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.

1. What I have done acceptable as a practice?

2. 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" ?

3. 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).

Answer 1

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.

Answer 2

My annotation lib for implementing Comparable and Comparator:

public class Person implements Comparable<Person> {         
    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

Answer 3

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]

Answer 4

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.

Answer 5

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

Answer 6

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.