An iterative implementation of Quick Sort in Java

QuickSort.java

import java.util.Queue;
import java.util.LinkedList;

public class QuickSort {

	private int[] arr;
	private Queue<int[]> queue;

	public QuickSort(int[] arr) {
		super();
		this.arr = arr;
		this.init();
	}

	public void init() {
		this.queue = new LinkedList<int[]>();
		queue.add(new int[] {0, arr.length - 1});
	}

	public static void main(String[] args) {
		int[] arr = new int[] {40,50,10,20,70,80,90,30,60};
		QuickSort sortApp = new QuickSort(arr);
		sortApp.print(arr);
		System.out.println("START");
		int[] sorted_arr = sortApp.sort();
		System.out.println("DONE");
		sortApp.print(sorted_arr);
	}

	public int[] sort() {
		while(queue.size() > 0) {
			int[] indexes = queue.remove();
			int left_index = indexes[0];
			int right_index = indexes[1];

			int index = partition(left_index, right_index);
			if(left_index < index - 1) {
				queue.add(new int[] {left_index, index - 1});
			}
			if(index < right_index) {
				queue.add(new int[] {index, right_index});
			}
		}
		return this.arr;
	}

	private int partition(int left, int right) {
		int i = left;
		int j = right;
		int pivot_value = arr[(left + right) / 2];
		while(i <= j) {
			while(arr[i] < pivot_value) {
				i++;
			}
			while(arr[j] > pivot_value) {
				j--;
			}
			if(i <= j) {
				swap(i, j);
				i++;
				j--;
			}
		}
		return i;
	}

	private void swap(int i, int j) {
		int tmp = arr[i];
		arr[i] = arr[j];
		arr[j] = tmp;
	}

	private void print(int[] a) {
		int[] array = a == null ? this.arr : a;
		for(int i = 0; i < array.length; i++) {
			System.out.print(array[i] + ", ");
		}
		System.out.println();
	}
	
}