StackOverflowError when sorting a list of 7 numbers with my quicksort algorithm

Question

For the following quicksort algorithm:

import java.util.Scanner;
import java.util.ArrayList;
public class quick_sort_demo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        ArrayList<Integer> my_arr = new ArrayList<Integer>();
        while (true) {
            System.out.println("Enter input value. '-1' to exit the adding process. ");
            int val = sc.nextInt();
            sc.nextLine();
            if (val == -1) {
                break;
            } else {
                my_arr.add(val);
            }
        }
        quick_sort(my_arr);
    }
    public static ArrayList<Integer> quick_sort(ArrayList<Integer> arr) {
        if (arr.size() <= 1) {
            return arr;
        }
        int pivot_index = (int) (Math.random() * arr.size());
        int pivot = arr.get(pivot_index);
        ArrayList<Integer> left_arr = new ArrayList<Integer>();
        ArrayList<Integer> right_arr = new ArrayList<Integer>();
        for (int i = 0; i < arr.size(); i++) {
            int element = arr.get(i);
            if (element > pivot) {
                right_arr.add(element);
            } else {
                left_arr.add(element);
            }
        }
        left_arr = quick_sort(left_arr);
        right_arr = quick_sort(right_arr);
        ArrayList<Integer> result = new ArrayList<Integer>();
        result.addAll(left_arr);
        result.add(pivot);
        result.addAll(right_arr);
        return result;
    }
}

I am getting this error repeatedly:

Exception in thread "main" java.lang.StackOverflowError
        at java.base/java.util.Random.next(Random.java:209)
        at java.base/java.util.Random.nextDouble(Random.java:463)
        at java.base/java.lang.Math.random(Math.java:865)
        at quick_sort_demo.quick_sort(quick_sort_demo.java:23)
        at quick_sort_demo.quick_sort(quick_sort_demo.java:35)

My exact inputs for the program were as follows: {6,15,32,643,6543,534232,232,-1}

I tried asking CHATGPT but I couldn't get a helpful answer for the specific error I encountered. I don't understand why I'm getting a stackoverflow error as the recursive method should be calling itself without issue until the 'arr.size()' returns something <= 1. Can someone explain what I'm doing wrong? Is my algorithm wrong and why am I getting a recursion error?

Answer 1

code runs fine

well almost fine, there is a bug in your code

left_arr = quick_sort(left_arr);
right_arr = quick_sort(right_arr);
ArrayList<Integer> result = new ArrayList<Integer>();
result.addAll(left_arr);
result.add(pivot); //BUG HERE
result.addAll(right_arr);

when you split left/right you already add your pivot into the left_arr - so no need to add it twice, since

if (element > pivot) {
    right_arr.add(element);
} else {
    left_arr.add(element); //pivot itself is NOT bigger than pivot and will be added here
}

alternatively you can remove the pivot from the list: int pivot = arr.remove(pivot_index);

about java.lang.StackOverflowError

well - your code is working fine, on my machine so i guess you have to setup your computer properly - see this answer for further details (sorry for that link-only answer)