Collections.reverseOrder() in Java with Examples

Difficulty Level : Easy
Last Updated : 07 Dec, 2018

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.

// Java program to demonstrate working of Collections.reveseOrder() 
// to sort a list in descending order 
import java.util.*; 
  
public class Collectionsorting 
{ 
    public static void main(String[] args) 
    { 
        // Create a list of Integers 
        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.

// Java program to demonstrate working of Collections.reveseOrder() 
// to sort an array in descending order 
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)
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

References:
https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#reverseOrder()

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