Sort stack in ascending order

I am using two more helper stacks to sort. Kindly suggest improvements in terms of space and time.

import java.util.ArrayDeque;
import java.util.Deque;

public class SortStackAscendingOrder {

    Deque stackhelper1 = new ArrayDeque<>();
    Deque stackHelper2 = new ArrayDeque<>();

    public Deque sortStack(Deque mainStack) {
        if (mainStack.isEmpty()) {
            return null;
        }

        while (!mainStack.isEmpty()) {
            int mainItem = mainStack.pop();
            int helperItem1 = -1;
            if (!stackhelper1.isEmpty()) {
                helperItem1 = stackhelper1.peek();
            }
            if (mainItem > helperItem1) {
                // push all helperItem1 to helperItem2, till it is in ascending
                // order
                while (!stackhelper1.isEmpty() && mainItem > helperItem1) {
                    stackHelper2.push(stackhelper1.pop());
                    helperItem1 = stackhelper1.peek();
                }
                // push mainItem.
                stackhelper1.push(mainItem);
                // bring back all the helperItem2
                while (!stackHelper2.isEmpty()) {
                    stackhelper1.push(stackHelper2.pop());
                }
            } else {
                // directly push in stackhelper1
                stackhelper1.push(mainItem);
            }
        }

        return stackhelper1;

    }

}

Possible bug:

The following demo will throw a NullPointerException:

public static void main(String[] args) {
    Deque stack = new ArrayDeque<>();

    stack.addLast(2);
    stack.addLast(-1);
    stack.addLast(-3);
    stack.addLast(1);

    System.out.println("Original stack: " + stack);
    SortStackAscendingOrder sorter = new SortStackAscendingOrder();

    Deque sortedStack = sorter.sortStack(stack);

    System.out.println("Sorted original stack: " + sortedStack);

    stack.clear();
    stack.addLast(11);
    stack.addLast(8);
    stack.addLast(9);
    stack.addLast(10);

    System.out.println("Another stack: " +
            sorter.sortStack(stack));

    System.out.println("Sorted original stack again: " + sortedStack);
}

Most likely the problem above is due to the fact that you return stackhelper1, which is an essential internal facility in your algorithm.

API design:

First of all, I would consider implementing the stack sort as a static method that does not "leak" any internals. The idea here is that you sort the input stack: just like sorting methods in java.util.Arrays, all are void and modify the input arrays instead of returning a sorted copy.

All in all, I had this in mind:

public static void sortStack(Deque mainStack) {
    Deque stackHelper1 = new ArrayDeque<>();
    Deque stackHelper2 = new ArrayDeque<>();

    while (!mainStack.isEmpty()) {
        int mainItem = mainStack.pop();
        int helperItem1 = -1;

        if (!stackHelper1.isEmpty()) {
            helperItem1 = stackHelper1.peek();
        }

        if (mainItem > helperItem1) {
            while (!stackHelper1.isEmpty() && mainItem > helperItem1) {
                stackHelper2.push(stackHelper1.pop());
                helperItem1 = stackHelper1.peek();
            }

            stackHelper1.push(mainItem);

            while (!stackHelper2.isEmpty()) {
                stackHelper1.push(stackHelper2.pop());
            }
        } else {
            stackHelper1.push(mainItem);
        }
    }

    mainStack.clear();
    mainStack.addAll(stackHelper1);
}

StackExchange Code Review Q#120842, answer score: 3