I study the Quicksort algorithm like this :

// Java program for implementation of QuickSort 
class QuickSort 
{ 
    /* This function takes last element as pivot, 
    places the pivot element at its correct 
    position in sorted array, and places all 
    smaller (smaller than pivot) to left of 
    pivot and all greater elements to right 
    of pivot */
    int partition(int arr[], int low, int high) 
    { 
        int pivot = arr[high]; 
        int i = (low-1); // index of smaller element 
        for (int j=low; j<high; j++) 
        { 
            // If current element is smaller than or 
            // equal to pivot 
            if (arr[j] <= pivot) 
            { 
                i++; 

                // swap arr[i] and arr[j] 
                int temp = arr[i]; 
                arr[i] = arr[j]; 
                arr[j] = temp; 
            } 
        } 

        // swap arr[i+1] and arr[high] (or pivot) 
        int temp = arr[i+1]; 
        arr[i+1] = arr[high]; 
        arr[high] = temp; 

        return i+1; 
    } 

    /* The main function that implements QuickSort() 
    arr[] --> Array to be sorted, 
    low --> Starting index, 
    high --> Ending index */
    void sort(int arr[], int low, int high) 
    { 

int partition(int arr[], int low, int high) 
{ 
    int pivot = arr[high];  
    int i = (low-1); // index of smaller element 
    for (int j=low; j<high; j++
        if (low < high) 
        { 
            /* pi is partitioning index, arr[pi] is 
            now at right place */
            int pi = partition(arr, low, high); 

            // Recursively sort elements before 
            // partition and after partition 
            sort(arr, low, pi-1); 
            sort(arr, pi+1, high); 
        } 
    } 

    /* A utility function to print array of size n */
    static void printArray(int arr[]) 
    { 
        int n = arr.length; 
        for (int i=0; i<n; ++i) 
            System.out.print(arr[i]+" "); 
        System.out.println(); 
    } 

    // Driver program 
    public static void main(String args[]) 
    { 
        int arr[] = {10, 7, 8, 9, 1, 5}; 
        int n = arr.length; 

        QuickSort ob = new QuickSort(); 
        ob.sort(arr, 0, n-1); 

        System.out.println("sorted array"); 
        printArray(arr); 
    } 
} 
/*This code is contributed by Rajat Mishra */

And precisely in this part :

int partition(int arr[], int low, int high) 
{ 
    int pivot = arr[high];  
    int i = (low-1); // index of smaller element 
    for (int j=low; j<high; j++

Would you have an idea why the code says: int i = (low-1) ? The range of "i" will not become -1 with this instruction? I mean it was initialized previously to 0? Then do think is it possible such instruction? Or what as you it has to understand?

Best regard,

Thanks, Intelego.

The thing about quicksort is the sheer number of implementations that you find on the web.
https://rosettacode.org/wiki/Sorting_algorithms/Quicksort shows over 120 implementations for your study.

While creating another quicksort may be educational, it's best to view these as wheels that you pick up and install so you don't lose time creating the real app you wanted to make.