import java.util.*;

public class LinkedList {
    private Link firstLink;
    private Link lastLink;
    private int count = 0;

    public LinkedList() {
	firstLink = lastLink = new Link(null,null,null);
    }

    private class Link {
	public Object value;
	public Link next;
	public Link prev;

	public Link(Object v, Link n, Link p) {
	    value = v; next = n; prev = p;
	}

	public void insert(Object newValue) {
	    Link newNode = new Link(newValue, this, prev);
	    count++;
	    if (prev == null) firstLink = newNode;
	    else prev.next = newNode;
	    prev = newNode;
	}

	public Object remove() {
	    if (next == null)
		return null;
	    count--;
	    next.prev = prev;
	    if (prev == null) firstLink = next;
	    else prev.next = next;
	    return value;
	}
    }

    private class ListEnumeration implements Enumeration {
	public Link link = null;

	public boolean hasMoreElements() {
	    if (link == null) link = firstLink;
	    else link = link.next;
	    return link.next != null;
	}
	 
	public Object nextElement() {return link.value;}
    }

    public boolean isEmpty() { return firstLink == lastLink;}

    public int size() { return count;}

    public Object firstElement() { return firstLink.value;}

    public Object lastElement() { return lastLink.prev.value;}

    public void addFront(Object newValue) {firstLink.insert(newValue);}

    public void addBack(Object newValue) {lastLink.insert(newValue);}

    public void addElement(Enumeration e, Object newValue) {
	ListEnumeration le = (ListEnumeration) e;
	le.link.insert(newValue);
    }

    public Object removeFront() {return firstLink.remove();}

    public Object removeBack() {return lastLink.prev.remove();}

    public Object removeElement(Enumeration e) {
	ListEnumeration le = (ListEnumeration) e;
	return le.link.remove();
    }

    public Enumeration elements() {return new ListEnumeration();}

    public static void main(String[] args) {
	LinkedList ll = new LinkedList();
	for(int i = 0; i < 5; i++) {
	    double x = Math.random();
	    System.out.println(x);
	    ll.addBack(new Double(x));
	}
	System.out.println("--- readout elements ---");
	Enumeration le = ll.elements();
	while(le.hasMoreElements()) 
	    System.out.println(le.nextElement());
	System.out.println("--- remove front elements ---");
	Object o;
	while ((o = ll.removeFront()) != null)
	    System.out.println(o);
    }
}