How to Reverse the Order of a List or an Array in Java

Learn to Use the Collections.reverse() Method

The Collections.reverse() method is a simple way to reverse the order of elements in a list or an array in Java. In this article, we will explore how to use this method step-by-step with examples.

Reversing an ArrayList or LinkedList

To reverse the order of elements in an ArrayList or LinkedList, follow these steps:

1. Create a list of elements using the ArrayList or LinkedList class.

sql
List<Integer> list = new ArrayList<Integer>();

or

sql
List<Integer> list = new LinkedList<Integer>();

2. Add elements to the list.

csharp
list.add(10);
list.add(20);
list.add(30);

3. Print the original order of the list.

go
System.out.println(list); // [10, 20, 30]

4. Use the Collections.reverse() method to reverse the order of elements.

lua
Collections.reverse(list);

5. Print the reversed order of the list.

go
System.out.println(list); // [30, 20, 10]

Reversing an Array

The Arrays class in Java does not have a built-in reverse() method. However, we can use the Collections.reverse() method to reverse an array by converting it to a list using the Arrays.asList() method.

1. Create an array of elements.

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Integer[] arr = {10, 20, 30};

2. Convert the array to a list.

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List<Integer> list = Arrays.asList(arr);

3. Print the original order of the array.

go
System.out.println(Arrays.toString(arr)); // [10, 20, 30]

4. Use the Collections.reverse() method to reverse the order of elements in the list.

lua
Collections.reverse(list);

5. Print the reversed order of the array.

go
System.out.println(Arrays.toString(arr)); // [30, 20, 10]

Example Code

Here is the complete code for both examples.

csharp
import java.util.*;

public class ReverseExample {
    public static void main(String[] args) {
        
        // Reversing an ArrayList or LinkedList
        List<Integer> list = new ArrayList<Integer>();
        list.add(10);
        list.add(20);
        list.add(30);
        System.out.println(list); // [10, 20, 30]
        Collections.reverse(list);
        System.out.println(list); // [30, 20, 10]
        
        // Reversing an Array
        Integer[] arr = {10, 20, 30};
        List<Integer> arrList = Arrays.asList(arr);
        System.out.println(Arrays.toString(arr)); // [10, 20, 30]
        Collections.reverse(arrList);
        System.out.println(Arrays.toString(arr)); // [30, 20, 10]
    }
}

In conclusion, the Collections.reverse() method is a convenient way to reverse the order of elements in a list or an array in Java. By following the steps outlined in this article, you can easily implement this method in your own code.