Sort a stack, using a stack, and only* a stack. It's not at all efficient.

StackSort.java

import java.util.Stack;

public class StackSort {
	/**
	 * Sort a stack, using a stack, and only* a stack. It's not at all efficient.
	 * @param asc The stack we want to sort into ascending order.
	 * @throws NullPointerException If any element in the stack is NULL.
	 */
	public static <T extends Comparable<? super T>> void sort(Stack<T> asc) {
		Stack<T> desc = new Stack<>();
		T outside = null; // Used to swap items in the stack

		while (asc != null) {
			while (!asc.empty()) {
				if(desc.empty() || asc.peek().compareTo(desc.peek()) > 0) desc.push(asc.pop());
				else if(outside == null) outside = asc.pop();
				else break;
			}

			if (asc.empty()) {
				while (!desc.empty()) asc.push(desc.pop());
				return;
			}

			do {
				if(outside == null && outside.compareTo(desc.peek()) > 0) {
					asc.push(outside);
					outside = null;
				} else asc.push(desc.pop());
			} while (!desc.empty());

			if(outside != null) {
				asc.push(outside);
				outside = null;
			}
		}
	}

	public static void main(String[] args) {
		for(int i = 0; i < 100; i++) {
			Stack<Double> stack = new Stack<>();
			Integer length = (int) Math.round(0 + Math.random() * 20);
			for(int j = 0; j < length; j++) stack.push(Math.random());

			System.out.println(stack);
			StackSort.sort(stack);
			System.out.println(stack + "\n");
		}
	}
}