/*
 *
 * Copyright (c) 1997-1999 Scott Oaks and Henry Wong. All Rights Reserved.
 *
 * Permission to use, copy, modify, and distribute this software
 * and its documentation for NON-COMMERCIAL purposes and
 * without fee is hereby granted.
 *
 * This sample source code is provided for example only,
 * on an unsupported, as-is basis. 
 *
 * AUTHOR MAKES NO REPRESENTATIONS OR WARRANTIES ABOUT THE SUITABILITY OF
 * THE SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
 * TO THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
 * PARTICULAR PURPOSE, OR NON-INFRINGEMENT. AUTHOR SHALL NOT BE LIABLE FOR
 * ANY DAMAGES SUFFERED BY LICENSEE AS A RESULT OF USING, MODIFYING OR
 * DISTRIBUTING THIS SOFTWARE OR ITS DERIVATIVES.
 *
 * THIS SOFTWARE IS NOT DESIGNED OR INTENDED FOR USE OR RESALE AS ON-LINE
 * CONTROL EQUIPMENT IN HAZARDOUS ENVIRONMENTS REQUIRING FAIL-SAFE
 * PERFORMANCE, SUCH AS IN THE OPERATION OF NUCLEAR FACILITIES, AIRCRAFT
 * NAVIGATION OR COMMUNICATION SYSTEMS, AIR TRAFFIC CONTROL, DIRECT LIFE
 * SUPPORT MACHINES, OR WEAPONS SYSTEMS, IN WHICH THE FAILURE OF THE
 * SOFTWARE COULD LEAD DIRECTLY TO DEATH, PERSONAL INJURY, OR SEVERE
 * PHYSICAL OR ENVIRONMENTAL DAMAGE ("HIGH RISK ACTIVITIES").  AUTHOR
 * SPECIFICALLY DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR
 * HIGH RISK ACTIVITIES.
 */


class CircularListNode {
    Object o;
    CircularListNode next;
    CircularListNode prev;
}

public class CircularList {
	private CircularListNode current;

	public synchronized void insert(Object o) {
		CircularListNode tn = new CircularListNode();
		tn.o = o;
		if (current == null) {
			tn.next = tn.prev = tn;
			current = tn;
		} else {	// Add Before Current Node
			tn.next = current;
			tn.prev = current.prev;
			current.prev.next = tn;
			current.prev = tn;
		}
	}

	public synchronized void delete(Object o) {
		CircularListNode p = find(o);
		CircularListNode next = p.next;
		CircularListNode prev = p.prev;
		if (p == p.next) {	   // Last Object on the list
			current = null;
			return;
		}
		prev.next = next;
		next.prev = prev;
		if (current == p) current = next;
	}

	private CircularListNode find(Object o) {
		CircularListNode p = current;
		if (p == null)
			throw new IllegalArgumentException();
		do {
			if (p.o == o) return p;
			p = p.next;
		} while (p != current);
		throw new IllegalArgumentException();
	}

	public synchronized Object locate(Object o) {
		CircularListNode p = current;
		do {
			if (p.o.equals(o)) return p.o;
			p = p.next;
		} while (p != current);
		throw new IllegalArgumentException();
	}

	public synchronized Object getNext() {
		if (current == null)
			return null;
		current = current.next;
		return current.o;
	}
}