The Observer design pattern is a way of structuring code so that one class can automatically update another class when it changes state. This is often used in GUI programming, so that when a button is clicked, for example, the display can be updated accordingly. The Observer design pattern has two main components: the Subject and the Observer. The Subject is the object being watched, and the Observer is the object doing the watching. In order for this to work, the Subject must provide a way for Observers to register themselves; this usually takes the form of an addObserver() method.   The Subject will then notify all of its Observers whenever it changes state, typically by calling a method such as update(). The Observers can then use this information to update their own state accordingly. The Observer design pattern is a powerful tool, but it should be used with care. If too many Observers are registered with a Subject, then notification messages can start to pile up, potentially leading to performance problems. Therefore, it is important to consider whether using this design pattern is really the best solution for a particular problem before implementing it.