Sometimes, we want to update a group of objects when the state of another object changes. A very popular example lies in the Model-View-Controller (MVC) pattern. Assume that we are using the data of the same model in two views, for instance in a pie chart and in a spreadsheet. Whenever the model is modified, both the views need to be updated. That's the role of the Observer design pattern [Eckel08, page 213].

The Observer pattern describes a publish-subscribe relationship between a single object, : the publisher, which is also known as the subject or observable, and one or more objects, : the subscribers, also known as observers. In the MVC example, the publisher is the model and the subscribers are the views. However, MVC is not the only publish-subscribe example. Subscribing to a news feed such as RSS or Atom is another example. Many readers can subscribe to the feed typically using a feed reader, and every time a new item is added, they receive the update...