Facade Pattern

The facade pattern is a software engineering design pattern commonly used with Object-oriented programming. (The name is by analogy to an architectural facade.)

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 and test, 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.

Examples

This is an abstract example of how a client (“you”) interacts with a facade (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) {
		...
	}
}

/* Facade */

class Computer {
	private CPU cpu=null;
	private Memory memory=null;
	private HardDrive hardDrive=null;

	public Computer() {
		this.cpu=new CPU();
		this.memory=new Memory();
		this.hardDrive=new HardDrive();
	}

	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();
	}
}