Collections.reverseOrder() in Java with Examples

Collections.reverseOrder() in Java with Examples

java.util.Collections.reverseOrder() method is a java.util.Collections class method.

// Returns a comparator that imposes the reverse of
// the natural ordering on a collection of objects
// that implement the Comparable interface.
// The natural ordering is the ordering imposed by
// the objects' own compareTo method

public static  Comparator reverseOrder()

We can the comparator returned by Collections.reverseOrder() to sort a list in descending order.

Sample Code:

import java.util.ArrayList;
import java.util.Collections;


/**
 * @author Abhinaw.Tripathi
 *
 */
public class ReverseOrderApp {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
ArrayList<Integer> al = new ArrayList<Integer>();
        al.add(30);
        al.add(20);
        al.add(10);
        al.add(40);
        al.add(50);

        /* Collections.sort method is sorting the
        elements of ArrayList in descending order. */
        Collections.sort(al, Collections.reverseOrder());

        // Let us print the sorted list
        System.out.println("List after the use of Collection.reverseOrder()"+
                           " and Collections.sort() :\n" + al);
}

}


Output:

List after the use of Collection.reverseOrder() and Collections.sort() :
[50, 40, 30, 20, 10]


We can use this method with Arrays.sort() also.

Sample Code:

import java.util.*;

public class Collectionsorting
{
    public static void main(String[] args)
    {
        // Create an array to be sorted in descending order.
        Integer [] arr = {30, 20, 40, 10};

        /* Collections.sort method is sorting the
        elements of arr[] in descending order. */
        Arrays.sort(arr, Collections.reverseOrder());

        // Let us print the sorted array
        System.out.println("Array after the use of Collection.reverseOrder()"+
                           " and Arrays.sort() :\n" + Arrays.toString(arr));
    }
}

Output:

Array after the use of Collection.reverseOrder() and Arrays.sort() :
[40, 30, 20, 10]



public static Comparator reverseOrder(Comparator c)


Sample Code:

It returns a Comparator that imposes reverse order of a passed Comparator object. We can use this method to sort a list in reverse order of user defined Comparator. For example, in the below program, we have created a reverse of user defined comparator to sort students in descending order of roll numbers.

// Java program to demonstrate working of
// reverseOrder(Comparator c) to sort students in descending
// order of roll numbers when there is a user defined comparator
// to do reverse.

import java.util.*;
import java.lang.*;
import java.io.*;

// A class to represent a student.
class Student
{
    int rollno;
    String name, address;

    // Constructor
    public Student(int rollno, String name,
                               String address)
    {
        this.rollno = rollno;
        this.name = name;
        this.address = address;
    }

    // Used to print student details in main()
    public String toString()
    {
        return this.rollno + " " + this.name +
                           " " + this.address;
    }
}

class Sortbyroll implements Comparator<Student>
{
    // Used for sorting in ascending order of
    // roll number
    public int compare(Student a, Student b)
    {
        return a.rollno - b.rollno;
    }
}

// Driver class
class Main
{
    public static void main (String[] args)
    {
        ArrayList<Student> ar = new ArrayList<Student>();
        ar.add(new Student(111, "bbbb", "london"));
        ar.add(new Student(131, "aaaa", "nyc"));
        ar.add(new Student(121, "cccc", "jaipur"));

        System.out.println("Unsorted");
        for (int i=0; i<ar.size(); i++)
            System.out.println(ar.get(i));

        // Sorting a list of students in descending order of
        // roll numbers using a Comparator that is reverse of
        // Sortbyroll()
        Comparator c = Collections.reverseOrder(new Sortbyroll());
        Collections.sort(ar, c);

        System.out.println("\nSorted by rollno");
        for (int i=0; i<ar.size(); i++)
            System.out.println(ar.get(i));
    }
}


Output :

Unsorted
111 bbbb london
131 aaaa nyc
121 cccc jaipur

Sorted by rollno
131 aaaa nyc
121 cccc jaipur
111 bbbb london