As systems evolve, they can get very complex. It is not unusual to end up with a very large (and sometimes confusing) collection of classes and interactions. In many cases, we don't want to expose this complexity to the client. The Facade (also known as Façade) design pattern helps us to hide the internal complexity of our systems and expose only what is necessary to the client through a simplified interface [Eckel08, page 209]. In essence, Facade is an abstraction layer implemented over an existing complex system.

The role of Facade is demonstrated in the following figure. The figure is a class diagram representation of Wikipedia's Java Facade example [j.mp/wikifac]. A computer is a complex machine that depends on several parts to be fully functional. To keep things simple, the word computer in this case, refers to an IBM derivative that uses a Von Neumann architecture. Booting a computer is a particularly complex procedure. The CPU, main memory, and hard disk...