How To Reverse An ArrayList In Java

In this post, we will see how to reverse an ArrayList in Java by creating a copy of it. In reverse order without altering the ordering of elements in the original list. ArrayList is nothing but a dynamic array, which can resize itself.

What is Collections Class

The specified list may or may not be mutable, but the returned list should be mutable. The Collections class provides a method named reverse() this method accepts a list and reverses the order of elements in it.

How to start Android app development for beginners

How Collections Reverse Work

The given ArrayList can be reversed using Collections.reverse() method. Collections is a class in java util package which contains various static methods for searching, sorting, reversing, finding min, max….etc.

Example

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

    public class ReverseArrayList
    {
        public static void main(String[] args)
        {
            ArrayList<String> list = new ArrayList<String>();

            list.add("Cupcake");

            list.add("Donut");

            list.add("Eclair");

            list.add("Froyo");

            list.add("Gingerbread");

            list.add("Honeycomb");

            list.add("Cream Sandwich");

            list.add("Jelly Bean");

            //Printing list before reverse

            System.out.println("ArrayList Before Reverse :");

            System.out.println(list);

            //Reversing the list using Collections.reverse() method

            Collections.reverse(list);

            //Printing list after reverse

            System.out.println("ArrayList After Reverse :");

            System.out.println(list);
        }
    }

Some Useful Methods Of java Util Collections

Collections copy

Collections.copy(): This method is used to copy all elements from one list to another list.

Collections replace All

Collections.replaceAll(): In addition, It replaces all occurrences of old value with new value in the given list.

How to add Firebase in android app

Collections synchronized List

Collections.synchronizedList(): Above method returns the synchronized i.e thread-safe list backed by the specified list.

Collections synchronized Collection

Collections.synchronizedCollection(): This method returns the synchronized version of the specified Collection.

Collections max

Collections.max(): Collections max returns the maximum element in the given Collection.

Collections min

Collections.min(): Similarly, It returns the minimum element in the given Collection.

Conclusion

There are many types of exercise you can do regularly. Above all code is fit for this example. In conclusion, you can see the output of the above code for reverse an ArrayList in Java :

ArrayList Before Reverse :
[Cupcake, Donut, Eclair, Froyo, Gingerbread, Honeycomb, Cream Sandwich, Jelly Bean]

ArrayList After Reverse :
[Jelly Bean, Cream Sandwich, Honeycomb, Gingerbread, Froyo, Eclair, Donut, Cupcake ]