QueueStack.java

class QueueStack<T> {

    private List<T> dataCollection;

    QueueStack() {
        dataCollection = new LinkedList<>();
    }

    void push(T item) {
        dataCollection.add(0, item);
    }

    Optional<T> pop() {
        if(dataCollection.size() > 0)
            return Optional.of(dataCollection.remove(dataCollection.size() - 1));
        else
            return Optional.empty();
    }

    void clear() {
        dataCollection.clear();
    }

}