A Notifier is an object which will be notified by a priority queue when the index in the queue viewed as array of some data item has been changed.
A Notifier is notified when the index in the queue viewed as array of some data item has been changed, this gives the Notifier object the ability to update its value of the index for that data item. As an example, in the search engine, the WebQueueBundle class implements Notifier. Web queue bundles store url together with their weights and allow one to get out the url of highest weight. This is implemented by storing in a PriorityQueue keys consisting of hashes of urls (as fixed length) and values consisting of the weight. Then in a web archive the url and its index in the priority queue is stored. When the index in the queue changes, the WebQueueBundle's notify method is called to adjust the index that is stored in the web archive.