Rotate and return the ArrayList<Integer> in java

Question

here in below program i am defining ArrayList of size 7 [1,2,3,4,5,6,7] and and rotating right by 3 places but when i print the numbers1 list it gives me my expected answer numbers: [5, 6, 7, 1, 2, 3, 4] but when i return this ArrayList and then print it, it gives me this answer numbers: [2, 3, 4, 5, 6, 7, 1] why is that please explain.

package QAIntvSprint;

import java.util.ArrayList;
import java.util.Scanner;

public class RotateArray {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int k = sc.nextInt();

    ArrayList<Integer> numbers = new ArrayList<>();

    for (int i = 0; i < n; i++) {
      numbers.add(sc.nextInt());
    }
    rotateArray(numbers, k);
    ArrayList<Integer> ans = rotateArray(numbers, k);
    for (Integer x : ans) {
      System.out.print(x + " ");
    }
  }

  static ArrayList<Integer> rotateArray(ArrayList<Integer> numbers, int k) {
    for (int i = 0; i < k; i++) {
      numbers.add(0, numbers.get(numbers.size() - 1));
      numbers.remove(numbers.size() - 1);
    }
    ArrayList<Integer> numbers1 = numbers;
    System.out.println("numbers: " + numbers1.toString());
    return numbers1;
  }
}

output

Answer 1

You are running your function twice:

rotateArray(numbers, k); // first
ArrayList<Integer> ans = rotateArray(numbers, k); // second

So the numbers are shifted by 6 instead of 3 positions

Besides, your function does not create a new copy of the array
You return a reference to the same array

ArrayList<Integer> numbers1 = numbers; // copies a reference to an object, not the object itself
return numbers1; // returns the original link

So i suggest refactoring your code like this:

public class RotateArray {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        
        ArrayList<Integer> numbers = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            numbers.add(sc.nextInt());
        }
        
        // the function will modify the ArrayList by reference, it will change
        rotateArray(numbers, k);
        for (Integer x : numbers) {
            System.out.print(x + " ");
        }
    }
    
    static void rotateArray(ArrayList<Integer> numbers, int k) {
        for (int i = 0; i < k; i++) {
            numbers.add(0, numbers.get(numbers.size() - 1));
            numbers.remove(numbers.size() - 1);
        }
    }
}