1












                              1








                              1







                              You may do it like so,



                              List<String> formattedOutput = outerMap
                              
    .entrySet().stream()
                              
    .flatMap(e -> e.getValue().entrySet().stream().map(se -> "In class " + e.getKey()
                              
        + " " + se.getKey() + " with roll number " + se.getValue().getRollNumber()))
                              
    .collect(Collectors.toList());


                              You have to use the flatMap operator instead of map operator.






                              share|improve this answer















                              You may do it like so,



                              List<String> formattedOutput = outerMap
                              
    .entrySet().stream()
                              
    .flatMap(e -> e.getValue().entrySet().stream().map(se -> "In class " + e.getKey()
                              
        + " " + se.getKey() + " with roll number " + se.getValue().getRollNumber()))
                              
    .collect(Collectors.toList());


                              You have to use the flatMap operator instead of map operator.







                              share|improve this answer














                              share|improve this answer



                              share|improve this answer








                              edited 31 mins ago

























                              answered 1 hour ago









                              Ravindra RanwalaRavindra Ranwala

                              8,71531634




                              8,71531634








                              • 1





                                @nullpointer Thanks for pointing that out. I have updated my answer.

                                – Ravindra Ranwala
                                43 mins ago














                              • 1





                                @nullpointer Thanks for pointing that out. I have updated my answer.

                                – Ravindra Ranwala
                                43 mins ago








                              1




                              1





                              @nullpointer Thanks for pointing that out. I have updated my answer.

                              – Ravindra Ranwala
                              43 mins ago





                              @nullpointer Thanks for pointing that out. I have updated my answer.

                              – Ravindra Ranwala
                              43 mins ago


















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