/*
* Copyright (C) 2004-2008 Jive Software. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.jivesoftware.util;
/**
* Doubly linked node in a LinkedList. Most LinkedList implementations keep the
* equivalent of this class private. We make it public so that references
* to each node in the list can be maintained externally.
* <p>
* Exposing this class lets us make remove operations very fast. Remove is
* built into this class and only requires two reference reassignments. If
* remove existed in the main LinkedList class, a linear scan would have to
* be performed to find the correct node to delete.</p>
* <p>
* The linked list implementation was specifically written for the Jive
* cache system. While it can be used as a general purpose linked list, for
* most applications, it is more suitable to use the linked list that is part
* of the Java Collections package.</p>
*
* @author Jive Software
* @see org.jivesoftware.util.LinkedList
*/
public class LinkedListNode<E> {
public LinkedListNode<E> previous;
public LinkedListNode<E> next;
public E object;
/**
* This class is further customized for the CoolServlets cache system. It
* maintains a timestamp of when a Cacheable object was first added to
* cache. Timestamps are stored as long values and represent the number
* of milleseconds passed since January 1, 1970 00:00:00.000 GMT.
* <p>
* The creation timestamp is used in the case that the cache has a
* maximum lifetime set. In that case, when
* [current time] - [creation time] > [max lifetime], the object will be
* deleted from cache.</p>
*/
public long timestamp;
/**
* Constructs an self-referencing node. This node acts as a start/end
* sentinel when traversing nodes in a LinkedList.
*/
public LinkedListNode() {
previous = next = this;
}
/**
* Constructs a new linked list node.
*
* @param object the Object that the node represents.
* @param next a reference to the next LinkedListNode in the list.
* @param previous a reference to the previous LinkedListNode in the list.
*/
public LinkedListNode(E object, LinkedListNode<E> next, LinkedListNode<E> previous) {
if (next != null && previous != null) {
this.insert(next, previous);
}
this.object = object;
}
/**
* Removes this node from the linked list that it was a part of.
* @return This node; next and previous references dropped
*/
public LinkedListNode<E> remove() {
previous.next = next;
next.previous = previous;
previous = next = null;
return this;
}
/**
* Inserts this node into the linked list that it will be a part of.
* @return This node, updated to reflect previous/next changes
*/
public LinkedListNode<E> insert(LinkedListNode<E> next, LinkedListNode<E> previous) {
this.next = next;
this.previous = previous;
this.previous.next = this.next.previous = this;
return this;
}
/**
* Returns a String representation of the linked list node by calling the
* toString method of the node's object.
*
* @return a String representation of the LinkedListNode.
*/
@Override
public String toString() {
return object == null ? "null" : object.toString();
}
}