A DESCRIPTION OF THE PROBLEM :
// stack iterator starts from bottom of stack instead of top of stack
import java.util.Stack;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
class Main {
    public static void main(String[] args) {
        Stack<Character> stack = new Stack<>();
        Collection<Character> list = new ArrayList<>();
        stack.push('a');
        list.add('b');
        stack.addAll(list);
        Iterator stackIterator = stack.iterator();
        // prints 'a' (bottom of stack) and not 'b' (top of stack)
        System.out.println(stackIterator.next());
        // prints 'b' (top of stack) and not 'a' (bottom of stack)
        System.out.println(stackIterator.next());
    }
}