The Facade Pattern

The façade pattern is a software engineering design pattern commonly used with Object-oriented programming.

A facade is an object that provides a simplified interface to a larger body of code, such as a class library. A facade can:

• make a software library easier to use and understand, since the facade has convenient methods for common tasks;
• make code that uses the library more readable, for the same reason;
• reduce dependencies of outside code on the inner workings of a library, since most code uses the facade, thus allowing more flexibility in developing the system;
• wrap a poorly-designed collection of APIs with a single well-designed API (as per task needs).

An Adapter is used when the wrapper must respect a particular interface and must support a polymorphic behavior. On the other hand, a facade is used when one wants an easier or simpler interface to work with

Structure

Facade

The facade class interacts Packages 1, 2, and 3 with the rest of the application.

Clients

The objects using the Facade Pattern to access resources from the Packages.

Packages

Software library / API collection accessed through the Facade Class.

Examples

Java

This is an abstract example of how a client ("you") interacts with a façade (the "computer") to a complex system (internal computer parts, like CPU and HardDrive).

/* Complex parts */

class CPU {
public void freeze() { ... }
public void jump(long position) { ... }
public void execute() { ... }
}

class Memory {
public void load(long position, byte[] data) {
...
}
}

class HardDrive {
public byte[] read(long lba, int size) {
...
}
}

/* Façade */

class Computer {
public void startComputer() {
cpu.freeze();
memory.load(BOOT_ADDRESS, hardDrive.read(BOOT_SECTOR, SECTOR_SIZE));
cpu.jump(BOOT_ADDRESS);
cpu.execute();
}
}

/* Client */

class You {
public static void main(String[] args) {
Computer facade = new Computer();
facade.startComputer();
}
}