Generate array of random numbers with quickSort in Java?

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class QuickSort {
/**
* Main method.
* @param args
*/
public static void main(String[] args) {

QuickSort app = new QuickSort();

//Generate an integer array of length 7
   List<Integer> input = app.generateRandomNumbers(7);

   //Before sort
   System.out.println(input);

   //After sort
   System.out.println(app.quicksort(input));

}

/**
* This method sort the input ArrayList using quick sort algorithm.
* @param input the ArrayList of integers.
* @return sorted ArrayList of integers.
*/
private List<Integer> quicksort(List<Integer> input){

if(input.size() <= 1){
return input;
}

int middle = (int) Math.ceil((double)input.size() / 2);
int pivot = input.get(middle);

List<Integer> less = new ArrayList<Integer>();
List<Integer> greater = new ArrayList<Integer>();

for (int i = 0; i < input.size(); i++) {
if(input.get(i) <= pivot){
if(i == middle){
continue;
}
less.add(input.get(i));
}
else{
greater.add(input.get(i));
}
}

return concatenate(quicksort(less), pivot, quicksort(greater));
}

/**
* Join the less array, pivot integer, and greater array
* to single array.
* @param less integer ArrayList with values less than pivot.
* @param pivot the pivot integer.
* @param greater integer ArrayList with values greater than pivot.
* @return the integer ArrayList after join.
*/
private List<Integer> concatenate(List<Integer> less, int pivot, List<Integer> greater){

List<Integer> list = new ArrayList<Integer>();

for (int i = 0; i < less.size(); i++) {
list.add(less.get(i));
}

list.add(pivot);

for (int i = 0; i < greater.size(); i++) {
list.add(greater.get(i));
}

return list;
}

/**
* This method generate a ArrayList with length n containing random integers .
* @param n the length of the ArrayList to generate.
* @return ArrayList of random integers with length n.
*/
private List<Integer> generateRandomNumbers(int n){

   List<Integer> list = new ArrayList<Integer>(n);
   Random random = new Random();

   for (int i = 0; i < n; i++) {
   list.add(random.nextInt(n * 10));
   }

   return list;
}
}

Below is the output....


[25, 58, 65, 19, 59, 8, 39]
[8, 19, 25, 39, 58, 59, 65]