Comparator vs Comparable in Java

Here are some of the common differences, which is worth remembering to answer this question if asked during a telephonic or face to face interview:

 

1) Comparator in Java is defined in java.util package while Comparable interface in Java is defined in java.lang package, which very much says that Comparator should be used as an utility to sort objects which Comparable should be provided by default.

 

2) Comparator interface in Java has method public int compare (Object o1, Object o2) which returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second. While Comparable interface has method public int compareTo(Object o) which returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

 

3) 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. I have shared lot of tips on how to override compareTo() method and avoid some common mistakes programmer makes while implementing Comparable interface.

 

4) Comparable in Java is used to implement natural ordering of object. In Java API String, Date and wrapper classes implements Comparable interface.Its always good practice to override compareTo() for value objects.

 

5) 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 Arrays.sort() method and object will be sorted based on there natural order defined by CompareTo method.

 

6) Objects which implement Comparable in Java  can be used as keys in a SortedMap like TreeMap or elements in a SortedSet  for example TreeSet, without specifying any Comparator.

 

These were combination of some theoretical and practical differences between Comparator and Comparator interface in Java. It does help you to decide when to use Comparator vs Comparable but things will be more clear when we some best practices around using both of these interfaces. Now let’s see an example of Comparator in Java:

 

 

Example of using Comparator and Comparable in Java

So in Summary if you want to sort objects based on natural order then use Comparable in Java and if you want to sort on some other attribute of object then use Comparator in Java. Now to understand these concepts lets see an example or real life coding:

 

 

1) There is class called Person, sort the Person based on person_id, which is primary key in database

2) Sort the Person based on there name.

 

For a Person class, sorting based on person_id can be treated as natural order sorting and sorting based on name field can be implemented using Comparator interface. To sort based on person_id we need to implement compareTo() method.

 

 

public class Person implements Comparable {
    private int person_id;
    private String name;
   
    /**
     * Compare current person with specified person
     * return zero if person_id for both person is same
     * return negative if current person_id is less than specified one
     * return positive if specified person_id is greater than specified one
     */
    @Override
    public int compareTo(Object o) {
        Person p = (Person) o;
        return this.person_id - o.person_id ;
    }
    ….
}