Collections.reverse() in Java with Examples

java.util.Collections.reverse() method is a java.util.Collections class method. It reverses the order of elements in a list passed as an argument.

// Reverses elements of myList and returns Nothing.
// For example, if list contains {1, 2, 3, 4}, it converts
// list to {4, 3, 2, 1}
public static void reverse(List myList)

It throws UnsupportedOperationException if the specified 
list or its list-iterator does not support the set operation.

Reversing an ArrayList or LinkedList.

// Java program to demonstrate working of java.utils.
// Collections.reverse()
import java.util.*;

public class ReverseDemo
{
    public static void main(String[] args)
    {
        // Let us create a list of strings
        List<String>  mylist = new ArrayList<String>();
        mylist.add("practice");
        mylist.add("code");
        mylist.add("quiz");
        mylist.add("geeksforgeeks");

        System.out.println("Original List : " + mylist);

        // Here we are using reverse() method
        // to reverse the element order of mylist
        Collections.reverse(mylist);

        System.out.println("Modified List: " + mylist);
    }
}

Output:

Original List : [practice, code, quiz, geeksforgeeks]
Modified List: [geeksforgeeks, quiz, code, practice]

For Linkedlist, we just need to replace ArrayList with LinkedList in “List mylist = new ArrayList();”.

Arrays class in Java doesn’t have reverse method. We can use Collections.reverse() to reverse an array also.

// Java program to demonstrate reversing of array
// with Collections.reverse()
import java.util.*;

public class ReverseDemo
{
    public static void main(String[] args)
    {
        // Let us create an array of integers
        Integer arr[] = {10, 20, 30, 40, 50};

        System.out.println("Original Array : " +
                                Arrays.toString(arr));
        
        // Please refer below post for details of asList()
        // https://www.geeksforgeeks.org/array-class-in-java/
        Collections.reverse(Arrays.asList(arr));
        
        System.out.println("Modified Array : " +
                                Arrays.toString(arr));
    }
}

Output:

Original Array : [10, 20, 30, 40, 50]
Modified Array : [50, 40, 30, 20, 10]

This article is contributed by Mohit Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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