public class ListQueue implements Queue {
    private class Node {
	Object item;
	Node next;
    }
    private Node head;
    private Node tail;

    public ListQueue() {
	clear();
    }
    public void clear() {
	head = null;
	tail = null;
    }
    public boolean isEmpty() {
	return head == null;
    }
    public void enqueue(Object x) {
	Node n = new Node();
	n.item = x;
	if (isEmpty())
	    head = n;
	else
	    tail.next = n;
	tail = n;
    }
    public Object dequeue() {
	if (isEmpty())
	    throw new java.util.EmptyStackException();
	Object x = head.item;
	head = head.next;
	return x;
    }
    public Object first() {
	if (isEmpty())
	    throw new java.util.EmptyStackException();
	return head.item;
    }
}