public class Node {
    private int data;
    private Node next;

    public Node(int d) {
	data = d;
	next = null; // can skip this line: null is the default value
    }

    public int getData() {
	return data;
    }

    // return the next node
    public Node getNext() {
	return next;
    }

    // set the node 
    public void setNext(Node n) {
	next = n;
    }
}

public class LinkedList {
    private Node first;

    public LinkedList() {
	first = null;
    }

    // returns 'true' for an empty list and 'false' otherwise
    public boolean isEmpty() {
	return (first == null);
    }

    // returns the reference to the first node in the list
    // if the list is empty, null is returned.
    public Node getFirst() {
	return first;
    }

    // the method adds the node to the front of the list.
    // the 'next' field of the node is replaced by the reference to 
    // the rest of the list
    // If the parameter is null, an error message is printed and
    // the program stops
    public void addFront(Node n) {
	// before calling a method on n, need to make sure 
	// that n is not null
	if (n == null) {
	    System.out.println("Cannot add a null element to the list");
	    System.exit(0);
	}
	// if we got here, n is not null
	n.setNext(first); 
	first = n;
    }

    // the method removes the first node from the list. 
    // if the list is empty, an error message is printed and
    // the program stops
    public void removeFront() {
	// before removing an element from a list, need to make sure that
	// the list is not empty
	if (first == null) {
	    System.out.println("Cannot remove an element from an empty list");
	    System.exit(0);
	}
	// if we got here, the list is not empty
	first = first.getNext();
    }
}

public class TestLinkedLists {
    public static void main(String [] args) {
	// testing nodes:
	Node first = new Node(5);
	Node second = new Node(6);

	LinkedList list = new LinkedList();
	
	// add the two nodes to the list
	list.addFront(first);
	list.addFront(second);

	// Something to think about for Wednesday:
	// how do we print the list? 
    }
}