Group by multiple field names in java 8

Question

I found the code for grouping the objects by some field name from POJO  Below is the code for that   public class Temp        static class Person            private String name          private int age          private long salary           Person String name  int age  long salary                 this name   name              this age   age              this salary   salary                      Override         public String toString                 return String format  Person name   s   age  d  salary  d    name  age  salary                        public static void main String   args            Stream lt Person gt  people   Stream of new Person  Paul   24  20000                   new Person  Mark   30  30000                   new Person  Will   28  28000                   new Person  William   28  28000            Map lt Integer  List lt Person gt  gt  peopleByAge          peopleByAge   people                  collect Collectors groupingBy p - gt  p age  Collectors mapping  Person p  - gt  p  toList               System out println peopleByAge             And the output is  which is correct     24  Person name  Paul   age 24  salary 20000    28  Person name  Will   age 28  salary 28000   Person name  William   age 28  salary 28000    30  Person name  Mark   age 30  salary 30000      But what if I want to group by multiple fields  I can obviously pass some POJO in groupingBy   method after implementing equals   method in that POJO but is there any other option like I can group by more than one fields from the given POJO   E g  here in my case  I want to group by name and age

User · Answer

Here look at the code   You can simply create a Function and let it do the work for you  kind of functional Style   Function lt Person  List lt Object gt  gt  compositeKey   personRecord - gt      Arrays  lt Object gt asList personRecord getName    personRecord getAge       Now you can use it as a map   Map lt Object  List lt Person gt  gt  map   people collect Collectors groupingBy compositeKey  Collectors toList        Cheers

User · Answer

I needed to make report for a catering firm which serves lunches for various clients  In other words  catering may have on or more firms which take orders from catering  and it must know how many lunches it must produce every single day for all it s clients    Just to notice  I didn t use sorting  in order not to over complicate this example   This is my code      Test public void test 2   throws Exception       Firm catering   DS firm   get 1       LocalDateTime ldtFrom   LocalDateTime of 2017  Month JANUARY  1  0  0       LocalDateTime ldtTo   LocalDateTime of 2017  Month MAY  2  0  0       Date dFrom   Date from ldtFrom atZone ZoneId systemDefault    toInstant         Date dTo   Date from ldtTo atZone ZoneId systemDefault    toInstant          List lt PersonOrders gt  LON   DS firm   getAllOrders catering  dFrom  dTo  false       Map lt Object  Long gt  M   LON stream   collect              Collectors groupingBy p                     - gt  Arrays asList p getDatum    p getPerson   getIdfirm    p getIdProduct                         Collectors counting           for  Map Entry lt Object  Long gt  e   M entrySet              Object key   e getKey            Long value   e getValue            System err println String format  Client firm   s  total   d   key  value

User · Answer

You have a few options here  The simplest is to chain your collectors  Map lt String  Map lt Integer  List lt Person gt  gt  gt  map   people      collect Collectors groupingBy Person  getName          Collectors groupingBy Person  getAge     Then to get a list of 18 year old people called Fred you would use  map get  quot Fred quot   get 18    A second option is to define a class that represents the grouping  This can be inside Person  This code uses a record but it could just as easily be a class  with equals and hashCode defined  in versions of Java before JEP 359 was added  class Person       record NameAge String name  int age           public NameAge getNameAge             return new NameAge name  age            Then you can use  Map lt NameAge  List lt Person gt  gt  map   people collect Collectors groupingBy Person  getNameAge     and search with map get new NameAge  quot Fred quot   18     Finally if you don t want to implement your own group record then many of the Java frameworks around have a pair class designed for this type of thing  For example  apache commons pair If you use one of these libraries then you can make the key to the map a pair of the name and age  Map lt Pair lt String  Integer gt   List lt Person gt  gt  map       people collect Collectors groupingBy p - gt  Pair of p getName    p getAge        and retrieve with  map get Pair of  quot Fred quot   18     Personally I don t really see much value in generic tuples now that records are available in the language as records display intent better and require very little code

User · Answer

You can use List as a classifier for many fields  but you need wrap null values into Optional    Function lt String  List gt  classifier    item  - gt  List of      item getFieldA        item getFieldB        Optional ofNullable item getFieldC        Map lt List  List lt Item gt  gt  grouped   items stream        collect Collectors groupingBy classifier

User · Answer

This is how I did grouping by multiple fields branchCode and prdId  Just posting it for someone in need      import java math BigDecimal      import java math BigInteger      import java util ArrayList      import java util Iterator      import java util LinkedList      import java util List      import java util Map      import java util stream Collectors                          author charudatta joshi             public class Product1            public BigInteger branchCode          public BigInteger prdId          public String accountCode          public BigDecimal actualBalance          public BigDecimal sumActBal          public BigInteger countOfAccts           public Product1                        public Product1 BigInteger branchCode  BigInteger prdId  String accountCode  BigDecimal actualBalance                this branchCode   branchCode              this prdId   prdId              this accountCode   accountCode              this actualBalance   actualBalance                     public BigInteger getCountOfAccts                 return countOfAccts                     public void setCountOfAccts BigInteger countOfAccts                this countOfAccts   countOfAccts                     public BigDecimal getSumActBal                 return sumActBal                     public void setSumActBal BigDecimal sumActBal                this sumActBal   sumActBal                     public BigInteger getBranchCode                 return branchCode                     public void setBranchCode BigInteger branchCode                this branchCode   branchCode                     public BigInteger getPrdId                 return prdId                     public void setPrdId BigInteger prdId                this prdId   prdId                     public String getAccountCode                 return accountCode                     public void setAccountCode String accountCode                this accountCode   accountCode                     public BigDecimal getActualBalance                 return actualBalance                     public void setActualBalance BigDecimal actualBalance                this actualBalance   actualBalance                      Override         public String toString                 return  Product      branchCode     branchCode      prdId     prdId      accountCode     accountCode      actualBalance     actualBalance      sumActBal     sumActBal      countOfAccts     countOfAccts                           public static void main String   args                List lt Product1 gt  al   new ArrayList lt Product1 gt                 System out println al               al add new Product1 new BigInteger  01    new BigInteger  11     001   new BigDecimal  10                  al add new Product1 new BigInteger  01    new BigInteger  11     002   new BigDecimal  10                  al add new Product1 new BigInteger  01    new BigInteger  12     003   new BigDecimal  10                  al add new Product1 new BigInteger  01    new BigInteger  12     004   new BigDecimal  10                  al add new Product1 new BigInteger  01    new BigInteger  12     005   new BigDecimal  10                  al add new Product1 new BigInteger  01    new BigInteger  13     006   new BigDecimal  10                  al add new Product1 new BigInteger  02    new BigInteger  11     007   new BigDecimal  10                  al add new Product1 new BigInteger  02    new BigInteger  11     008   new BigDecimal  10                  al add new Product1 new BigInteger  02    new BigInteger  12     009   new BigDecimal  10                  al add new Product1 new BigInteger  02    new BigInteger  12     010   new BigDecimal  10                  al add new Product1 new BigInteger  02    new BigInteger  12     011   new BigDecimal  10                  al add new Product1 new BigInteger  02    new BigInteger  13     012   new BigDecimal  10                    Map lt BigInteger  Long gt  counting   al stream   collect Collectors groupingBy Product1  getBranchCode  Collectors counting                     System out println counting                  group by branch code             Map lt BigInteger  List lt Product1 gt  gt  groupByBrCd   al stream   collect Collectors groupingBy Product1  getBranchCode  Collectors toList                  System out println   n n n    groupByBrCd                 Map lt BigInteger  List lt Product1 gt  gt  groupByPrId   null                   Create a final List to show for output containing one element of each group             List lt Product gt  finalOutputList   new LinkedList lt Product gt                 Product newPrd   null                 Iterate over resultant  Map Of List             Iterator lt BigInteger gt  brItr   groupByBrCd keySet   iterator                Iterator lt BigInteger gt  prdidItr   null                     BigInteger brCode   null              BigInteger prdId   null               Map lt BigInteger  List lt Product gt  gt  tempMap   null              List lt Product1 gt  accListPerBr   null              List lt Product1 gt  accListPerBrPerPrd   null               Product1 tempPrd   null              Double sum   null              while  brItr hasNext                      brCode   brItr next                      get  list per branch                 accListPerBr   groupByBrCd get brCode                       group by br wise product wise                 groupByPrId accListPerBr stream   collect Collectors groupingBy Product1  getPrdId  Collectors toList                       System out println                                          System out println groupByPrId                    prdidItr   groupByPrId keySet   iterator                    while prdidItr hasNext                         prdId prdidItr next                           get list per brcode product code                     accListPerBrPerPrd groupByPrId get prdId                       newPrd   new Product                            Extract zeroth element to put in Output List to represent this group                     tempPrd   accListPerBrPerPrd get 0                       newPrd setBranchCode tempPrd getBranchCode                         newPrd setPrdId tempPrd getPrdId                            Set accCOunt by using size of list of our group                     newPrd setCountOfAccts BigInteger valueOf accListPerBrPerPrd size                            Sum actual balance of our  of list of our group                      sum   accListPerBrPerPrd stream   filter o - gt  o getActualBalance      null  mapToDouble o - gt  o getActualBalance   doubleValue    sum                        newPrd setSumActBal BigDecimal valueOf sum                           Add product element in final output list                      finalOutputList add newPrd                                                  System out println                                         System out println finalOutputList                      Output is as below                            Product branchCode 1  prdId 11  accountCode null  actualBalance null  sumActBal 20 0  countOfAccts 2   Product branchCode 1  prdId 12  accountCode null  actualBalance null  sumActBal 30 0  countOfAccts 3   Product branchCode 1  prdId 13  accountCode null  actualBalance null  sumActBal 10 0  countOfAccts 1   Product branchCode 2  prdId 11  accountCode null  actualBalance null  sumActBal 20 0  countOfAccts 2   Product branchCode 2  prdId 12  accountCode null  actualBalance null  sumActBal 30 0  countOfAccts 3   Product branchCode 2  prdId 13  accountCode null  actualBalance null  sumActBal 10 0  countOfAccts 1     After Formatting it      Product branchCode 1  prdId 11  accountCode null  actualBalance null  sumActBal 20 0  countOfAccts 2    Product branchCode 1  prdId 12  accountCode null  actualBalance null  sumActBal 30 0  countOfAccts 3    Product branchCode 1  prdId 13  accountCode null  actualBalance null  sumActBal 10 0  countOfAccts 1    Product branchCode 2  prdId 11  accountCode null  actualBalance null  sumActBal 20 0  countOfAccts 2    Product branchCode 2  prdId 12  accountCode null  actualBalance null  sumActBal 30 0  countOfAccts 3    Product branchCode 2  prdId 13  accountCode null  actualBalance null  sumActBal 10 0  countOfAccts 1

User · Answer

The groupingBy method has the first parameter is Function lt T K gt  where    param  lt T gt  the type of the input elements  param  lt K gt  the type of the keys  If we replace lambda with the anonymous class in your code  we can see some kind of that  people stream   collect Collectors groupingBy new Function lt Person  int gt                   Override             public int apply Person person                    return person getAge                                Just now change output parameter lt K gt   In this case  for example  I used a pair class from org apache commons lang3 tuple for grouping by name and age  but you may create your own class for filtering groups as you need  people stream   collect Collectors groupingBy new Function lt Person  Pair lt Integer  String gt  gt                       Override                 public YourFilter apply Person person                        return Pair of person getAge    person getName                                         Finally  after replacing with lambda back  code looks like that  Map lt Pair lt Integer String gt   List lt Person gt  gt  peopleByAgeAndName   people collect Collectors groupingBy p - gt  Pair of person getAge    person getName     Collectors mapping  Person p  - gt  p  toList

User · Answer

Hi You can simply concatenate your groupingByKey such as  Map lt String  List lt Person gt  gt  peopleBySomeKey   people                  collect Collectors groupingBy p - gt  getGroupingByKey p   Collectors mapping  Person p  - gt  p  toList            write getGroupingByKey   function private String getGroupingByKey Person p   return p getAge    -  p getName

User · Answer

Define a class for key definition in your group   class KeyObj        ArrayList lt Object gt  keys       public KeyObj  Object    objs             keys   new ArrayList lt Object gt              for  int i   0  i  lt  objs length  i                  keys add  objs i                             Add appropriate isEqual       you IDE should generate this      Now in your code   peopleByManyParams   people              collect Collectors groupingBy p - gt  new KeyObj  p age  p other1  p other2    Collectors mapping  Person p  - gt  p  toList

[java] Group by multiple field names in java 8

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