A Quicksort implementation in Java which uses the first number as pivot.

quicksort.java

/* Inspired by: http://interactivepython.org/runestone/static/pythonds/SortSearch/TheQuickSort.html */

void quickSort(int[] a, int first, int last) {
  if (first < last) {
    int p = partition(a, first, last); // split point
    quickSort(a, first, p - 1); // sort numbers on the left side of the split point
    quickSort(a, p + 1, last); // sort numbers on the right side of the split point
  }
}

int partition(int[] a, int first, int last) {
  int pivot = a[first]; // choose the first number as pivot value
  int left = first + 1;
  int right = last;
  
  while (left <= right) {
    // find numbers that should be swapped
    while (a[left] < pivot) left++;
    while (a[right] > pivot) right--;
    
    if (left < right) {
      // swap left and right
      swap(a, left, right);
    } else {
      // swap pivot and right, and return right index as the split point
      swap(a, first, right); 
      return right;
    }
  }
}

void swap(int[] a, int i, int j) {
  int temp = a[i];
  a[i] = a[j];
  a[j] = temp;
}