Java provides two interfaces to sort objects using data members of the class:
Using Comparable Interface
A comparable object is capable of comparing itself with another object. The class itself must implements the java.lang.Comparable interface to compare its instances.
Consider a Movie class that has members like, rating, name, year. Suppose we wish to sort a list of Movies based on year of release. We can implement the Comparable interface with the Movie class, and we override the method compareTo() of Comparable interface.
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
Now, suppose we want sort movies by their rating and names also. When we make a collection element comparable(by having it implement Comparable), we get only one chance to implement the compareTo() method. The solution is using Comparator.
Unlike Comparable, Comparator is external to the element type we are comparing. It’s a separate class. We create multiple separate classes (that implement Comparator) to compare by different members.
Collections class has a second sort() method and it takes Comparator. The sort() method invokes the compare() to sort objects.
To compare movies by Rating, we need to do 3 things :
- Create a class that implements Comparator (and thus the compare() method that does the work previously done by compareTo()).
- Make an instance of the Comparator class.
- Call the overloaded sort() method, giving it both the list and the instance of the class that implements 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
- Comparable is meant for objects with natural ordering which means the object itself must know how it is to be ordered. For example Roll Numbers of students. Whereas, Comparator interface sorting is done through a separate class.
- Logically, Comparable interface compares “this” reference with the object specified and Comparator in Java compares two different class objects provided.
- If any class implements 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 objects will be sorted based on there natural order defined by CompareTo method.
To summarize, if sorting of objects needs to be based on natural order then use Comparable whereas if you sorting needs to be done on attributes of different objects, then use Comparator in Java.