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?

Solution 1

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.

Solution 2

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.

Solution 3

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<Object> 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<Itself> 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).

Solution 4

java.lang.Comparable

1. To implement Comparable interface, class must implement a single method compareTo()

   int a.compareTo(b)

2. 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

1. To implement Comparator interface, class must implement a single method compare()

   int compare (a,b)

2. You build a class separate from class whose instance you want to sort. So that multiple sort sequence can be created per class.

Solution 5

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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Comments

[skaffman]

Have you read the javadoc on them? It describes the different quite clearly.

[RockStar]

What is comparable and comparator and when to use comparable and comparator. To know these read this link. this will help you to understand their behavior and usage. http://iandjava.blogspot.in/2012/10/comparable-and-comparato‌​r.html

[Russell Silva]

This is a good question with a good, objective answer. I am dismayed that it is closed.

[Pete]

Other questions on StackOverflow refer to this as the definitive "What's the difference between comparator and comparable" question. Why is it marked as "not constructive"? As a Java Newbie, this was a very useful question!

[niceToExist]

experts-exchange.com/Programming/Languages/Java/…

[rahulnikhare]

Comparable for Natural/Default sorting. And Comparator for Customized sorting

[MasterJoe]

I think of it this way. Comparable provides a default way to a class to compare two objects of itself. Comparator provides custom ways to compare two objects of same type. Hotel has Customers. Customer who spends more money is more important. Special offers go to top 10% customers only. But sometimes, we want to encourage the lowest spending customers to spend more. Then, we can compare by lesser money spent, get bottom 5% users & send special offers to them also.

[brady]

I'm not sure I agree with Comparable being preferred. Some objects have a strong sense of natural order—namely numbers, in all their forms: natural numbers, real numbers, dates, etc. But even other relatively primitive objects like character strings lack a universally applicable order. In the case of more complex objects like an entity from an application domain model, implementing Comparable is usually a mistake. Their many properties make it too difficult to anticipate what order will be wanted most often.

[Mdhar9e]

Hello skeet, can you please drop the code using comparable and comparator.

[Jon Skeet]

@mdhar9e: There are lots of examples around - if you're finding it difficult to translate them into your specific scenario, please give more information in a new question.

[oers]

Welcome to stackoverflow. This question is old and was already answered. Typically, it is best not to resurrect stale threads unless your response contributes something significantly new or different over previous answers.

[Alireza Farahani]

@JonSkeet Can you please give an example that a scenario implemented by Comparator can't be implemented by Comparable easily? I don't see much difference between them

[Jon Skeet]

@alireza: I've already given an example in the second paragraph: by having different comparators, you could sort the same collection (people) by different properties (age, name etc). You can't do that just by making Person implement Comparable, because you can't then change how two people are compared.

[Nagappa L M]

@Jon Skeet Thank you very much for your explaination.Can you provide some real time example ?

[Nagappa L M]

@jon Skeet Sorting logic is in separate class. Hence we can write different sorting based on different attributes of objects to be sorted.Is this mean applying differant sorting methods like bubble,quick etc sorts ?

[Jon Skeet]

@feelgoodandprogramming: No, those are different sorting algorithms. There are potentially three pieces of code here, in three different classes: 1) The entity itself, e.g. Person. 2) The comparator, e.g. PersonAgeComparator which is able to compare two different entities and decide which should come first in that particular sort order. 3) The sort code, which takes a collection of entities and a comparator, and sorts that collection using the comparator to determine the order.

[RishiKesh Pathak]

@JonSkeet let me know my understanding is correct or not. 1) use Comparable when i want to compare only on any one of them, age or name. And 2) use Comparator when i want to use age and name both. means if Comparator will find the age to be same it will compare name.

[Jon Skeet]

@RishiKeshPathak: Well it's more that you use Comparable if there is only one single obvious thing to compare on. And even then you can just write a Comparator. If you have two different programs using the same class, and one wants to sort by just age and the other by just name, at least one of them will need to use a Comparator.

[RishiKesh Pathak]

@JonSkeet you mean, we are good even without Comparable. Only advantage it gives is more readable code?

[Jon Skeet]

@RishiKeshPathak: Yes, most APIs support providing an explicit comparator - it's just that it's a bit of extra cruft to do so, if there's one obvious ordering.

[mel3kings]

for detailed explanation of the uses of both Comparator and comparable: sysdotoutdotprint.com/index.php/2017/03/28/…

[EMM]

Another good article: web.archive.org/web/20180707182116/http://codecramp.com/…

[CJay Horton]

@mel3kings Can you please update the link in your comment? It currently provides no information towards this question.