How to reverse a List in Java

In this blog post, I will be explaining how you can reverse a Set or List via Java.  Consider the following code snippet:

package learnjava.collections;import java.util.ArrayList;import java.util.Collections;import java.util.List;public class ReverseDemo {public static void main(String[] args) {List<Integer>inputList = new ArrayList<Integer>();inputList.add(5);inputList.add(15);inputList.add(20);inputList.add(25);List<Integer>reverserdList1 = reverseList1(inputList);List<Integer>reverserdList2 = reverseList2(inputList);System.out.println("Input List is "+inputList);System.out.println("reverserdList using method1 "+reverserdList1);System.out.println("reverserdList using method2 "+reverserdList2);}private static List<Integer> reverseList2(List<Integer>inputList ){List<Integer> reverserdList = new ArrayList<Integer>();for(int i =inputList.size()-1;i >=0;i--){reverserdList.add(inputList.get(i));}return reverserdList;}private static List<Integer> reverseList1(List<Integer>inputList ){Collections.reverse(inputList);return inputList;}}&lt;pre&gt;

 

This code snippet demonstrates two ways to reverse a List.

 

Method 1 – Using Collections.reverse

The Collections class has a number of useful utility methods for collections. One such method provided by this class is the Collections.reverse method. This method accepts a Collection and reverses its contents. So any Collection like Set, List, etc can be passed to this method. It has no return value, it returns a void. So the Collection that is passed in is modified by this method.

Method 2 – Using Manual method

In the manual method, the code iterates through the input List in the reverse order. It then adds each element in the List to a new List and returns the new List object.

So when you run this code, it will produce the following output:

Input List is [25, 20, 15, 5]reverserdList using method1 [25, 20, 15, 5]reverserdList using method2 [5, 15, 20, 25]

 

 

 

 

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