When to use Comparable and Comparator

Question

I have a list of objects I need to sort on a field  say Score  Without giving much thought I wrote a new class that implements Comparator  that does the task and it works    Now looking back at this  I am wondering if I should have instead have the my class implement Comparable instead of creating a new class that implements Comparator  The score is the only field that the objects will be ordered on     What I have done acceptable as a practice   Is the right approach  First have the class implement Comparable  for the natural ordering  and if an alternative field comparison is required  then create a new class that implements Comparator    If  2  above is true  then does it mean that one should implement Comparator only after they have the class implement Comparable   Assuming I own the original class

User · Answer

Very simple approach is to assume that the entity class in question be represented in database and then in database table would you need index made up of fields of entity class  If answer is yes then implement comparable and use the index field s  for natural sorting order  In all other cases use comparator

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I would say    if the comparison is intuitive  then by all means implement Comparable if it is unclear wether your comparison is intuitive  use a Comparator as it s more explicit and thus more clear for the poor soul who has to maintain the code if there is more than one intuitive comparison possible I d prefer a Comparator  possibly build by a factory method in the class to be compared  if the comparison is special purpose  use Comparator

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If you own the class better go with Comparable  Generally Comparator is used if you dont own the class but you have to use it a TreeSet or TreeMap because Comparator can be passed as a parameter in the conctructor of TreeSet or TreeMap  You can see how to use Comparator and Comparable in http   preciselyconcise com java collections g comparator php

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Use Comparable    if the object is in your control  if the comparing behaviour is the main comparing behaviour    Use Comparator     if the object is outside your control and you cannot make them implement Comparable  when you want comparing behaviour different from the default  which is specified by Comparable  behaviour

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I would say that an object should implement Comparable if that is the clear natural way to sort the class  and anyone would need to sort the class would generally want to do it that way   If  however  the sorting was an unusual use of the class  or the sorting only makes sense for a specific use case  then a Comparator is a better option   Put another way  given the class name  is it clear how a comparable would sort  or do you have to resort to reading the javadoc  If it is the latter  odds are every future sorting use case would require a comparator  at which point the implementation of comparable may slow down users of the class  not speed them up

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I have been asked sorting of a definite range of numbers in better than nlogn time in one of interview   Not using Counting sort   Implementing Comparable interface over an object allows implicit sorting algos to use overridden compareTo method to order sort elements and that would be linear time

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My need was sort based on date   So  I used Comparable and it worked easily for me   public int compareTo GoogleCalendarBean o           TODO Auto-generated method stub     return eventdate compareTo o getEventdate         One restriction with Comparable is that they cannot used for Collections other than List

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Comparable is the default natural sorting order provided for numerical values are ascending and for strings are alphabetical order  for eg   Treeset t new Treeset    t add 2   t add 1   System out println t     1 2    Comparator is the custom sorting order  implemented in custom myComparator class by overriding a compare method for eg   Treeset t new Treeset new myComparator     t add 55   t add 56   class myComparator implements Comparator  public int compare Object o1 Object o2     Descending Logic     System out println t     56 55

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Comparable   Whenever we want to store only homogeneous elements and default natural sorting order required  we can go for class implementing  comparable interface   Comparator  Whenever we want to store homogeneous and heterogeneous elements and we want to sort in default customized sorting order  we can go for comparator interface

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Comparator does everything that comparable does  plus more                                                                 Comparable   Comparator                                                                                      Is used to allow Collections sort to work                 yes            yes   Can compare multiple fields                               yes            yes   Lives inside the class you   re comparing and serves                       as a    default    way to compare                             yes            yes   Can live outside the class you   re comparing               no             yes   Can have multiple instances with different method names   no             yes   Input arguments can be a list of                          just Object    Any type   Can use enums                                             no             yes      I found the best approach to use comparators as anonymous classes as follows   private static void sortAccountsByPriority List lt AccountRecord gt  accounts        Collections sort accounts  new Comparator lt AccountRecord gt                Override         public int compare AccountRecord a1  AccountRecord a2                return a1 getRank   compareTo a2 getRank                           You can create multiple versions of such methods right inside the class you   re planning to sort    So you can have    sortAccountsByPriority  sortAccountsByType sortAccountsByPriorityAndType   etc      Now  you can use these sort methods anywhere and get code reuse  This gives me everything a comparable would  plus more     so I don   t see any reason to use comparable at all

User · Answer

Comparable should be used when you compare instances of the same class  Comparator can be used to compare instances of different classes  Comparable is implemented by the class which needs to define a natural ordering for its objects  For example  String implements Comparable  In case a different sorting order is required  then  implement comparator and define its own way of comparing two instances

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If sorting of objects needs to be based on natural order then use Comparable whereas if your sorting needs to be done on attributes of different objects  then use Comparator in Java  Main differences between Comparable and Comparator   ------------------------------------------------------------------------------------                   Comparable                                  Comparator                     ----------------------------------------- ------------------------------------------      java lang Comparable                       java util Comparator                          ----------------------------------------- ------------------------------------------      int objOne compareTo objTwo                int compare objOne  objTwo                    ----------------------------------------- ------------------------------------------      Negative  if objOne  lt  objTwo               Same as Comparable                             Zero   if objOne    objTwo                                                                Positive   if objOne  gt  objTwo                                                            ----------------------------------------- ------------------------------------------      You must modify the class whose            You build a class separate from to sort        instances you want to sort                 the class whose instances you want            ----------------------------------------- ------------------------------------------      Only one sort sequence can be created      Many sort sequences can be created            ----------------------------------------- ------------------------------------------      Implemented frequently in the API by       Meant to be implemented to sort                String  Wrapper classes  Date  Calendar    instances of third-party classes             ------------------------------------------------------------------------------------

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Use Comparable if you want to define a default  natural  ordering behaviour of the object in question  a common practice is to use a technical or natural  database   identifier of the object for this   Use Comparator if you want to define an external controllable ordering behaviour  this can override the default ordering behaviour

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My annotation lib for implementing Comparable and Comparator   public class Person implements Comparable lt Person gt                 private String firstName        private String lastName               private int age               private char gentle                 Override               CompaProperties    CompaProperty property    lastName                           CompaProperty property    age    order   Order DSC                    public int compareTo Person person                             return Compamatic doComparasion this  person                        Click the link to see more examples  http   code google com p compamatic wiki CompamaticByExamples

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The following points help you in deciding in which situations one should use Comparable and in which Comparator   1  Code Availabilty  2  Single Versus Multiple Sorting Criteria  3  Arays sort   and Collection sort    4  As keys in SortedMap and SortedSet  5  More Number of classes Versus flexibility  6  Interclass comparisions  7  Natural Order  For more detailed article you can refer When to use comparable and when to use comparator

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If at the moment of writing the class you have only one use case of sorting use Comparable   Only when you have    more than one strategy of sorting    implement a Comparator

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There had been a similar question here  When should a class be Comparable and or Comparator   I would say the following  Implement Comparable for something like a natural ordering  e g  based on an internal ID  Implement a Comparator if you have a more complex comparing algorithm  e g  multiple fields and so on

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If you need natural order sorting -- User Comparable IF you need Custom Order Sorting - Use Comparator  Example   Class Employee  private int id  private String name  private String department      Natural order Sorting would be based on id because it would be unique and custom order sortin g would be name and department   Refrences  When should a class be Comparable and or Comparator   http   javarevisited blogspot com 2011 06 comparator-and-comparable-in-java html

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Comparable - java lang Comparable  int compareTo Object o1   A comparable object is capable of comparing itself with another object  The class itself must implements the java lang Comparable interface in order to be able to compare its instances    Capable of comparing current object with the provided object  By using this we can implement only one sort sequence based on the instances properties  EX  Person id Some of the Predefined Classes like String  Wrapper classes  Date  Calendar has implemented Comparable interface      Comparator - java util Comparator  int compare Object o1  Object o2   A comparator object is capable of comparing two different objects  The class is not comparing its instances  but some other class   s instances  This comparator class must implement the java util Comparator interface    Capable of comparing any two Objects of Same Type  By using this we can implement many sort sequence and name each  based on the instances properties  EX  Person id  Person name  Person age We can implement Comparator interface to our Pre-defined classes for Customized sorting      Example   public class Employee implements Comparable lt Employee gt         private int id      private String name      private int age      private long salary          Many sort sequences can be created with different names      public static Comparator lt Employee gt  NameComparator   new Comparator lt Employee gt                        Override         public int compare Employee e1  Employee e2                return e1 getName   compareTo e2 getName                          public static Comparator lt Employee gt  idComparator   new Comparator lt Employee gt                      Override         public int compare Employee e1  Employee e2                return Integer valueOf e1 getId    compareTo Integer valueOf e2 getId                            public Employee           public Employee int id  String name  int age  long salary           this id   id          this name   name          this age   age          this salary   salary               setters and getters          Only one sort sequence can be created with in the class       Override     public int compareTo Employee e          return Integer valueOf this id  compareTo Integer valueOf e id          return Character toString this name charAt 0   compareToIgnoreCase Character toString e name charAt 0             if  this id  gt  e id                return 1           else if this id  lt  e id               return -1           else               return Character toString this name charAt 0   compareToIgnoreCase Character toString e name charAt 0                              public static void main String   args             Employee e1   new Employee 5   Yash   22  1000           Employee e2   new Employee 8   Tharun   24  25000            List lt Employee gt  list   new ArrayList lt Employee gt             list add e1           list add e2           Collections sort list      call  compareTo o1          Collections sort list  Employee nameComparator      call  compare  o1 o2          Collections sort list  Employee idComparator      call  compare  o1 o2             For customized sorting we go for comparator  compare o1  o2  for other scenarios we go for comparable  compareTo o1    with out changing code if we want to sort more than one field then we use comparator     For Java 8 Lambda   Comparator refer to my post

[java] When to use Comparable and Comparator

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