How to Use Collections.reverse in Java: Simple Guide

Use Collections.reverse(List<T> list) to reverse the order of elements in a list in Java. This method modifies the original list in place, so no new list is created.

Syntax

The method Collections.reverse takes a single argument, which is a List of any type. It reverses the order of elements inside the list directly.

• Collections.reverse(list): Reverses the elements of list in place.
• list: A List object whose elements you want to reverse.

java

Collections.reverse(list);

Example

This example shows how to reverse a list of strings using Collections.reverse. The original list is changed after calling the method.

java

import java.util.*;

public class ReverseExample {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>(Arrays.asList("Apple", "Banana", "Cherry", "Date"));
        System.out.println("Original list: " + fruits);
        Collections.reverse(fruits);
        System.out.println("Reversed list: " + fruits);
    }
}

Output

Original list: [Apple, Banana, Cherry, Date]
Reversed list: [Date, Cherry, Banana, Apple]

Common Pitfalls

Common mistakes when using Collections.reverse include:

• Trying to reverse an immutable or fixed-size list, which causes UnsupportedOperationException if you try to add or remove elements.
• Expecting Collections.reverse to return a new list instead of modifying the original.
• Passing null as the list, which causes NullPointerException.

Always ensure the list is mutable and not null.

java

import java.util.*;

public class ReversePitfall {
    public static void main(String[] args) {
        // Arrays.asList returns a fixed-size list
        List<String> fixedList = Arrays.asList("A", "B", "C");
        try {
            Collections.reverse(fixedList); // This works because reverse swaps elements, but adding/removing fails
            System.out.println("Reversed fixedList: " + fixedList);
        } catch (UnsupportedOperationException e) {
            System.out.println("Cannot modify fixed-size list");
        }

        // Use ArrayList for mutable list
        List<String> mutableList = new ArrayList<>(fixedList);
        Collections.reverse(mutableList);
        System.out.println("Reversed mutableList: " + mutableList);
    }
}

Output

Reversed fixedList: [C, B, A]
Reversed mutableList: [C, B, A]

Quick Reference

Collections.reverse modifies the list in place and does not return a value.

• Input: List<T>
• Output: void (list reversed in place)
• Throws: NullPointerException if list is null
• Works only on mutable lists

Method	Description
Collections.reverse(list)	Reverses the order of elements in the given list in place.
Throws NullPointerException	If the list passed is null.
Works on mutable lists	The list must support element modification.

Key Takeaways

Collections.reverse reverses the elements of a mutable list in place without creating a new list.

Always pass a mutable and non-null list to avoid exceptions.

The method does not return a new list; it changes the original list directly.

Arrays.asList returns a fixed-size list; wrap it with ArrayList to safely reverse.

NullPointerException occurs if you pass null instead of a list.