Facade Pattern

Facade pattern is a structural pattern. Like Adapter pattern, it also alters an interface but with the purpose of simplifying it.

Many times we have to work with a complex software system which consists of various parts. These individual parts have their interfaces for clients to work with. We need to use system as a whole to get some work done and this work is done by interacting with different individual parts of the system. Interacting with so many subparts and hence so many interfaces makes our task quite complex and involves many steps.

We can make use of Facade pattern in such situations to simplify interaction with a complex system. We design a simple interface, called as Facade, for client to work with, and Facade internally interacts with different parts of the complex system. So now all the complex implementation of interacting with various interfaces of the complex system is within the Facade and client just needs to interact with this simple interface. Facade doesn't restrict access to low level functionalities of the system and client can still access low level classes if needed. 

Code structure

To implement a Facade, we design a class which is composed of different subparts of the complex system we plan to provide simple interface for. This class is our Facade for that complex system.

Facade is composed of different subparts of the complex system needed to get some work done. Client just interacts with Facade.

Facade pattern is also a way to implement OO design principle of least knowledge(also known as Law of Demeter) in a system. This principle seeks to reduce number of classes an object interacts with as much as possible so that our system is less fragile and less complex to understand. With Facade pattern, client now only interacts with Facade instead of all those subparts in the complex system.