public class ActionHistory {
    private class Node<T> {
        private T content = null;
        private Node nextNode = null;

        public Node(T content) {
          setContent(content);
        }

        private void setContent(T content) {
            this.content = content;
        }

        private T getContent() {
            return this.content;
        }

        private void setNextNode(Node nextNode) {
            this.nextNode = nextNode;
        }

        private Node getNextNode() {
            return this.nextNode;
        }
    }

    private Node head = null;

    public ActionHistory() {}

    public boolean isEmpty() {
      if (head == null) {
        return true;
      }
      return false;
    }

    public void push(Object content) {
      if (isEmpty()) {
         head = new Node(content);
      } else {
        Node newNode = new Node(content);
        newNode.setNextNode(head);
        head = newNode;
      }
    }

    public Object pop() {
      if (isEmpty()) {
        return null;
      }
      Object content = head.getContent();
      head = head.getNextNode();
      return content;
    }

    public Object top() {
      return ((isEmpty()) ? null : head.getContent());
    }
}