How to sort by two fields in Java

Question

I have array of objects person  int age  String name     How can I sort this array alphabetically by name and then by age   Which algorithm would you use for this

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You can use generic serial Comparator to sort collections by multiple fields   import org apache commons lang3 reflect FieldUtils  import java util Arrays  import java util Comparator  import java util List          author MaheshRPM    public class SerialComparator lt T gt  implements Comparator lt T gt    List lt String gt  sortingFields   public SerialComparator List lt String gt  sortingFields        this sortingFields   sortingFields     public SerialComparator String    sortingFields        this sortingFields   Arrays asList sortingFields       Override public int compare T o1  T o2        int result   0      try           for  String sortingField   sortingFields                if  result    0                    Object value1   FieldUtils readField o1  sortingField  true                   Object value2   FieldUtils readField o2  sortingField  true                   if  value1 instanceof Comparable  amp  amp  value2 instanceof Comparable                        Comparable comparable1    Comparable  value1                      Comparable comparable2    Comparable  value2                      result   comparable1 compareTo comparable2                     else                       throw new RuntimeException  Cannot compare non Comparable fields      value1 getClass                                getName       must implement Comparable lt     value1 getClass   getName       gt                                     else                   break                                catch  IllegalAccessException e            throw new RuntimeException e             return result

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You need to implement your own Comparator  and then use it  for example  Arrays sort persons  new PersonComparator       Your Comparator could look a bit like this   public class PersonComparator implements Comparator lt   extends Person gt       public int compare Person p1  Person p2         int nameCompare   p1 name compareToIgnoreCase p2 name        if  nameCompare    0            return nameCompare         else          return Integer valueOf p1 age  compareTo Integer valueOf p2 age                   The comparator first compares the names  if they are not equals it returns the result from comparing them  else it returns the compare result when comparing the ages of both persons   This code is only a draft  because the class is immutable you could think of building an singleton of it  instead creating a new instance for each sorting

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I would be careful when using Guava s ComparisonChain because it creates an instance of it per element been compared so you would be looking at a creation of N x Log N comparison chains just to compare if you are sorting  or N instances if you are iterating and checking for equality   I would instead create a static Comparator using the newest Java 8 API if possible or Guava s Ordering API which allows you to do that  here is an example with Java 8   import java util Comparator  import static java util Comparator naturalOrder  import static java util Comparator nullsLast   private static final Comparator lt Person gt  COMPARATOR   Comparator    comparing Person  getName  nullsLast naturalOrder        thenComparingInt Person  getAge     Override public int compareTo  NotNull Person other      return COMPARATOR compare this  other       Here is how to use the Guava s Ordering API  https   github com google guava wiki OrderingExplained

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Use Comparator and then put objects into Collection   then Collections sort     class Person        String fname      String lname      int age       public Person                public int getAge             return age             public void setAge int age            this age   age             public String getFname             return fname             public void setFname String fname            this fname   fname             public String getLname             return lname             public void setLname String lname            this lname   lname             public Person String fname  String lname  int age            this fname   fname          this lname   lname          this age   age              Override     public String toString             return fname         lname         age           public class Main       public static void main String   args            List lt Person gt  persons   new java util ArrayList lt Person gt             persons add new Person  abc3    def3   10            persons add new Person  abc2    def2   32            persons add new Person  abc1    def1   65            persons add new Person  abc4    def4   10            System out println persons           Collections sort persons  new Comparator lt Person gt                    Override             public int compare Person t  Person t1                    return t getAge   - t1 getAge                                      System out println persons

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For those able to use the Java 8 streaming API  there is a neater approach that is well documented here  Lambdas and sorting  I was looking for the equivalent of the C  LINQ    ThenBy        I found the mechanism in Java 8 on the Comparator    thenComparing        So here is the snippet that demonstrates the algorithm       Comparator lt Person gt  comparator   Comparator comparing person - gt  person name       comparator   comparator thenComparing Comparator comparing person - gt  person age      Check out the link above for a neater way and an explanation about how Java s type inference makes it a bit more clunky to define compared to LINQ   Here is the full unit test for reference    Test public void testChainedSorting            Create the collection of people      ArrayList lt Person gt  people   new ArrayList lt  gt         people add new Person  Dan   4        people add new Person  Andi   2        people add new Person  Bob   42        people add new Person  Debby   3        people add new Person  Bob   72        people add new Person  Barry   20        people add new Person  Cathy   40        people add new Person  Bob   40        people add new Person  Barry   50            Define chained comparators         Great article explaining this and how to make it even neater         http   blog jooq org 2014 01 31 java-8-friday-goodies-lambdas-and-sorting      Comparator lt Person gt  comparator   Comparator comparing person - gt  person name       comparator   comparator thenComparing Comparator comparing person - gt  person age            Sort the stream      Stream lt Person gt  personStream   people stream   sorted comparator           Make sure that the output is as expected      List lt Person gt  sortedPeople   personStream collect Collectors toList         Assert assertEquals  Andi    sortedPeople get 0  name   Assert assertEquals 2   sortedPeople get 0  age       Assert assertEquals  Barry   sortedPeople get 1  name   Assert assertEquals 20  sortedPeople get 1  age       Assert assertEquals  Barry   sortedPeople get 2  name   Assert assertEquals 50  sortedPeople get 2  age       Assert assertEquals  Bob     sortedPeople get 3  name   Assert assertEquals 40  sortedPeople get 3  age       Assert assertEquals  Bob     sortedPeople get 4  name   Assert assertEquals 42  sortedPeople get 4  age       Assert assertEquals  Bob     sortedPeople get 5  name   Assert assertEquals 72  sortedPeople get 5  age       Assert assertEquals  Cathy   sortedPeople get 6  name   Assert assertEquals 40  sortedPeople get 6  age       Assert assertEquals  Dan     sortedPeople get 7  name   Assert assertEquals 4   sortedPeople get 7  age       Assert assertEquals  Debby   sortedPeople get 8  name   Assert assertEquals 3   sortedPeople get 8  age          Andi       2        Barry      20        Barry      50        Bob        40        Bob        42        Bob        72        Cathy      40        Dan        4        Debby      3           A person in our system      public static class Person                  Creates a new person          param name The name of the person          param age The age of the person              public Person String name  int age                this age   age          this name   name                        The name of the person              public String name                  The age of the person              public int age        Override     public String toString                 if  name    null  return super toString            else return String format   s    d   this name  this age

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Guava s ComparisonChain provides a clean way of doing it  Refer to this link   A utility for performing a chained comparison statement  For example      public int compareTo Foo that         return ComparisonChain start             compare this aString  that aString            compare this anInt  that anInt            compare this anEnum  that anEnum  Ordering natural   nullsLast              result

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You can use Java 8 Lambda approach to achieve this   Like this   persons sort Comparator comparing Person  getName  thenComparing Person  getAge

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Using the Java 8 Streams approach       Creates and sorts a stream  does not sort the original list         persons stream   sorted Comparator comparing Person  getName  thenComparing Person  getAge      And the Java 8 Lambda approach       Sorts the original list Lambda style persons sort  p1  p2  - gt            if  p1 getName   compareTo p2 getName       0                return p1 getAge   compareTo p2 getAge               else               return p1 getName   compareTo p2 getName                          Lastly       This is similar SYNTAX to the Streams above  but it sorts the original list   persons sort Comparator comparing Person  getName  thenComparing Person  getAge

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Arrays sort persons  new PersonComparator        import java util Comparator   public class PersonComparator implements Comparator lt   extends Person gt          Override     public int compare Person o1  Person o2            if null    o1    null    o2     null    o1 getName      null   o2 getName                  throw new NullPointerException             else              int nameComparisonResult   o1 getName   compareTo o2 getName                 if 0    nameComparisonResult                   return o1 getAge  -o2 getAge                 else                  return nameComparisonResult                                    class Person      int age  String name       public int getAge             return age             public void setAge int age            this age   age             public String getName             return name             public void setName String name            this name   name             Updated version   public class PersonComparator implements Comparator lt   extends Person gt         Override    public int compare Person o1  Person o2           int nameComparisonResult   o1 getName   compareToIgnoreCase o2 getName           return 0    nameComparisonResult o1 getAge  -o2 getAge   nameComparisonResult

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You can use Collections sort as follows   private static void order List lt Person gt  persons         Collections sort persons  new Comparator              public int compare Object o1  Object o2                 String x1     Person  o1  getName                String x2     Person  o2  getName                int sComp   x1 compareTo x2                if  sComp    0                   return sComp                              Integer x1     Person  o1  getAge                Integer x2     Person  o2  getAge                return x1 compareTo x2                List lt Persons gt  is now sorted by name  then by age   String compareTo  Compares two strings lexicographically  - from the docs   Collections sort is a static method in the native Collections library  It does the actual sorting  you just need to provide a Comparator which defines how two elements in your list should be compared  this is achieved by providing your own implementation of the compare method

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For a class Book like this    package books   public class Book        private Integer id      private Integer number      private String name       public Integer getId             return id             public void setId Integer id            this id   id             public Integer getNumber             return number             public void setNumber Integer number            this number   number             public String getName             return name             public void setName String name            this name   name              Override     public String toString             return  book                      id     id                      number     number                      name      name            n                                   sorting main class with mock objects   package books   import java util ArrayList  import java util Collections  import java util Comparator  import java util List    public class Main        public static void main String   args            System out println  Hello World              Book b   new Book             Book c   new Book             Book d   new Book             Book e   new Book             Book f   new Book             Book g   new Book            Book g1   new Book            Book g2   new Book            Book g3   new Book            Book g4   new Book                b setId 1           b setNumber 12           b setName  gk             c setId 2           c setNumber 12           c setName  gk             d setId 2           d setNumber 13           d setName  maths             e setId 3           e setNumber 3           e setName  geometry             f setId 3           f setNumber 34           b setName  gk             g setId 3           g setNumber 11           g setName  gk             g1 setId 3           g1 setNumber 88           g1 setName  gk            g2 setId 3           g2 setNumber 91           g2 setName  gk            g3 setId 3           g3 setNumber 101           g3 setName  gk            g4 setId 3           g4 setNumber 4           g4 setName  gk                 List lt Book gt  allBooks   new ArrayList lt Book gt              allBooks add b           allBooks add c           allBooks add d           allBooks add e           allBooks add f           allBooks add g           allBooks add g1           allBooks add g2           allBooks add g3           allBooks add g4              System out println allBooks size               Collections sort allBooks  new Comparator lt Book gt                    Override             public int compare Book t  Book t1                    int a    t getId  - t1 getId                     if a    0                       int a1   t getNumber   - t1 getNumber                        return a1                                    else                     return a                                    System out println allBooks

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I m not sure if it s ugly to write the compartor inside the Person class in this case  Did it like this   public class Person implements Comparable  lt Person gt         private String lastName      private String firstName      private int age       public Person String firstName  String lastName  int BirthDay            this firstName   firstName          this lastName   lastName          this age   BirthDay             public int getAge             return age             public String getFirstName             return firstName             public String getLastName             return lastName              Override     public int compareTo Person o               default compareTo             Override     public String toString             return firstName         lastName         age                  public static class firstNameComperator implements Comparator lt Person gt             Override         public int compare Person o1  Person o2                return o1 firstName compareTo o2 firstName                        public static class lastNameComperator implements Comparator lt Person gt             Override         public int compare Person o1  Person o2                return o1 lastName compareTo o2 lastName                        public static class ageComperator implements Comparator lt Person gt             Override         public int compare Person o1  Person o2                return o1 age - o2 age                    public class Test       private static void print            ArrayList lt Person gt  list   new ArrayList            list add new Person  Diana    Agron   31            list add new Person  Kay    Panabaker   27            list add new Person  Lucy    Hale   28            list add new Person  Ashley    Benson   28            list add new Person  Megan    Park   31            list add new Person  Lucas    Till   27            list add new Person  Nicholas    Hoult   28            list add new Person  Aly    Michalka   28            list add new Person  Adam    Brody   38            list add new Person  Chris    Pine   37            Collections sort list  new Person lastNameComperator             Iterator lt Person gt  it   list iterator            while it hasNext                 System out println it next   toString

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Create as many comparators as necessary  After  call the method  thenComparing  for each order category  It s a way of doing by Streams  See     Sort by first and last name System out println   n2 Sort list of person objects by firstName then                                              by lastName then by age    Comparator lt Person gt  sortByFirstName                                 p  o  - gt  p firstName compareToIgnoreCase o firstName   Comparator lt Person gt  sortByLastName                                 p  o  - gt  p lastName compareToIgnoreCase o lastName   Comparator lt Person gt  sortByAge                                 p  o  - gt  Integer compare p age o age      Sort by first Name then Sort by last name then sort by age personList stream   sorted      sortByFirstName          thenComparing sortByLastName           thenComparing sortByAge         forEach person- gt          System out println person              Look  Sort user defined object on multiple fields     Comparator  lambda stream

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You can do like this   List lt User gt  users   Lists newArrayList    new User  Pedro   12      new User  Maria   10      new User  Rafael  12      users sort    Comparator comparing User  getName  thenComparing User  getAge

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Or you can exploit the fact that Collections sort    or Arrays sort    is stable  it doesn t reorder elements that are equal  and use a Comparator to sort by age first and then another one to sort by name   In this specific case this isn t a very good idea but if you have to be able to change the sort order in runtime  it might be useful

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Have your person class implement Comparable lt Person gt  and then implement the compareTo method  for instance   public int compareTo Person o        int result   name compareToIgnoreCase o name       if result  0            return Integer valueOf age  compareTo o age             else           return result            That will sort first by name  case insensitively  and then by age  You can then run Arrays sort   or Collections sort   on the collection or array of Person objects

[java] How to sort by two fields in Java?

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