Comparable Vs. Comparator

Comparable and Comparator are two interfaces provided by Java Core API. From their names, we can tell they may be used for comparing stuff in some way. But what exactly are they and what is the difference between them? The following are two examples for answering this question. The simple examples compare two HDTV's size. How to use Comparable vs. Comparator is obvious after reading the code.

1. Comparable

Comparable is implemented by a class in order to be able to comparing object of itself with some other objects. The class itself must implement the interface in order to be able to compare its instance(s). The method required for implementation is compareTo(). Here is an example:

// A Java program to demonstrate use of Comparable

import java.io.*;

import java.util.*;

// A class 'Movie' that implements Comparable

class Movie implements Comparable<Movie>

{

    private double rating;

    private String name;

    private int year;

    // Used to sort movies by year

    public int compareTo(Movie m)

    {

        return this.year - m.year;

    }

    // Constructor

    public Movie(String nm, double rt, int yr)

    {

        this.name = nm;

        this.rating = rt;

        this.year = yr;

    }

    // Getter methods for accessing private data

    public double getRating() { return rating; }

    public String getName()   {  return name; }

    public int getYear()      {  return year;  }

}

// Driver class

class ComparableTest

{

    public static void main(String[] args)

    {

        ArrayList<Movie> list = new ArrayList<Movie>();

        list.add(new Movie("Force Awakens", 8.3, 2015));

        list.add(new Movie("Star Wars", 8.7, 1977));

        list.add(new Movie("Empire Strikes Back", 8.8, 1980));

        list.add(new Movie("Return of the Jedi", 8.4, 1983));

        Collections.sort(list);

        System.out.println("Movies after sorting : ");

        for (Movie movie: list)

        {

            System.out.println(movie.getName() + " " +

                               movie.getRating() + " " +

                               movie.getYear());

        }

    }

}

Output(Comparable):

Movies after sorting : 

Star Wars 8.7 1977

Empire Strikes Back 8.8 1980

Return of the Jedi 8.4 1983

Force Awakens 8.3 2015

2. Comparator

In some situations, you may not want to change a class and make it comparable. In such cases, Comparatorcan be used if you want to compare objects based on certain attributes/fields. For example, 2 persons can be compared based on `height` or `age` etc. (this can not be done using comparable.)

The method required to implement is compare(). Now let's use another way to compare those TV by size. One common use of Comparator is sorting. Both Collections and Arrays classes provide a sort method which use a Comparator.

//A Java program to demonstrate Comparator interface

import java.io.*;

import java.util.*;

// A class 'Movie' that implements Comparable

class Movie1 implements Comparable<Movie1>

{

private double rating;

private String name;

private int year;

// Used to sort movies by year

public int compareTo(Movie1 m)

{

return this.year - m.year;

}

// Constructor

public Movie1(String nm, double rt, int yr)

{

this.name = nm;

this.rating = rt;

this.year = yr;

}

// Getter methods for accessing private data

public double getRating() { return rating; }

public String getName() { return name; }

public int getYear() { return year; }

}

// Class to compare Movies by ratings

class RatingCompare implements Comparator<Movie>

{

public int compare(Movie m1, Movie m2)

{

if (m1.getRating() < m2.getRating()) return -1;

if (m1.getRating() > m2.getRating()) return 1;

else return 0;

}

}

// Class to compare Movies by name

class NameCompare implements Comparator<Movie>

{

public int compare(Movie m1, Movie m2)

{

return m1.getName().compareTo(m2.getName());

}

}

// Driver class

class ComparatorTest

{

public static void main(String[] args)

{

ArrayList<Movie> list = new ArrayList<Movie>();

list.add(new Movie("Force Awakens", 8.3, 2015));

list.add(new Movie("Star Wars", 8.7, 1977));

list.add(new Movie("Empire Strikes Back", 8.8, 1980));

list.add(new Movie("Return of the Jedi", 8.4, 1983));

// Sort by rating : (1) Create an object of ratingCompare

// (2) Call Collections.sort

// (3) Print Sorted list

System.out.println("Sorted by rating");

RatingCompare ratingCompare = new RatingCompare();

Collections.sort(list, ratingCompare);

for (Movie movie: list)

System.out.println(movie.getRating() + " " +

movie.getName() + " " +

movie.getYear());

// Call overloaded sort method with RatingCompare

// (Same three steps as above)

System.out.println("\nSorted by name");

NameCompare nameCompare = new NameCompare();

Collections.sort(list, nameCompare);

for (Movie movie: list)

System.out.println(movie.getName() + " " +

movie.getRating() + " " +

movie.getYear());

// Uses Comparable to sort by year

System.out.println("\nSorted by year");

Collections.sort(list);

for (Movie movie: list)

System.out.println(movie.getYear() + " " +

movie.getRating() + " " +

movie.getName()+" ");

}

} 

Output(Comparator):

Sorted by rating

8.3 Force Awakens 2015

8.4 Return of the Jedi 1983

8.7 Star Wars 1977

8.8 Empire Strikes Back 1980

Sorted by name

Empire Strikes Back 8.8 1980

Force Awakens 8.3 2015

Return of the Jedi 8.4 1983

Star Wars 8.7 1977

Sorted by year

1977 8.7 Star Wars 

1980 8.8 Empire Strikes Back 

1983 8.4 Return of the Jedi 

2015 8.3 Force Awakens 

Difference between Comparable and Comparator

Comparable and Comparator both are interfaces and can be used to sort collection elements.

But there are many differences between Comparable and Comparator interfaces that are given below.

Comparable	Comparator
1) Comparable provides single sorting sequence. In other words, we can sort the collection on the basis of single element such as id or name or price etc.	Comparator provides multiple sorting sequence. In other words, we can sort the collection on the basis of multiple elements such as id, name and price etc.
2) Comparable affects the original class i.e. actual class is modified.	Comparator doesn't affect the original class i.e. actual class is not modified.
3) Comparable provides compareTo() method to sort elements.	Comparator provides compare() method to sort elements.
4) Comparable is found in java.lang package.	Comparator is found in java.util package.
5) We can sort the list elements of Comparable type byCollections.sort(List) method.	We can sort the list elements of Comparator type byCollections.sort(List,Comparator) method.