Group a list of objects by an attribute

Question

I need to group a list of objects  Student  using an attribute  Location  of the particular object  The code is like below  public class Grouping       public static void main String   args             List lt Student gt  studlist   new ArrayList lt Student gt             studlist add new Student  quot 1726 quot    quot John quot    quot New York quot             studlist add new Student  quot 4321 quot    quot Max quot    quot California quot             studlist add new Student  quot 2234 quot    quot Andrew quot    quot Los Angeles quot             studlist add new Student  quot 5223 quot    quot Michael quot    quot New York quot             studlist add new Student  quot 7765 quot    quot Sam quot    quot California quot             studlist add new Student  quot 3442 quot    quot Mark quot    quot New York quot               class Student       String stud id      String stud name      String stud location       Student String sid  String sname  String slocation            this stud id   sid          this stud name   sname          this stud location   slocation           Please suggest me a clean way to do it

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You can use the following   Map lt String  List lt Student gt  gt  groupedStudents   new HashMap lt String  List lt Student gt  gt     for  Student student  studlist        String key   student stud location      if  groupedStudents get key     null            groupedStudents put key  new ArrayList lt Student gt                groupedStudents get key  add student         print   Set lt String gt  groupedStudentsKeySet   groupedCustomer keySet    for  String location  groupedStudentsKeySet       List lt Student gt  stdnts   groupedStudents get location      for  Student student   stdnts            System out println  ID     student stud id   t   Name     student stud name   t   Location     student stud location

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Map lt String  List lt Student gt  gt  map   new HashMap lt String  List lt Student gt  gt      for  Student student   studlist        String key    student stud location      if map containsKey key            List lt Student gt  list   map get key           list add student         else          List lt Student gt  list   new ArrayList lt Student gt             list add student           map put key  list

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Java 8 groupingBy Collector Probably it s late but I like to share an improved idea to this problem  This is basically the same of  Vitalii Fedorenko s answer but more handly to play around  You can just use the Collectors groupingBy   by passing the grouping logic as function parameter and you will get the splitted list with the key parameter mapping  Note that using Optional is used to avoid the unwanted NPE when the provided list is null public static  lt E  K gt  Map lt K  List lt E gt  gt  groupBy List lt E gt  list  Function lt E  K gt  keyFunction        return Optional ofNullable list               orElseGet ArrayList  new               stream                collect Collectors groupingBy keyFunction       Now you can groupBy anything with this  For the use case here in the question Map lt String  List lt Student gt  gt  map   groupBy studlist  Student  getLocation   Maybe you would like to look into this also Guide to Java 8 groupingBy Collector

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You can sort like this       Collections sort studlist  new Comparator lt Student gt                Override         public int compare Student o1  Student o2                return o1 getStud location   compareTo o2 getStud location                         Assuming you also have the getter for location on your Student class

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Using Java 8  import java util List  import java util Map  import java util stream Collectors  import java util stream Stream   class Student        String stud id      String stud name      String stud location       public String getStud id             return stud id             public String getStud name             return stud name             public String getStud location             return stud location               Student String sid  String sname  String slocation             this stud id   sid          this stud name   sname          this stud location   slocation            class Temp       public static void main String args                   Stream lt Student gt  studs            Stream of new Student  1726    John    New York                    new Student  4321    Max    California                    new Student  2234    Max    Los Angeles                    new Student  7765    Sam    California             Map lt String  Map lt Object  List lt Student gt  gt  gt  map  studs collect Collectors groupingBy Student  getStud name Collectors groupingBy Student  getStud location                     System out println map    print by name and then location            The result will be         Max           Los Angeles  Student 214c265e            California  Student 448139f0              John           New York  Student 7cca494b              Sam           California  Student 7ba4f24f

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Function lt Student  List lt Object gt  gt  compositKey   std - gt                  Arrays asList std stud location             studentList stream   collect Collectors groupingBy compositKey  Collectors toList        If you want to add multiple objects for group by you can simply add the object in compositKey method separating by a comma   Function lt Student  List lt Object gt  gt  compositKey   std - gt                  Arrays asList std stud location   std stud name             studentList stream   collect Collectors groupingBy compositKey  Collectors toList

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you can use guava s Multimaps   Canonical class Persion        String name      Integer age   List lt Persion gt  list        new Persion  qianzi   100      new Persion  qianzi   99      new Persion  zhijia   99    println Multimaps index list    Persion p - gt  return p name      it print       qianzi  com ctcf message Persion qianzi  100  com ctcf message Persion qianzi  88   zhijia  com ctcf message Persion zhijia  99

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Implement SQL GROUP BY Feature in Java using Comparator  comparator will compare your column data  and sort it  Basically if you keep sorted data that looks as grouped data  for example if you have same repeated column data then sort mechanism sort them keeping same data one side and then look for other data which is dissimilar data  This indirectly viewed as GROUPING of same data   public class GroupByFeatureInJava        public static void main String   args            ProductBean p1   new ProductBean  P1   20  new Date             ProductBean p2   new ProductBean  P1   30  new Date             ProductBean p3   new ProductBean  P2   20  new Date             ProductBean p4   new ProductBean  P1   20  new Date             ProductBean p5   new ProductBean  P3   60  new Date             ProductBean p6   new ProductBean  P1   20  new Date              List lt ProductBean gt  list   new ArrayList lt ProductBean gt             list add p1           list add p2           list add p3           list add p4           list add p5           list add p6            for  Iterator iterator   list iterator    iterator hasNext                   ProductBean bean    ProductBean  iterator next                System out println bean                     System out println           AFTER GROUP BY PRODUCT ID                   Collections sort list  new ProductBean   new CompareByProductID             for  Iterator iterator   list iterator    iterator hasNext                   ProductBean bean    ProductBean  iterator next                System out println bean                      System out println           AFTER GROUP BY PRICE                   Collections sort list  new ProductBean   new CompareByProductPrice             for  Iterator iterator   list iterator    iterator hasNext                   ProductBean bean    ProductBean  iterator next                System out println bean                      class ProductBean       String productId      int price      Date date        Override     public String toString             return  ProductBean      productId         price         date                  ProductBean               ProductBean String productId  int price  Date date            this productId   productId          this price   price          this date   date            class CompareByProductID implements Comparator lt ProductBean gt            public int compare ProductBean p1  ProductBean p2                if  p1 productId compareTo p2 productId   gt  0                    return 1                            if  p1 productId compareTo p2 productId   lt  0                    return -1                               at this point all a b c d are equal    so return  equal              return 0                     Override         public boolean equals Object obj                   TODO Auto-generated method stub             return super equals obj                        class CompareByProductPrice implements Comparator lt ProductBean gt             Override         public int compare ProductBean p1  ProductBean p2                   this mean the first column is tied in thee two rows             if  p1 price  gt  p2 price                    return 1                            if  p1 price  lt  p2 price                    return -1                            return 0                    public boolean equals Object obj                   TODO Auto-generated method stub             return super equals obj                        class CompareByCreateDate implements Comparator lt ProductBean gt             Override         public int compare ProductBean p1  ProductBean p2                if  p1 date after p2 date                     return 1                            if  p1 date before p2 date                     return -1                            return 0                     Override         public boolean equals Object obj                   TODO Auto-generated method stub             return super equals obj                       Output is here for the above ProductBean list is done GROUP BY criteria  here if you see the input data that is given list of ProductBean to Collections sort list  object of Comparator for your required column  This will sort based on your comparator implementation and you will be able to see the GROUPED data in below output  Hope this helps                   BEFORE GROUPING INPUT DATA LOOKS THIS WAY            ProductBean  P1 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P1 30 Mon Nov 17 09 31 01 IST 2014      ProductBean  P2 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P1 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P3 60 Mon Nov 17 09 31 01 IST 2014      ProductBean  P1 20 Mon Nov 17 09 31 01 IST 2014               AFTER GROUP BY PRODUCT ID            ProductBean  P1 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P1 30 Mon Nov 17 09 31 01 IST 2014      ProductBean  P1 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P1 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P2 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P3 60 Mon Nov 17 09 31 01 IST 2014                AFTER GROUP BY PRICE            ProductBean  P1 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P1 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P2 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P1 20 Mon Nov 17 09 31 01 IST 2014      ProductBean  P1 30 Mon Nov 17 09 31 01 IST 2014      ProductBean  P3 60 Mon Nov 17 09 31 01 IST 2014

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You could do this   Map lt String  List lt Student gt  gt  map   new HashMap lt String  List lt Student gt  gt     List lt Student gt  studlist   new ArrayList lt Student gt     studlist add new Student  1726    John    New York     map put  New York   studlist     the keys will be locations and the values list of students  So later you can get a group of students just by using   studlist   map get  New York

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This will add the students object to the HashMap with locationID as key   HashMap lt Integer  List lt Student gt  gt  hashMap   new HashMap lt Integer  List lt Student gt  gt       Iterate over this code and add students to the HashMap   if   hashMap containsKey locationId         List lt Student gt  list   new ArrayList lt Student gt         list add student        hashMap put locationId  list     else       hashMap get locationId  add student       If you want all the student with particular location details then you can use this   hashMap get locationId     which will get you all the students with the same the location ID

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In Java 8   Map lt String  List lt Student gt  gt  studlistGrouped       studlist stream   collect Collectors groupingBy w - gt  w stud location

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public class Test9        static class Student            String stud id          String stud name          String stud location           public Student String stud id  String stud name  String stud location                super                this stud id   stud id              this stud name   stud name              this stud location   stud location                     public String getStud id                 return stud id                     public void setStud id String stud id                this stud id   stud id                     public String getStud name                 return stud name                     public void setStud name String stud name                this stud name   stud name                     public String getStud location                 return stud location                     public void setStud location String stud location                this stud location   stud location                      Override         public String toString                 return    stud id     stud id      stud name     stud name                              public static void main String   args             List lt Student gt  list   new ArrayList lt Student gt             list add new Student  1726    John Easton    Lancaster             list add new Student  4321    Max Carrados    London             list add new Student  2234    Andrew Lewis    Lancaster             list add new Student  5223    Michael Benson    Leeds             list add new Student  5225    Sanath Jayasuriya    Leeds             list add new Student  7765    Samuael Vatican    California             list add new Student  3442    Mark Farley    Ladykirk             list add new Student  3443    Alex Stuart    Ladykirk             list add new Student  4321    Michael Stuart    California              Map lt String  List lt Student gt  gt  map1                     list                  stream                 sorted Comparator comparing Student  getStud id                       thenComparing Student  getStud name                       thenComparing Student  getStud location                                          collect Collectors groupingBy                   ch - gt  ch stud location                       System out println map1         Output     Ladykirk    stud id 3442  stud name Mark Farley      stud id 3443  stud name Alex Stuart       Leeds    stud id 5223  stud name Michael Benson       stud id 5225  stud name Sanath Jayasuriya        London    stud id 4321  stud name Max Carrados         Lancaster    stud id 1726  stud name John Easton          stud id 2234  stud name Andrew Lewis          California    stud id 4321  stud name Michael Stuart         stud id 7765  stud name Samuael Vatican                 main

[java] Group a list of objects by an attribute

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