JAVA Comparable vs Comparator

The java.lang.Comparable and java.util.Comparator interfaces provide similar function and there is sometimes confusion as to which interface to use where. Some documentation adds to the confusion by implying that the Comparable interface should be used for natural ordering where as Comparator should be used for total ordering without explaining what that means.

The Comparable interface is defined as

public interface Comparable<T> {

   int compareTo(T o) ;
}

The compareTo method is for comparing this object with the object that is passed in. Instances of classes that implement Comparable use this method to compare other instances to themselves.

Let us say class Employee implements Comparable. If an array of Employees is passed to Arrays.sort(Object[] o) methods, the sorting code calls the compareTo method to compare Employee objects with one another and put them in the correct order.  In fact the Arrays method sort(Object[] o) expects the objects to have implemented the Comparable interface and an exception is thrown if the objects do not implement Comparable.

class Employee implements Comparable<Employee> {

    private int id ;
    private String name ;

    // natural ordering  compare by id and by name
    // simplistic 
    public int compareTo(Employee e) {

        if ( id == e.id)
             return name.compareTo(e.name) ;
        
        return (id - e.id) ; 
   }

}

Generally the Comparable interface is used when

(1) You have control of the class and can make the class implement Comparable
(2) The objects need to be sorted in their natural order. Natural order is the order defined by the JVM. For number types everyone understand the order - 23 is greater than 22. For Strings it is the lexicographic order.
(3) Generally there is only one natural order for each type. In the above example, you can order by id,name. If you needed a second kind of ordering by just the id or just the name, it would not be possible with Comparable. But you could write multiple Comparators .

If you need to sort an array of objects which do not implement Comparable and the code for the classes is not under your control, you can write a new class that implements the Comparator interface and call the following method in java.util.Arrays.

public static void sort(T[] a , Comparator <? super T>) ;

interface Comparator<T> {

   int compare(T o1, T o2) ;

   boolean equals(Object o) ;
}

public class EmpIdComparator implements Comparator<Employee> {

   public int compare(Employee e1, Employee e2) {

          return e1.id - e2.id ;
    }

}

public class EmpNameComparator implements  Comparator<Employee> {

   public int compare(Employee e1, Employee e2) {

         return e1.name.compareTo(e2.name) ;

   }

}

If you need to sort employees by id , you can pass an instance of the EmpIdComparator to the sort methods. If you need employees sorted by name, use the EmpNameComparator.

Here the compare method lets you compare any two objects. The Comparator interface is used when

(1) You do not have control of the class or the class does not implement Comparable for some reason.
(2) You want ordering that is different from what is generally accepted as the natural order.
(3)  You need many different kinds of ordering.

All the sorting methods and collections that maintain order ( SortedMap, SortedSet ) expect the contained objects to either implement Comparable or you need to pass in another instance of a Comparator Object.

Lastly the Comparable and Comparator interfaces are useful design patterns that you can copy whenever you need to write code that applies to generics. Given a collection of type T and you need to do some operation on each type T. Consider defining a generic interface with a a generic method. The caller of your API can pass in an implementation for the interface for the appropriate type.