How to Reverse an ArrayList in Java

Reversing an ArrayList means changing the order of its elements so that the first element becomes the last, the second becomes the second-last, and so on. Java provides a simple method to reverse the order of elements in an ArrayList using Collections.reverse(). The following are the ways to reverse an ArrayList

• Using Collections.reverse() method

• Manually reversing an ArrayList

Reverse an ArrayList Using Collections.reverse() Method

The reverseOrder() method of Java Collections obtains a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface.

Syntax

Following is the declaration for java.util.Collections.reverseOrder() method.

public static  Comparator reverseOrder()

Example

The following example reverses an array list by using Collections.reverse(ArrayList)method

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

public class Main {
   public static void main(String[] args) {
      ArrayList arrayList = new ArrayList();
      arrayList.add("A");
      arrayList.add("B");
      arrayList.add("C");
      arrayList.add("D");
      arrayList.add("E");
      System.out.println("Before Reverse Order: " + arrayList);
      Collections.reverse(arrayList);
      System.out.println("After Reverse Order: " + arrayList);
   }
}

Output

Before Reverse Order: [A, B, C, D, E]
After Reverse Order: [E, D, C, B, A]

Reverse an ArrayList Manually

The process involves swapping elements from both ends of the array until they meet in the middle.

Example

The following is another example of reversing an array

public class HelloWorld {
   public static void main(String[] args) {
      int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
      System.out.println("Array before reverse:");
      
      for (int i = 0; i < numbers.length; i++) {
         System.out.print(numbers[i] + " ");
      } 
      for (int i = 0; i < numbers.length / 2; i++) {
         int temp = numbers[i];
         numbers[i] = numbers[numbers.length - 1 - i];
         numbers[numbers.length - 1 - i] = temp;
      } 
      System.out.println("\nArray after reverse:");
      for (int i = 0; i < numbers.length; i++) {
         System.out.print(numbers[i] + " ");
      } 
   }
}

Output

Array before reverse:
1 2 3 4 5 6 7 8 9 10 
Array after reverse:
10 9 8 7 6 5 4 3 2 1