class ListStack {

    int value;
    int size;
    ListStack next;

    public ListStack () {
        this.size = 0;
    }

    private ListStack (int add, ListStack head) {
        this.value = add;
        this.next = head;
    }

    public void push (int add) {
        this.next = new ListStack(this.value, this.next);
        this.value = add;
        this.size++;
    }

    public int pop() {
        if (this.size > 0) {
            int i = this.value;
            this.value = this.next.value;
            this.next = this.next.next;
            this.size--;
            return i;
        } else {
            throw new EmptyListStackException();
        }
    }

    public int peek() {
        return this.value;
    }

    public void print() {
        String aString = "";
        ListStack p = this;
        for (int i = 1; i < this.size; i++, p = p.next) {
            aString += p.value+" -> ";
        }
        aString += "null";
        System.out.println(aString);
    }

    public class EmptyListStackException extends NullPointerException {
        public EmptyListStackException() {
            super("The list stack is empty, you cannot .pop() from it.");
        }
    }
}