Collections sort with multiple fields

Question

I have a list of  Report  objects with three fields  All String type -  ReportKey StudentNumber School   I have a sort code goes like-  Collections sort reportList  new Comparator lt Report gt        Override public int compare final Report record1  final Report record2          return  record1 getReportKey     record1 getStudentNumber     record1 getSchool                                   compareTo record2 getReportKey     record2 getStudentNumber     record2 getSchool                    For some reason  I don t have the sorted order  One advised to put spaces in between fields  but why   Do you see anything wrong with the code

User · Answer

A lot of answers above have fields compared in single comparator method which is not actually working. There are some answers though with different comparators implemented for each field, I am posting this because this example would be much more clearer and simple to understand I am believing.

class Student{
    Integer bornYear;
    Integer bornMonth;
    Integer bornDay;
    public Student(int bornYear, int bornMonth, int bornDay) {

        this.bornYear = bornYear;
        this.bornMonth = bornMonth;
        this.bornDay = bornDay;
    }
    public Student(int bornYear, int bornMonth) {

        this.bornYear = bornYear;
        this.bornMonth = bornMonth;

    }
    public Student(int bornYear) {

        this.bornYear = bornYear;

    }
    public Integer getBornYear() {
        return bornYear;
    }
    public void setBornYear(int bornYear) {
        this.bornYear = bornYear;
    }
    public Integer getBornMonth() {
        return bornMonth;
    }
    public void setBornMonth(int bornMonth) {
        this.bornMonth = bornMonth;
    }
    public Integer getBornDay() {
        return bornDay;
    }
    public void setBornDay(int bornDay) {
        this.bornDay = bornDay;
    }
    @Override
    public String toString() {
        return "Student [bornYear=" + bornYear + ", bornMonth=" + bornMonth + ", bornDay=" + bornDay + "]";
    }


}
class TestClass
{       

    // Comparator problem in JAVA for sorting objects based on multiple fields 
    public static void main(String[] args)
    {
        int N,c;// Number of threads

        Student s1=new Student(2018,12);
        Student s2=new Student(2018,12);
        Student s3=new Student(2018,11);
        Student s4=new Student(2017,6);
        Student s5=new Student(2017,4);
        Student s6=new Student(2016,8);
        Student s7=new Student(2018);
        Student s8=new Student(2017,8);
        Student s9=new Student(2017,2);
        Student s10=new Student(2017,9);

        List<Student> studentList=new ArrayList<>();
        studentList.add(s1);
        studentList.add(s2);
        studentList.add(s3);
        studentList.add(s4);
        studentList.add(s5);
        studentList.add(s6);
        studentList.add(s7);
        studentList.add(s8);
        studentList.add(s9);
        studentList.add(s10);

        Comparator<Student> byMonth=new Comparator<Student>() {
            @Override
            public int compare(Student st1,Student st2) {
                if(st1.getBornMonth()!=null && st2.getBornMonth()!=null) {
                    return st2.getBornMonth()-st1.getBornMonth();
                }
                else if(st1.getBornMonth()!=null) {
                    return 1;
                }
                else {
                    return -1;
                }
        }};

        Collections.sort(studentList, new Comparator<Student>() {
            @Override
            public int compare(Student st1,Student st2) {
                return st2.getBornYear()-st1.getBornYear();
        }}.thenComparing(byMonth));

        System.out.println("The sorted students list in descending is"+Arrays.deepToString(studentList.toArray()));



    }

}

OUTPUT

The sorted students list in descending is[Student [bornYear=2018, bornMonth=null, bornDay=null], Student [bornYear=2018, bornMonth=12, bornDay=null], Student [bornYear=2018, bornMonth=12, bornDay=null], Student [bornYear=2018, bornMonth=11, bornDay=null], Student [bornYear=2017, bornMonth=9, bornDay=null], Student [bornYear=2017, bornMonth=8, bornDay=null], Student [bornYear=2017, bornMonth=6, bornDay=null], Student [bornYear=2017, bornMonth=4, bornDay=null], Student [bornYear=2017, bornMonth=2, bornDay=null], Student [bornYear=2016, bornMonth=8, bornDay=null]]

User · Answer

This is an old question so I don t see a Java 8 equivalent  Here is an example for this specific case   import java util ArrayList  import java util Collections  import java util Comparator  import java util List          Compares multiple parts of the Report object      public class SimpleJava8ComparatorClass        public static void main String   args            List lt Report gt  reportList   new ArrayList lt  gt             reportList add new Report  reportKey2    studentNumber2    school1             reportList add new Report  reportKey4    studentNumber4    school6             reportList add new Report  reportKey1    studentNumber1    school1             reportList add new Report  reportKey3    studentNumber2    school4             reportList add new Report  reportKey2    studentNumber2    school3              System out println  pre-sorting            System out println reportList           System out println             Collections sort reportList  Comparator comparing Report  getReportKey               thenComparing Report  getStudentNumber               thenComparing Report  getSchool             System out println  post-sorting            System out println reportList              private static class Report            private String reportKey          private String studentNumber          private String school           public Report String reportKey  String studentNumber  String school                this reportKey   reportKey              this studentNumber   studentNumber              this school   school                     public String getReportKey                 return reportKey                     public void setReportKey String reportKey                this reportKey   reportKey                     public String getStudentNumber                 return studentNumber                     public void setStudentNumber String studentNumber                this studentNumber   studentNumber                     public String getSchool                 return school                     public void setSchool String school                this school   school                      Override         public String toString                 return  Report                         reportKey      reportKey                                studentNumber      studentNumber                                school      school

User · Answer

Do you see anything wrong with the code    Yes  Why are you adding the three fields together before you compare them   I would probably do something like this   assuming the fields are in the order you wish to sort them in    Override public int compare final Report record1  final Report record2        int c      c   record1 getReportKey   compareTo record2 getReportKey         if  c    0         c   record1 getStudentNumber   compareTo record2 getStudentNumber         if  c    0         c   record1 getSchool   compareTo record2 getSchool         return c

User · Answer

Sorting with multiple fields in Java8  package com java8 chapter1   import java util Arrays  import java util Comparator  import java util List  import static java util Comparator        public class Example1        public static void main String   args            List lt Employee gt  empList   getEmpList                 Before Java 8          empList sort new Comparator lt Employee gt                    Override             public int compare Employee o1  Employee o2                    int res   o1 getDesignation   compareTo o2 getDesignation                     if  res    0                        return o1 getSalary    gt  o2 getSalary     1   o1 getSalary    lt  o2 getSalary     -1   0                    else                       return res                                                       for  Employee emp   empList                System out println emp                     System out println  ---------------------------------------------------------------------------                In Java 8          empList sort comparing Employee  getDesignation  thenComparing Employee  getSalary            empList stream   forEach System out  println              private static List lt Employee gt  getEmpList             return Arrays asList new Employee  Lakshman A    Consultent   450000                   new Employee  Chaitra S    Developer   250000   new Employee  Manoj PVN    Developer   250000                   new Employee  Ramesh R    Developer   280000   new Employee  Suresh S    Developer   270000                   new Employee  Jaishree    Opearations HR   350000             class Employee       private String fullName      private String designation      private double salary       public Employee String fullName  String designation  double salary            super            this fullName   fullName          this designation   designation          this salary   salary             public String getFullName             return fullName             public String getDesignation             return designation             public double getSalary             return salary              Override     public String toString             return  Employee  fullName     fullName      designation     designation      salary     salary

User · Answer

Use Comparator interface with methods introduced in JDK1 8  comparing and thenComparing  or more concrete methods  comparingXXX and thenComparingXXX   For example  if we wanna sort a list of persons by their id firstly  then age  then name               Comparator lt Person gt  comparator   Comparator comparingLong Person  getId                       thenComparingInt Person  getAge                       thenComparing Person  getName               personList sort comparator

User · Answer

If you want to sort based on ReportKey first then Student Number then School  you need to compare each String instead of concatenating them  Your method might work if you pad the strings with spaces so that each ReportKey is the same length and so on  but it is not really worth the effort   Instead just change the compare method to compare the ReportKeys  if compareTo returns 0 then try StudentNumber  then School

User · Answer

If the StudentNumber is numeric it will not be sorted numeric but alphanumeric  Do not expect    2   lt   11    it will be    11   lt   2

User · Answer

I suggest to use Java 8 Lambda approach   List lt Report gt  reportList   new ArrayList lt Report gt     reportList sort Comparator comparing Report  getRecord1  thenComparing Report  getRecord2

User · Answer

I d make a comparator using Guava s ComparisonChain   public class ReportComparator implements Comparator lt Report gt      public int compare Report r1  Report r2        return ComparisonChain start            compare r1 getReportKey    r2 getReportKey             compare r1 getStudentNumber    r2 getStudentNumber             compare r1 getSchool    r2 getSchool             result

User · Answer

I had the same issue and I needed an algorithm using a config file  In This way you can use multiple fields define by a configuration file  simulate just by a List lt String  config    public static void test              Associate your configName with your Comparator      Map lt String  Comparator lt DocumentDto gt  gt  map   new HashMap lt  gt         map put  quot id quot   new IdSort         map put  quot createUser quot   new DocumentUserSort         map put  quot documentType quot   new DocumentTypeSort                   In your config yml file  you ll have something like       sortlist           - documentType         - createUser         - id            List lt String gt  config   new ArrayList lt  gt         config add  quot documentType quot        config add  quot createUser quot        config add  quot id quot              List lt Comparator lt DocumentDto gt  gt  sorts   new ArrayList lt  gt          for  String comparator   config            sorts add map get comparator                  Begin creation of the list      DocumentDto d1   new DocumentDto        d1 setDocumentType new DocumentTypeDto         d1 getDocumentType   setCode  quot A quot        d1 setId 1       d1 setCreateUser  quot Djory quot         DocumentDto d2   new DocumentDto        d2 setDocumentType new DocumentTypeDto         d2 getDocumentType   setCode  quot A quot        d2 setId 2       d2 setCreateUser  quot Alex quot         DocumentDto d3   new DocumentDto        d3 setDocumentType new DocumentTypeDto         d3 getDocumentType   setCode  quot A quot        d3 setId 3       d3 setCreateUser  quot Djory quot         DocumentDto d4   new DocumentDto        d4 setDocumentType new DocumentTypeDto         d4 getDocumentType   setCode  quot A quot        d4 setId 4       d4 setCreateUser  quot Alex quot         DocumentDto d5   new DocumentDto        d5 setDocumentType new DocumentTypeDto         d5 getDocumentType   setCode  quot D quot        d5 setId 5       d5 setCreateUser  quot Djory quot         DocumentDto d6   new DocumentDto        d6 setDocumentType new DocumentTypeDto         d6 getDocumentType   setCode  quot B quot        d6 setId 6       d6 setCreateUser  quot Alex quot         DocumentDto d7   new DocumentDto        d7 setDocumentType new DocumentTypeDto         d7 getDocumentType   setCode  quot B quot        d7 setId 7       d7 setCreateUser  quot Alex quot         List lt DocumentDto gt  documents   new ArrayList lt  gt          documents add d1       documents add d2       documents add d3       documents add d4       documents add d5       documents add d6       documents add d7          End creation of the list          The Sort      Stream lt DocumentDto gt  docStream   documents stream            we need to reverse this list in order to sort by documentType first because stream are pull-based  last sorted   will have the priority     Collections reverse sorts        for Comparator lt DocumentDto gt  entitySort   sorts           docStream   docStream sorted entitySort             documents   docStream collect Collectors toList            documents has been sorted has you configured        in case of equality second sort will be used       System out println documents         Comparator objects are really simple  public class IdSort implements Comparator lt DocumentDto gt          Override     public int compare DocumentDto o1  DocumentDto o2            return o1 getId   compareTo o2 getId             public class DocumentUserSort implements Comparator lt DocumentDto gt          Override     public int compare DocumentDto o1  DocumentDto o2            return o1 getCreateUser   compareTo o2 getCreateUser             public class DocumentTypeSort implements Comparator lt DocumentDto gt          Override     public int compare DocumentDto o1  DocumentDto o2            return o1 getDocumentType   getCode   compareTo o2 getDocumentType   getCode              Conclusion   this method isn t has efficient but you can create generic sort using a file configuration in this way

User · Answer

originally from Ways to sort lists of objects in Java based on multiple fields   Original working code in this gist  Using Java 8 lambda s  added April 10  2019   Java 8 solves this nicely by lambda s  though Guava and Apache Commons might still offer more flexibility    Collections sort reportList  Comparator comparing Report  getReportKey               thenComparing Report  getStudentNumber               thenComparing Report  getSchool      Thanks to  gaoagong s answer below   Note that one advantage here is that the getters are evaluated lazily  eg  getSchool   is only evaluated if relevant    Messy and convoluted  Sorting by hand  Collections sort pizzas  new Comparator lt Pizza gt             Override       public int compare Pizza p1  Pizza p2              int sizeCmp   p1 size compareTo p2 size             if  sizeCmp    0                  return sizeCmp                        int nrOfToppingsCmp   p1 nrOfToppings compareTo p2 nrOfToppings             if  nrOfToppingsCmp    0                  return nrOfToppingsCmp                        return p1 name compareTo p2 name                     This requires a lot of typing  maintenance and is error prone  The only advantage is that getters are only invoked when relevant   The reflective way  Sorting with BeanComparator  ComparatorChain chain   new ComparatorChain Arrays asList     new BeanComparator  size        new BeanComparator  nrOfToppings        new BeanComparator  name       Collections sort pizzas  chain       Obviously this is more concise  but even more error prone as you lose your direct reference to the fields by using Strings instead  no typesafety  auto-refactorings   Now if a field is renamed  the compiler won   t even report a problem  Moreover  because this solution uses reflection  the sorting is much slower   Getting there  Sorting with Google Guava   s ComparisonChain  Collections sort pizzas  new Comparator lt Pizza gt             Override       public int compare Pizza p1  Pizza p2              return ComparisonChain start   compare p1 size  p2 size  compare p1 nrOfToppings  p2 nrOfToppings  compare p1 name  p2 name  result                 or in case the fields can be null                        return ComparisonChain start                compare p1 size  p2 size  Ordering natural   nullsLast                 compare p1 nrOfToppings  p2 nrOfToppings  Ordering natural   nullsLast                 compare p1 name  p2 name  Ordering natural   nullsLast                 result                                  This is much better  but requires some boiler plate code for the most common use case  null-values should be valued less by default  For null-fields  you have to provide an extra directive to Guava what to do in that case  This is a flexible mechanism if you want to do something specific  but often you want the default case  ie  1  a  b  z  null    And as noted in the comments below  these getters are all evaluated immediately for each comparison   Sorting with Apache Commons CompareToBuilder  Collections sort pizzas  new Comparator lt Pizza gt             Override       public int compare Pizza p1  Pizza p2              return new CompareToBuilder   append p1 size  p2 size  append p1 nrOfToppings  p2 nrOfToppings  append p1 name  p2 name  toComparison                      Like Guava   s ComparisonChain  this library class sorts easily on multiple fields  but also defines default behavior for null values  ie  1  a  b  z  null   However  you can   t specify anything else either  unless you provide your own Comparator   Again  as noted in the comments below  these getters are all evaluated immediately for each comparison   Thus  Ultimately it comes down to flavor and the need for flexibility  Guava   s ComparisonChain  vs  concise code  Apache   s CompareToBuilder    Bonus method  I found a nice solution that combines multiple comparators in order of priority on CodeReview in a MultiComparator   class MultiComparator lt T gt  implements Comparator lt T gt        private final List lt Comparator lt T gt  gt  comparators       public MultiComparator List lt Comparator lt   super T gt  gt  comparators            this comparators   comparators             public MultiComparator Comparator lt   super T gt     comparators            this Arrays asList comparators               public int compare T o1  T o2            for  Comparator lt T gt  c   comparators                int result   c compare o1  o2               if  result    0                    return result                                  return 0             public static  lt T gt  void sort List lt T gt  list  Comparator lt   super T gt     comparators            Collections sort list  new MultiComparator lt T gt  comparators              Ofcourse Apache Commons Collections has a util for this already   ComparatorUtils chainedComparator comparatorCollection   Collections sort list  ComparatorUtils chainedComparator comparators

User · Answer

If you want to sort by report key  then student number  then school  you should do something like this   public class ReportComparator implements Comparator lt Report gt        public int compare Report r1  Report r2                int result   r1 getReportKey   compareTo r2 getReportKey             if  result    0                        return result                    result   r1 getStudentNumber   compareTo r2 getStudentNumber             if  result    0                        return result                    return r1 getSchool   compareTo r2 getSchool               This assumes none of the values can be null  of course - it gets more complicated if you need to allow for null values for the report  report key  student number or school   While you could get the string concatenation version to work using spaces  it would still fail in strange cases if you had odd data which itself included spaces etc  The above code is the logical code you want    compare by report key first  then only bother with the student number if the report keys are the same  etc

User · Answer

Here is a full example comparing 2 fields in an object  one String and one int  also using Collator to sort   public class Test        public static void main String   args             Collator myCollator          myCollator   Collator getInstance Locale US            List lt Item gt  items   new ArrayList lt Item gt              items add new Item  costrels   1039737                items add new Item  Costs   1570019                items add new Item  costs   310831                items add new Item  costs   310832                 Collections sort items  new Comparator lt Item gt                   Override             public int compare final Item record1  final Item record2                    int c                    c   record1 item1 compareTo record2 item1     optional comparison without Collator                                 c   myCollator compare record1 item1  record2 item1                   if  c    0                                         return record1 item2  lt  record2 item2   -1                                record1 item2  gt  record2 item2   1                               0                                    return c                                          for  Item item   items                        System out println item item1               System out println item item2                                public static class Item               public String item1          public int item2          public String item3           public Item String item1  int item2  String item3                        this item1   item1              this item2   item2              this item3   item3                              Output   costrels 1039737  costs 310831  costs 310832  Costs 1570019

[java] Collections.sort with multiple fields

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