// making everything public for clarity (but bad style!)
// see C&S 23.35-23.38
public class TestDLL {
    public static void main(String[] args) {
	DLL list = new DLL();
	list.append("D");
	list.append("C");
	list.append("B");
	list.append("A");
	System.out.println(list);
	list.removeFront();
	System.out.println(list);
    }
}
class DLL {
    public DLLCell head;
    public DLLCell tail;
    
    public void append(Object o) {
	if (head == null) {
	    head = new DLLCell(o,null,null);
	    tail = head;
	}
	else {
	    tail.next = new DLLCell(o,tail,null);
	    tail = tail.next;
	}
    }

    public Object removeFront() {
	Object front = null;
	if (head != null) {
	    front = head.data;
	    head = head.next;
	    if ( head == null )
		tail = null;
	    else 
		head.prev = null;
	}
	return front;
    }

    public String toString() {
	String list = "";
	for (DLLCell cell = head; cell != null; cell = cell.next) {
	    list = list + cell;
	    if ( cell.next != null ) list += "<-->";
	}
	return list;
    }
}
class DLLCell {
    public DLLCell next;
    public DLLCell prev;
    public Object data;
    public DLLCell(Object data, DLLCell prev, DLLCell next) {
	this.data = data;
	this.prev = prev;
	this.next = next;
    }
    public String toString() { return ""+data; }
}