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;
    }
}

// WARNING: this is not the best way of writing a list class
// in Java. It's better to make nodes invisible from outside
// and let the outside world see the data only. 
// We choose a more transparent implementation so that it's 
// easier to test. 

public class LinkedList {
    private Node first;

    public LinkedList() {
	first = null;
    }

    public boolean isEmpty() {
	return (first == null);
    }

    public Node getFirst() {
	return first;
    }

    public void setFirst(Node n) {
	first = n;
    }

    public void addFront(Node n) {
	n.setNext(first); 
	first = n;
    }
}

public class TestLinkedLists {
    public static void main(String [] args) {
	// testing nodes:
	Node first = new Node(5);
	Node second = new Node(6);
	first.setNext(second);
	System.out.println( (first.getNext()).getData() );
	first.setNext(null);
	System.out.println(first.getNext());

	// testing linked lists:
	LinkedList list = new LinkedList();

	// add the two nodes to the list
	list.setFirst(first);
	list.addFront(second);

	// print the list
	System.out.println("The list elements are:");
	Node n = list.getFirst(); // if the list is empty, first is null
	while(n != null) {
	    int data = n.getData();
	    System.out.println(data);
	    n = n.getNext();
	}

	// adding an element to the back of the list
	Node oneMore = new Node(7);
	n = list.getFirst(); // if the list is empty, first is null
	if (n == null) {
	    list.setFirst(oneMore);
	} else {
	    Node last = null; // initialize last to keep the compiler happy
	    while(n != null) {
		last = n; // last is "one step behind" n 
		n = n.getNext();
	    }
	    last.setNext(oneMore);
	}

	// print the list after adding an element
	System.out.println("The list elements after adding one more are:");
	n = list.getFirst(); // if the list is empty, first is null
	while(n != null) {
	    int data = n.getData();
	    System.out.println(data);
	    n = n.getNext();
	}
    }
}