When should a class be Comparable and or Comparator

Question

I have seen classes which implement both Comparable and Comparator   What does this mean   Why would I use one over the other

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java lang Comparable       To implement Comparable interface  class must implement a single method compareTo    int a compareTo b  You must modify the class whose instance you want to sort  So that only one sort sequence can be created per class    java util Comparator   To implement Comparator interface  class must implement a single method compare    int compare  a b  You build a class separate from class whose instance you want to sort  So that multiple sort sequence can be created per class

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Comparable lets a class implement its own comparison    it s in the same class  it is often an advantage  there can be only one implementation  so you can t use that if you want two different cases    By comparison  Comparator is an external comparison    it is typically in a unique instance  either in the same class or in another place  you name each implementation with the way you want to sort things you can provide comparators for classes that you do not control the implementation is usable even if the first object is null     In both implementations  you can still choose to what you want to be compared  With generics  you can declare so  and have it checked at compile-time  This improves safety  but it is also a challenge to determine the appropriate value   As a guideline  I generally use the most general class or interface to which that object could be compared  in all use cases I envision    Not very precise a definition though    -    Comparable lt Object gt  lets you use it in all codes at compile-time  which is good if needed  or bad if not and you loose the compile-time error    your implementation has to cope with objects  and cast as needed but in a robust way  Comparable lt Itself gt  is very strict on the contrary       Funny  when you subclass Itself to Subclass  Subclass must also be Comparable and be robust about it  or it would break Liskov Principle  and give you runtime errors

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Implementing Comparable means  I can compare myself with another object   This is typically useful when there s a single natural default comparison   Implementing Comparator means  I can compare two other objects   This is typically useful when there are multiple ways of comparing two instances of a type - e g  you could compare people by age  name etc

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Comparable is for objects with a natural ordering  The object itself knows how it is to be ordered  Comparator is for objects without a natural ordering or when you wish to use a different ordering

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Comparable is for providing a default ordering on data objects  for example if the data objects have a natural order   A Comparator represents the ordering itself for a specific use

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Difference between Comparator and Comparable interfaces  Comparable is used to compare itself by using with another object   Comparator is used to compare two datatypes are objects

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The text below comes from Comparator vs Comparable  Comparable  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   Comparator  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

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here are few differences between Comparator and Comparable I found on web     If you see then logical difference between these two is Comparator in Java compare two objects provided to him  while Comparable interface compares  this  reference with the object specified  Comparable in Java is used to implement natural ordering of object  In Java API String  Date and wrapper classes implement Comparable interface  If any class implement Comparable interface in Java then collection of that object either List or Array can be sorted automatically by using  Collections sort   or Array sort   method and object will be sorted based on there natural order defined by CompareTo method  Objects which implement Comparable in Java  can be used as keys in a sorted map or elements in a sorted set for example TreeSet  without specifying any Comparator    site How to use Comparator and Comparable in Java  With example  Read more  How to use Comparator and Comparable in Java  With example

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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  compamatic

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If you see then logical difference between these two is Comparator in Java compare two objects provided to him  while Comparable interface compares  this  reference with the object specified   Comparable in Java is used to implement natural ordering of object  In Java API String  Date and wrapper classes implement Comparable interface   If any class implement Comparable interface in Java then collection of that object either List or Array can be sorted automatically by using Collections sort   or Array sort   method and object will be sorted based on there natural order defined by compareTo method   Objects which implement Comparable in Java can be used as keys in a sorted map or elements in a sorted set for example TreeSet  without specifying any Comparator

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Comparable is usually preferred  But sometimes a class already implements Comparable  but you want to sort on a different property  Then you re forced to use a Comparator   Some classes actually provide Comparators for common cases  for instance  Strings are by default case-sensitive when sorted  but there is also a static Comparator called CASE INSENSITIVE ORDER

[java] When should a class be Comparable and/or Comparator?

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