How To Rotate A List In Java – Two Ways

In this guide, you will learn how to rotate a list in Java. Rotation means shifting of elements, for example rotating a list to right by 2 position means 1st element becomes 3rd element, 2nd element becomes 4th element and so on. We will see two examples to rotate a list:

1. Using Collections.rotate()

You can use Collections.rotate() method to rotate a List. This method belongs to java.util.Collections class. Let’s see an example to understand the usage:

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class RotateList {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
        System.out.println("Original list: " + list);
        
        // Rotate the list by 2 positions to the right
        Collections.rotate(list, 2);
        System.out.println("Rotated list: " + list);
    }
}

In this example, the list [1, 2, 3, 4, 5] is rotated by 2 positions to the right, the output list is [4, 5, 1, 2, 3].

2. Manual Rotation

If you do not want to use the Collections.rotate() method, you can write a logic to manually rotate the list as shown below:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class RotateListManual {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
        System.out.println("Original list: " + list);
        
        // Rotate the list by 2 positions to the right
        int k = 2;
        rotate(list, k); // user defined method
        System.out.println("Rotated list: " + list);
    }
    
    public static void rotate(List<Integer> list, int k) {
        int size = list.size();
        k = k % size;  // In case k is greater than size of the list
        if (k < 0) {
            k += size;  // In case k is negative
        }
        
        reverse(list, 0, size - 1);
        reverse(list, 0, k - 1);
        reverse(list, k, size - 1);
    }
    
    private static void reverse(List<Integer> list, int start, int end) {
        while (start < end) {
            Integer temp = list.get(start);
            list.set(start, list.get(end));
            list.set(end, temp);
            start++;
            end--;
        }
    }
}

The output is same as above. It is best to use the rotate() method instead of using this manual code, however sometimes, this logic is asked in interviews and tests.